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Long-term productivity of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya
Forest Ecology and Management 139 (2000) 187±201
Long-term productivity of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya II. Crop growth and system performance J.E. Lotta,b,1, S.B. Howarda,b,2, C.K. Ongb, C.R. Blacka,* a
School of Biological Sciences, University of Nottingham, Sutton Bonington Campus, Loughborough, LE12 5RD, UK b International Centre for Research in Agroforestry (ICRAF), PO Box 30677, ICRAF House, Gigiri, Nairobi, Kenya Received 30 March 1999; received in revised form 10 December 1999; accepted 14 December 1999
Abstract Maize and cowpea were grown as sole stands or in agroforestry systems containing grevillea trees (Grevillea robusta A. Cunn.). Crop and system performance were examined over a 4.5-year-period (nine growing seasons) commencing in October 1991; failure of the rains caused the loss of one cropping season. A rotation of cowpea (Vigna unguiculata L. Walp.) and maize (Zea mays L.) was grown during the ®rst ®ve seasons after planting the trees, while maize was grown continuously during the ®nal four seasons. Sole maize was also grown under spectrally neutral shade netting which reduced incident radiation by 25, 50 or 75% to establish the relative importance of shade and below-ground competition for water and nutrients in determining the performance of understorey crops. The above-ground biomass and grain yield of understorey crops were not signi®cantly affected by the presence of grevillea during the ®rst four seasons, but were greatly reduced in subsequent seasons as the trees became increasingly dominant; maize yields reached 50% of the sole crop values only once during the ®nal four seasons, when rainfall was unusually high. The hypothesis that competition for water was the primary limiting factor for understorey crops was supported by the observation that above-ground biomass and grain yield were greater in the shade net treatments than in agroforestry maize, demonstrating that shade was not solely responsible for the substantial yield losses in the latter treatment. Performance ratios (ratio of values for the agroforestry system relative to sole stands) for total above-ground and trunk biomass in grevillea were initially low, re¯ecting the impact of competition with associated crops during tree establishment, but increased to unity within 2.5 years. Performance ratios for the understorey crops exhibited the reverse trend, initially being close to unity but approaching zero for three of the ®nal four seasons. Performance ratios were never close to unity for both trees and crops during the same season, indicating that there was always competition for available resources irrespective of crop species or tree size. The implications for agroforestry system design and future research are discussed. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Agroforestry; Cowpea; Grevillea robusta; Kenya; Maize; Productivity; Semi-arid
* Corresponding author. Tel.: 44-115-9516337; fax: 44-1159516334 E-mail address: [email protected] (C.R. Black). 1 Present address: British Waterways, Llanthony Warehouse, Gloucester, GL1 2EJ, UK. 2 Present address: WWF-UK, Panda House, Weyside Park, Catteshall Lane, Godalming, Surrey, GU7 1XR, UK.
1. Introduction Agroforestry has attracted considerable interest in recent years because of its potential to maintain or increase agricultural productivity in areas where high energy input, large-scale agriculture is impractical
0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 2 6 7 - X
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(Kidd and Pimental, 1992). Cannell et al. (1996) proposed that agroforestry may increase productivity provided the trees capture resources that are underutilised by annual crops. This hypothesis is supported by studies at Hyderabad, India which demonstrated that substantial improvements in productivity resulting from an increase in the utilisation of annual rainfall from 40 to 80% were achieved in agroforestry systems containing perennial pigeonpea (Cajanus cajan L.) and groundnut (Arachis hypogaea L.; Ong et al., 1996). However, experiments at Machakos, Kenya involving root trenching and tree species such as Leucaena leucocephala (Govindarajan et al., 1996) demonstrated that alley cropping may adversely affect crop productivity in semi-arid environments because increased competition for water outweighs potential soil fertility bene®ts resulting from applications of tree mulch, nitrogen ®xation or increased root turnover. Other studies have shown that regular pruning of the trees in alley cropping systems encourages the proliferation of ®ne roots in the surface soil horizons, so decreasing spatial niche separation between the tree and crop roots and hence the potential for complementarity in the use of below-ground resources (Van Noordwijk and Purnomosidhi, 1995). It is therefore important to select for trees with appropriate root architecture in order to achieve spatial complementarity and avoid major crop yield losses. The tree species examined in the present study, Grevillea robusta (grevillea), offers promise for use in agroforestry systems because observations of its root distribution (Jonsson et al., 1988; Mwihomeke, 1993) and measurements of water uptake (Lott et al., 1996; Howard et al., 1997) indicate that it is capable of extracting substantial quantities of water from beneath the crop rooting zone. Its widespread popularity with farmers in East Africa also suggests that adverse effects on associated crops are limited. Several mechanisms may enable agroforestry systems to use available water more effectively than sole crops and improve microclimatic conditions for understorey crops (Ong et al., 2000). These include: reductions in soil evaporation resulting from shading by the tree canopy and reduced air movement through the understorey environment; alterations in microclimatic conditions arising from reductions in air temperature, wind speed and saturation de®cit which
decrease crop water use (Monteith et al., 1991); and reductions in tissue temperature which bene®t the phenology and productivity of understorey crops by minimising exposure to heat stress (Monteith et al., 1991; Vandenbelt and Williams, 1992; Jonsson et al., 1999). The key question is whether these potential bene®ts may be outweighed by the detrimental effects of competition for light, water and nutrients between the trees and crops. The objective of the present study was to quantify the changing impact on understorey crops as grevillea trees established and matured. In Part I, it was shown that treegrowth in the agroforestry system was adversely affected by competition with the crops during the establishment period, and that tree height, leaf area and leaf, branch and trunk biomass were signi®cantly reduced relative to sole stands. Although tree height in the agroforestry treatment regained parity with sole grevillea within 2.5 years, total above-ground and trunk biomass and leaf area did not recover fully within the 4.5-year observation period. As the economic value of the trees was permanently affected, the viability of this grevillea-based system hinges on the extent to which the presence of trees affected crop productivity and overall system performance. This paper examines the changing impact of grevillea on crop and system performance as the trees matured. 2. Materials and methods 2.1. Experimental design Location, climate, soil characteristics, experimental design and the planting arrangement and management of grevillea (Grevillea robusta A. Cunn.) trees within the Complementarity In Resource Use on Sloping land (CIRUS) trial were described in Part I; details of the wide range of physical, biological and physiological measurements made are given by Ong et al. (2000). Relevant details of crop management, treatments imposed and growth analysis procedures are outlined below. 2.2. Treatments The three treatments reported here comprised sole crops (Cg) of maize (Zea mays L.) or cowpea (Vigna
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unguiculata L. Walp.), a sole tree treatment (Td) and an agroforestry treatment (CTd). Tree and crop spacings were identical in the monocrop and agroforestry treatments; grevillea was grown in a 3 m4 m dispersed arrangement (833 trees haÿ1), while maize and cowpea were grown at spacings of 0.3 m1.0 m (33 333 plants haÿ1) and 0.15 m50.0 m (13 333 plants haÿ1), respectively (Ong et al., 2000). 2.3. Growing seasons Rainfall distribution in Machakos District is bimodal, with the short rains extending from October to the end of December and the long rains from March to the end of May. These periods correspond to the short growing season (S) extending from ca. 1 October to 28/29 February, the long growing season (L) between ca. 1 March and 31 July and the dry season (D) between ca. 1 August and 30 September. Each season is de®ned according to the year in which it occurred; thus the 1993/1994 short growing season is identi®ed as S93/94. 2.4. Shading treatments The productivity of understorey crops in agroforestry systems is affected by shading and competition for water and nutrients. To distinguish between the effects of shading and below-ground competition with grevillea, areas of sole maize were covered during all seasons between the 1994 long (L94) and 1995/1996 short growing seasons (S95/96) with spectrally neutral shade netting (Lowes of Dundee, UK) to remove 25, 50 or 75% of the incident radiation. The netting was attached to 3 m4 m steel frames, equivalent in size to individual cells (de®ned as the 12 m2 of ground area enclosed by four adjacent trees) in the sole grevillea and agroforestry treatments (Lott et al., 2000a).
189
Equivalent unshaded control areas were demarcated in the sole crop plots at the beginning of the season to allow comparison of shaded and unshaded sole stands. The shade nets treatments were de®ned as Cg 25, 50 and 75% depending on the proportion of incident radiation removed; the unshaded control cells are referred to as Cg 0%. 2.5. Growth analysis Cowpea and maize were grown on a rotational basis during the ®rst ®ve seasons (S91/92±S93/94). However, this was replaced by continuous maize cultivation during the ®nal four seasons (L94±S95/96) to avoid infection of cowpea by root-rot (Fusarium udum) and re¯ect local farming practice. Growth analysis procedures for the ®rst ®ve seasons are described by Howard (1997) and for the ®nal four seasons by Lott (1998), but are brie¯y outlined here for clarity. 2.5.1. Cowpea and maize: S91/92±S93/94 seasons Measurements during the ®rst two cropping seasons were con®ned to ®nal harvest, when fresh and dry weights were determined on a row-wise basis for the pods and haulms of cowpea (S91/92) and the stover, cobs and grain of maize (L92). Poor rainfall (<25% of the seasonal average; Table 1) caused complete failure of the maize during the L93 season; detailed comparison of performance of the same crop during equivalent cropping seasons was therefore possible only for S92/93 and S93/94, and hence only for cowpea. As the planting density for cowpea was four times greater than for maize, regular destructive analyses were possible. During S92/93, all 36 plants from one quarter of one cell in each replicate agroforestry plot (Fig. 1a) and an equivalent area of the sole cowpea plots were harvested at ca. 10-day-intervals. Leaf,
Table 1 Rainfall (mm) in CIRUS during the long and short growing seasons and annual total rainfall for the period between planting (October 1991) and completion of the crop growth measurements reported here (March 1996) Time period
Meana
1991
1992
1993
1994
1995
Long growing season (1 March±31 July ) Short growing season (1 October±28/29 February) Annual total (1 January±30 December)
359 350 782
229 404 653
261 773 675
112 381 799
199 628 810
302 337 666
a
Data for 9-year-period for Machakos Maruba Dam Station.
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Fig. 1. (a and b) sampling areas for destructive analyses of cowpea growth and development in the dispersed agroforestry (CTd) treatment during the 1992/1993 (a) 1993/1994 (b) short growing seasons; (c), measurement locations for maize growth and development, meristem and soil temperatures and total shortwave radiation in the dispersed agroforestry (CTd) treatment during the 1994/1995 and 1995/1996 short growing seasons.
stem and pod fresh weights were recorded for six randomly selected plants from each plot before recombining these with the remainder of the sample to determine total fresh and dry weights. Leaf area was estimated using the dry weight:area ratio of disks cut from 40 randomly selected leaves. Measurements at ®nal harvest were made on a row-wise basis for each plot after separating the plants into pods and haulms.
A more detailed sampling strategy was adopted during S93/94 to analyse the spatial variation in crop growth at different locations relative to trees in the agroforestry treatment. Three plants were harvested at 12 locations within one cell (Fig. 1b) of each replicate agroforestry plot on six occasions; plants were also sampled at six locations in the sole crop plots. Leaf, stem and pod fresh and dry weights and leaf area,
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determined using the punched disk method described above, were measured for each plant. At maturity, all cells in the agroforestry plots not previously examined were sampled at the same locations. The remaining plants were harvested on a per cell (agroforestry treatment) or row-wise basis (sole crops). 2.5.2. Maize: L94±S95/96 seasons As the lower planting density of maize precluded routine destructive analysis, allometric methods were developed to allow regular non-destructive estimation of biomass and leaf area. These estimates were based on regression equations established between allometric parameters and destructive measurements involving limited numbers of plants. Allometric relationships for determining leaf area and above-ground biomass were established from weekly non-destructive and destructive measurements of ®ve plants sampled from each of the four replicate agroforestry and sole crop plots; plants were chosen to re¯ect the full range of sizes present. Parameters measured during non-destructive analysis were the smallest and largest basal stem diameters to permit calculation of the ellipsoidal cross-sectional area, height to the tip of the youngest leaf, height to the top of the canopy and the number of green leaves; destructive measurements included above-ground fresh and dry weights and leaf area. Linear regression analysis was used to establish equations describing the relationship between the non-destructively and destructively determined values, with the non-destructive variables being combined to form appropriate allometric parameters (AP) chosen on the basis of previous work (B. McIntyre, personal communication) and goodness of ®t. Two allometric parameters were used, APa for leaf area and leaf dry weight estimation: APa hLN e
(1)
and APb for stem dry weight estimation: APb he x
(2)
where he, LN and CSAm, respectively, denote height to the tip of the youngest fully expanded leaf, leaf number and the mean basal stem cross-sectional area. Total above-ground dry biomass was calculated as the sum of the values for leaf and stem biomass. The allometric relationships obtained were validated
Fig. 2. Relationship between allometric and measured values for maize plants grown as sole crops (Cg) or in the dispersed agroforestry system (CTd) during the 1995 long and 1995/1996 short growing seasons: (a) total leaf area for Cg (r20.85) and CTd maize (r20.72); (b) above-ground biomass for Cg (r20.86) and CTd maize (r20.89).
against destructive harvests carried out at thinning (ca. 25 DAS) and on at least one other occasion during each season. Fig. 2 illustrates the reliability of the estimates for leaf area (Cg: r20.85; CTd, r20.72) and above-ground biomass (Cg: r20.86; CTd, r20.89). The in¯uence of distance and direction from trees on crop performance was examined using nondestructive measurements made at ca. 7-day-intervals between thinning and ®nal harvest in the agroforestry and sole maize treatments. Sixteen sampling locations were chosen in the former treatment to represent the full range of interactions between trees and crops (Fig. 1c); each position was replicated around seven
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trees. Four plants were also examined at each sampling date in all replicate plots of the shade net (Cg 25, 50 and 75%) and unshaded sole crop treatments (Cg 0%) at positions equivalent to those in the agroforestry (CTd) treatment. These measurements were used to determine leaf area and biomass using the allometric procedures described above. At maturity, the plants used for non-destructive growth analysis were harvested and the fresh weights of leaves, stems and cobs were determined separately prior to drying at 708C. The remaining plants in all treatments were harvested on a row-wise basis and fresh and dry weights determined as described above. The timing of germination, ¯oral initiation, anthesis, silking and physiological maturity (Fischer and Palmer, 1984) were determined to establish treatment effects on the duration of the vegetative, reproductive and grain-®lling periods. 2.6. System productivity When the combined yield of the tree and crop components exceeds that of equivalent sole stands, agroforestry systems may be described as over-yielding and demonstrating complementarity of resource use (Ong et al., 1996). Of the various approaches developed to establish whether over-yielding is occurring in agroforestry and intercropping systems, the most common is the land equivalent ratio (LER; Willey, 1985; Rao et al., 1990, 1991). LER may be de®ned as the land area required under sole cropping to produce the yield achieved by intercropping, and corresponds to the sum of the performance ratios for the tree and crop components. Performance ratios were calculated for both components as: PRt
Ma Ms
(3)
where PRt represents the performance ratio for grevillea, and Ma and Ms denote the above-ground biomass of trees (t haÿ1) in the agroforestry and sole tree treatments. To calculate performance ratios for trunk biomass (PRtt), the appropriate values for the agroforestry and sole tree stands were substituted for Ma and Ms in Eq. (2). Performance ratios for total above-ground biomass (PRb) and grain yield (PRg) in maize and cowpea were calculated in an analogous manner.
2.7. Statistical analysis The results were subjected to analysis of variance using GENSTAT 5 to identify signi®cant treatment effects. 3. Results and discussion 3.1. Crop growth and final yields Fig. 3 shows the timecourses for leaf area index (LAI) and above-ground dry biomass (AGB) for cowpea in the agroforestry and sole crop systems during S92/93 and S93/94 as determined by destructive growth analyses. There was no signi®cant treatment effect during S92/93, when LAI and AGB reached maximum values of ca. 60 and 75 days after sowing (DAS; Fig. 3a and b). However, both variables were consistently lower in the agroforestry treatment than in sole cowpea during S93/94 (p<0.05), although the maximum values occurred at approximately the same time as in S92/93. LAI values for sole cowpea (Fig. 3a) were consistently lower between 35±65 DAS during S93/94, re¯ecting the much lower seasonal rainfall (381 mm versus 773 mm; Table 1), but above-ground biomass was less affected (Fig. 3b). The differences in maximum LAI and AGB between treatments were signi®cantly greater (p<0.01) during S93/94, re¯ecting the increasing dominance of the trees after ®ve cropping seasons and the lower rainfall. Fig. 4 shows allometric measurements of LAI and AGB for maize in the sole and agroforestry treatments during S94/95 and S95/96. The timecourses extend to maturity for biomass but only to anthesis for leaf area because the allometric approach for determining leaf area could not be used after appreciable foliar senescence began. The performance of maize in the agroforestry treatment differed greatly between seasons, with the reductions in LAI and AGB relative to sole maize being much smaller during the very wet S94/95 season than during the near-average S95/96 season (628 mm versus 317 mm; Table 1); development was also delayed by ca. 30 days in the agroforestry treatment maize during S95/96. This effect was not attributable to slower accumulation of thermal time under the trees (Lott, 1998), and may have originated from severe water stress resulting from a combination of
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Fig. 3. Timecourses for leaf area index and above-ground biomass of cowpea grown as sole crops (Cg) or in the dispersed agroforestry system (CTd) during the 1992/1993 and 1993/1994 short growing seasons (S92/93 and S93/94). Double standard errors of the mean are shown.
low rainfall and competition with the trees for available soil moisture. This conclusion is supported by the observation that the timing of ¯owering was similar in the agroforestry and sole maize stands during S94/95, when water stress was alleviated by high rainfall. Final yield data for maize and cowpea are shown for all cropping seasons in Fig. 5. Cowpea was grown during S91/92, S92/93 and S93/94 and maize during all other seasons, although poor rains caused crop failure during L93. Grain yield did not differ signi®cantly between treatments during the ®rst three seasons despite rapid tree growth (Lott et al., 2000a) and highly variable seasonal rainfall (Table 1). Signi®cant treatment effects ®rst became apparent during S93/94, when above-ground biomass and grain yield were greater in sole cowpea than in the agroforestry treatment (p<0.001). It is conceivable that the unusually
high rainfall during the preceding S92/93 season (773 mm versus long-term average of 350 mm; Table 1) may have precluded any competitive impact of the relatively large trees within the agroforestry system. The results for the ®nal four cropping seasons (L94± S95/96) clearly demonstrate the adverse effect of the established grevillea trees on the growth and productivity of maize. Above-ground biomass and grain yield were greatly reduced in the agroforestry treatment and almost no grain yield was obtained in three of the four seasons; yield reached ca. 50% of that for sole maize only when rainfall was much above average during S94/95 (628 mm; Table 1). These results suggest that water availability was the primary limitation for maize growth, a conclusion supported by the shade net experiments which simulated the shading effect of
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Fig. 4. Timecourses for leaf area index and above-ground biomass of maize growing as sole crops (Cg) or in the dispersed agroforestry system (CTd) during the 1994/1995 and 1995/1996 short growing seasons (S94/95 and S95/96). Double standard errors of the mean are shown.
the trees in the absence of competition for water and nutrients. Above-ground biomass and grain yield were consistently greater (p<0.05) in all shade net treatments than in the agroforestry treatment with exception of grain yield in the Cg 50% treatment during the very wet S94/95 season (Fig. 5). Above-ground biomass and grain yield in the Cg 25 and 50% treatments were not signi®cantly different from unshaded sole maize during the driest seasons (L94 and S95/96), but were signi®cantly reduced during seasons of high rainfall (S94/95 and L95; p<0.05), demonstrating the existence of an interaction between the effects
of shade and water availability. Shade was therefore not solely responsible for the substantial reduction in maize growth in the agroforestry system, as the mean intensity of shade imposed by grevillea on the understorey maize was intermediate between the Cg 25 and 50% shade net treatments. Harvest index was less affected than biomass or grain yield in the agroforestry treatment except during S95/96 (Fig. 5). Crop productivity is often assessed in terms of the quantity of grain obtained at ®nal harvest because economic value accrues mainly from grain production. However, farmers in the dry tropics frequently
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Fig. 5. Above-ground biomass, grain yield and harvest index for maize and cowpea crops during the short and long growing seasons and annual totals for the period between the 1991/1992 and 1995/1996 short growing seasons (S91/92±S95/96). Values are shown for crops grown as sole stands (Cg), in the dispersed agroforestry treatment (CTd), or as sole stands under 0, 25 or 50% artificial shade (Cg 0, 25 and 50%). Cowpea was grown during the 1991/1992, 1992/1993 and 1993/1994 short growing seasons and maize in all other seasons. Single standard errors of the mean are shown. Letters denote significant treatment differences (p<0.05).
attribute considerable value to maize stover by using it for animal fodder, barriers against erosion (trash lines), or as mulch (Okoba et al., 1998). Indeed, a market for maize stover exists in many dryland areas of Kenya, India and West Africa (E. Franzel, personal communication), resulting in a net export of crop
residues from the farm. It is therefore important for farmers that neither total above-ground biomass nor grain yield are compromised when crops are grown in agroforestry systems. The observation that aboveground biomass and grain yield in the Cg 25% shade treatment were generally comparable to unshaded sole
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maize (Fig. 5) indicates that the intensity of shade cast by overstorey grevillea trees was not in itself detrimental within this dryland environment, and hence that system productivity may be increased by the introduction of appropriate tree species. Belsky (1994) also used arti®cial shade treatments in the savanna of eastern Kenya to demonstrate that shading by Acacia nilotica and Adamsonia digitata did not adversely affect the understorey vegetation. However, it is clear that limitations imposed by below-ground competition, particularly for water, must be minimised if farmers are to adopt the technology. This conclusion is in marked contrast with earlier observations of root distribution (Jonsson et al., 1988; Mwihomeke, 1993) in eastern Africa and measurements of water uptake by 2±3-year-old trees at Machakos (Lott et al., 1996; Howard et al., 1997) which suggested that grevillea may extract a substantial proportion of its water requirements from beneath the crop rooting zone and therefore offers considerable promise of complementarity with associated crops. However, the potential of grevillea to exploit deep moisture reserves would have been severely constrained by the limited soil depth above the bedrock in the present trial (typically 0.5±1.4 m; Howard, 1997; Ong et al., 2000). Measurements showed that the roots of 4±6year-old trees were present throughout the soil pro®le and there was no spatial separation of the rooting zones of grevillea and maize (Smith et al., 1999); competition for water in the surface horizons was therefore unavoidable and complementarity of water use would have been possible only if alternative sources had been available 3.2. Complementarity of resource use and system productivity The overall height of the histograms shown in Fig. 6 represents the LER values for total above-ground biomass (Fig. 6a) and the main economic components (trunks and grain; Fig. 6b) for each cropping season. The stippled and solid areas within each column indicate the performance ratios for the tree and crop components within the agroforestry system. Missing values are apparent for the crop component during S91/92 and L92, when total above-ground biomass was not recorded (Fig. 6a), and L93, when poor rainfall caused crop failure (Fig. 6a and b). The
Fig. 6. Seasonal land equivalent ratios (overall column height) for the dispersed agroforestry (CTd) treatment calculated for (a) total above-ground biomass and (b) the main economic products, grain yield and trunk biomass. Single standard errors of the mean are shown. The stippled and solid areas within each column represent the performance ratios for the tree and crop components of the CTd system.
LER values were signi®cantly above or not signi®cantly different from unity in all except these instances, indicating either that a larger proportion of the available resources was captured, or that the captured resources were used more effectively for dry matter production. This situation occurs when there is niche differentiation between the system components or green area duration is extended (Loomis and Conner, 1992), and provides evidence of complementarity. The performance ratios of grevillea expressed either as total above-ground biomass (PRt) or trunk biomass (PRtt) were low (0.14 and 0.15, respectively) during
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the ®rst cropping season (S91/92), but subsequently increased steadily to reach unity by L94 (Fig. 6a); similar values were obtained for all subsequent seasons. These results indicate that seasonal performance was poorer when grevillea was grown in the agroforestry treatment than in sole stands during the ®rst 2 years, but thereafter was comparable in both treatments, supporting the conclusions of Lott et al. (2000a). Performance ratios for above-ground crop biomass (PRb) and grain yield (PRg) exhibited the reverse trend, as the values were close to unity during the ®rst three cropping seasons but approached zero during three of the ®nal four seasons (Fig. 6), although performance improved during the very wet S94/95 season (PRg0.39; PRb0.46). However, it should be remembered that cowpea was grown during three of the four seasons between S91/92 and S93/94 and that maize was grown continuously thereafter. The decline in crop performance in the agroforestry treatment may therefore have been at least partly attributable to the change in crop species from a C3 legume to a C4 cereal rather than changing tree/crop interactions resulting from tree growth. Performance ratios never approached unity for both the tree and crop components during the same season, demonstrating that there was always competition for the same resource pool irrespective of crop species or tree size. The observed LER values are consistent with alley cropping studies in India and Kenya (Ong et al., 1991, 1992; Rao et al., 1990, 1991; Jama et al., 1995), in which resource capture was dominated by the tree component because of the relatively high planting densities used. In their comparison of agroforestry and savanna systems, Ong and Leakey (1999) concluded that agroforestry research has typically focussed on fast growing tree species planted at close spacings and high densities, with the result that the trees captured most of the available resources and the potential microclimatic bene®ts for understorey crops were negated by reductions in soil moisture resulting from increased interception losses and water use by the trees. Results from other experiments at Machakos were used to show that, when seasonal rainfall was below 250 mm, maize yields were linearly correlated with the quantity of water transpired by ®ve tree species including grevillea: crop failure occurred when tree transpiration exceeded 100 mm. Independent assessments of transpiration based on a light
197
interception model (McIntyre et al., 1996) suggested that only 50% of the seasonal rainfall was used for transpiration and the remainder was lost as soil evaporation in both the sole crop and agroforestry systems. Further evaluation of water use by the trees and crops during the latter stages of the CIRUS trial is reported elsewhere (Lott et al., 2000b). 3.3. Implications for future research The overstorey agroforestry system reported here demonstrated important and changing interactions between the tree and crop components which favoured crop growth during tree establishment, but increasingly favoured the tree component as the system matured. The nature of these interactions and their in¯uence on system performance are examined elsewhere (Lott, 1998). However, it is apparent that the extent to which these interactions in¯uence the economic potential of the system may depend on the size of the interface zone between trees and crops and the proportion of farmland affected. It is also important to emphasise that current understanding of resource capture in agroforestry systems is based largely on well-managed small plots, often situated on research stations, in which 30±50% of rainfall is used for transpiration. For example, plot level studies at Machakos reported rainfall utilisation values for maize and cowpea of 40±45% (McIntyre et al., 1996), suggesting that the opportunities for agroforestry were limited. However, such levels of rainfall utilisation are rarely achieved either in farmers' ®elds or at the landscape level in sub-Saharan Africa because of the prevailing low input agricultural practices, suggesting that there is ample scope for increasing water use by incorporating trees into existing landuse systems. For instance, Rockstrom (1997) showed that only 6±16% of the rainfall on a watershed in Niger was used for transpiration by pearl millet, the remainder being lost by soil evaporation (40%) or deep drainage (33±40%). Further studies are required to establish the mechanisms underpinning the success of recently developed indigenous agroforestry systems. An interesting example is the indigenous simultaneous agroforestry systems based on Melia volkensii (Gurke) developed by farmers in semi-arid Kenya, which have minimal effects on crop yields (Stewart and Bromley, 1994). The widely dispersed trees (10±
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15 m spacing) are regularly pruned to minimise competition and provide high quality timber and other products including durable termite resistant poles, beehives, fodder and fuelwood (Kidundo, 1997). Clearly, there are important lessons to be learnt from such systems; these are currently being examined by ICRAF in association with the Kenyan Forestry Research Institute (KEFRI) and the UK Institute of Terrestrial Ecology (ITE). Ideally, planting densities in agroforestry systems should be manipulated to maximise productivity and economic returns. However, as agroforestry generally involves long payback periods, crop performance must be maintained to the greatest possible extent. The crop component is therefore likely to continue to be planted at optimal or near-optimal densities for sole crops and the trees at sub-optimal densities, even though this may not provide the most productive utilisation of resources within the system. There is a considerable need for research aimed at optimising system densities for the tree and crop components and establishing the extent to which these should be adjusted, perhaps by reducing tree populations, as the system matures. Pruning also represents a viable management option to limit competition and encourage temporal complementarity as pruning prior to the cropping season reduces water use by the trees during crop establishment and may maintain favourable conditions for crop growth throughout the season (Jones and Sinclair, 1995). In areas of bimodal rainfall such as Machakos, pruning would most appropriately be carried out immediately prior to the season of most reliable rainfall to minimise competition at a time when crop yields are potentially greatest; during the less reliable rainy season, crop growth may be limited by erratic or inadequate rainfall, to which the trees are more resistant owing to their more extensive root systems (Schroth, 1995). Regular pruning of the tree canopy provides a compromise between improving crop growth and limiting tree productivity. However, there is scope to limit detrimental effects on tree growth while still providing a favourable environment for understorey crops; for example, it is conceivable that tree canopies pruned to give differing shapes may transpire at similar rates but exhibit differing aboveground interactions with understorey crops. Alternatively, Brenner (1991) and Onyewotu et al. (1994)
demonstrated that root pruning in the upper 1 m of the soil pro®le may be used to limit competition with associated crops. However, such practices are justi®able only when the productive potential of the tree is high and crop yield losses are severe, although root pruning has been adopted by farmers in Bangladesh (Hocking, 1998). There is a need for further ®eld studies of water use and productivity under various root and shoot pruning regimes. Previous selection programmes for grevillea provenances for use in agroforestry have focussed largely on tree height and growth rate (Kallinganire and Hall, 1993), while ICRAF's own programme has concentrated on height, trunk diameter, straightness, bole form, wood density and crown diameter (Esegu and Odoul, 1992). However, Howard (1997) suggested that characteristics which improve complementarity may represent more appropriate selection criteria, while Harwood and Owino (1992) proposed that a sparse and narrow crown and deep rooting habit were desirable traits. ICRAF is currently testing a simple technique for assessing root architecture and root functioning based on allometric approaches and fractal branching principles which may be used to predict the severity of root competition (Ong et al., 1999). Although attributes which improve complementarity are desirable, it is essential that the search for such characteristics does not compromise the economic potential of the system; thus tree species with limited economic value are of little bene®t to resource-poor farmers irrespective of the extent of complementarity with understorey crops. The crop component of agroforestry systems is almost inevitably the species and cultivar best suited to the prevailing climatic conditions when grown as a sole crop. This choice might nevertheless be poorly adapted to the understorey microclimatic conditions experienced by crops in agroforestry systems, particularly in semi-arid environments. For example, any delay in development resulting from modi®cation of the thermal environment and increased soil water de®cits in the crop rooting zone may make it necessary to adopt shorter duration varieties or species, especially in areas such as Machakos where the period between consecutive cropping seasons is short. The increased incidence of water stress resulting from below-ground competition also suggests that cultivars which emerge and establish rapidly and have deep and
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extensive root systems may compete more effectively for available water and nutrients with the established root systems of trees. However, such attributes may not be ideal during tree establishment when competition from associated crops may irreversibly alter the productive potential of the trees (Lott et al., 2000a). A sequence of crop species or cultivars may therefore be preferable for different stages during the life cycle of speci®c agroforestry systems. By characterising the understorey environment at each stage, it may be possible to identify crops or cultivars grown as sole stands in areas where climatic conditions are comparable to those within agroforestry systems which might be suitable for use in speci®c systems. 4. Conclusions Long-term studies of overstorey agroforestry systems in semi-arid environments are rare owing to the substantial ®nancial, labour and time investments involved. The present study has shown that the nature and extent of the interactions between the tree and crop components change greatly as the system matures, and that the intensity of these interactions depends on the prevailing environmental conditions, particularly seasonal rainfall. Although it is impossible to provide recommendations with multi-site applicability for optimising the design of agroforestry systems in semi-arid regions from a single study, the work has highlighted areas where further research is needed. As experimental research and subsequent technology transfer to farmers are hampered by the long lead times required for agroforestry systems to establish and mature, the development of simulation models providing quantitative assessments of the impact of speci®c tree/crop combinations and management strategies on system function and productivity may offer a rapid and cost-effective alternative for screening potential agroforestry systems. The output from such models would permit future ®eld trials to focus on systems identi®ed as showing promise. Validation of such models requires substantial experimental datasets providing detailed information on resource capture and productivity over extended periods, such as that reported here. Recent tests of the HyPAR model developed by Lawson et al. (1996) using the present dataset are described by Lott et al. (1998). However,
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continued development of HyPAR and other models such as WaNuLuCAS (Van Noordwijk and Lusiana, 1998) using experimental datasets from a range of sites is essential to establish the likely impact of local conditions on the success of speci®c agroforestry systems. As several appropriate datasets already exist, there is a need for these to be made available in suitable formats to support the development of reliable and robust simulation models. Acknowledgements This publication is an output from a research project funded by the Department for International Development of the United Kingdom. However, the Department for International Development can accept no responsibility for any information provided or views expressed. This work was funded from project R5810 of the Forestry Research Programme. Additional technical and material support was provided by ICRAF, The University of Nottingham, The Royal Society, the UK Natural Environment Research Council and the Swedish International Development Agency. We wish to thank Ahmed Khan, Nick Jackson, Raphael Maweu and ®eld staff at Machakos Research Station for their support, and Jackie Humphreys for excellent secretarial assistance. References Belsky, A.J., 1994. Influence of trees on savanna productivity: test of shade, nutrients and tree-grass competition. Ecology 75, 922±932. Brenner, A.J., 1991. Tree±Crop Interactions within a Sahelian Windbreak System. PhD Thesis, University of Edinburgh, UK, 284 pp. Cannell, M.G.R., Van Noordwijk, M., Ong, C.K., 1996. The central agroforestry hypothesis: the trees must acquire resources that the crop would not otherwise acquire. Agrofor. Syst. 34, 27±31. Esegu, O.F.I., Odoul, P.A., 1992. Baseline selection of Grevillea robusta in Western Kenya. In: Harwood, C.E. (Ed.), Grevillea robusta in Agroforestry Systems. ICRAF, Nairobi, pp. 183± 188. Fischer, K.S., Palmer, A.F.E., 1984. Tropical maize. In: Goldsworthy, P.R., Fisher, N.M. (Eds.), The Physiology of Tropical Field Crops. Wiley, Chichester, UK, pp. 213±248. Govindarajan, M., Rao, M.R., Mathuva, M.N., Nair, R.K., 1996. Soil-water and root dynamics under hedgerow intercropping in semi arid Kenya. Agron. J. 88, 513±520.
200
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Harwood, C.E., Owino, F., 1992. Design of a genetic improvement strategy for Grevillea robusta. In: Harwood, C.E. (Ed.), Proceedings of an International Grevillea robusta in Agroforestry Systems. ICRAF, Nairobi, pp. 141±150. Hocking, D., 1998. Trees in wetland rice fields: an exciting project in Bangladesh. Agrofor. Today 10, 4±6. Howard, S.B., 1997. Resource Capture and Productivity of Agroforestry Systems in Kenya. PhD Thesis, University of Nottingham, UK, 259 pp. Howard, S.B., Ong, C.K., Black, C.R., Khan, A.A.H., 1997. Using sap-flow gauges to quantify water uptake by tree roots from beneath the crop rooting zone in agroforestry systems. Agrofor. Syst. 35, 15±29. Jama, B., Nair, P.K.R., Rao, M.R., 1995. Productivity of hedgerow shrubs and maize under alley cropping and block planting systems in semiarid Kenya. Agrofor. Syst. 31, 257±274. Jones, M., Sinclair, F., 1995. Differences in root system responses of two semi-arid tree species to crown pruning. Agrofor. For. 7, 24±27. Jonsson, K., Fidjeland, L., Maghembe, J.A., Hogberg, P., 1988. The vertical distribution of fine roots of five tree species and maize in Morogoro, Tanzania. Agrofor. Syst. 6, 63±69. Jonsson, K., Ong, C.K., Odongo, J.C.W., 1999. Influence of scattered nere and karite trees on microclimate. Exper. Agric. 35, 39±53. Kallinganire, A., Hall, J.B., 1993. Growth and biomass production of young Grevillea robusta provenances in Rwanda. For. Ecol. Manage. 62, 73±84. Kidd, C.V., Pimental, D., 1992. Integrated Resource Management: Agroforestry for Development. Academic Press, San Diego, USA. Kidundo, M., 1997. Melia volkensii Ð propagating the tree of knowledge. Agrofor. Today 9, 21±22. Lawson, G.J., Cannell, M.G.R., Mobbs, D.C., Crout, N.M.J., Muetzelfeldt, R., Wallace, J.S., Gregory, P.J., Thomas, T.H., Jagtap, S., Ludlow, A.R., Cobbina, J., Arah, J., Macdonald, K.J., Taylor, J., Sharp, L., Wiggins, G., Flynn, L., Livesley, S, Willis, R.W., Friend, A.D., 1996. Agroforestry Modelling and Research Coordination Annual Report: June 1995±May 1996. Institute of Terrestrial Ecology, Penicuik, UK, 178 pp. Loomis, R.S., Conner, D.J., 1992. Crop Ecology: Productivity and Management in Agricultural Systems. Cambridge University Press, Cambridge, UK, 600 pp. Lott, J.E., 1998. Resource Capture and Use in Semi-arid Overstorey Agroforestry Systems. PhD Thesis, University of Nottingham, UK, 259 pp. Lott, J.E., Black, C.R., Ong, C.K., 1998. Comparison of Output from HyPAR with Observed Maize Yields in Kenya. Report prepared for the UK Department of International Development, Mounted on Website http://www.ac.uk/ite/edin/agro/ampnews7pdf, 45 pp. Lott, J.E., Howard, S.B., Ong, C.K., Black, C.R., 2000a. Long term productivity of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya: I. Tree growth. For. Ecol. Manage. 139, 175±186. Lott, J.E., Khan, A.A.H., Ong, C.K., Black, C.R., 1996. Sap flow measurements of lateral tree roots in agroforestry systems. Tree Physiol. 16, 995±1001.
Lott, J.E., Ong, C.K., Black, C.R., 2000b. Long term water use of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya. For. Ecol. Manage., submitted for publication. McIntyre, B.D., Riha, S., Ong, C.K., 1996. Light interception and evapotranspiration in hedgerow agroforestry systems. Agric. For. Meteorol. 81, 31±40. Monteith, J.L., Ong, C.K., Corlett, J.E., 1991. Microclimatic interactions in agroforestry systems. For. Ecol. Manage. 45, 31± 44. Mwihomeke, S.T., 1993. A comparative study of the rooting depth of Grevillea robusta interplanted with sugar cane along contour strips. In: Harwood, C.E. (Ed.), Grevillea robusta in Agroforestry Systems. ICRAF, Nairobi, pp. 117±124. Okoba, B., Twomlow, S., Mugo, C., 1998. Evaluation of indigenous soil and water conservation technologies for runoff and soil loss control in semi-arid Mbeere District, Kenya. In: Proceedings of DFID Land Management Workshop on Modern Methods From Traditional Soil and Water Conservation Technologies, 13±15 January 1998. Kabale, Uganda, pp. 135±156. Ong, C.K., Leakey, R.R.B., 1999. Why tree±crop interactions in agroforestry appear at odds with tree±grass interactions in tropical savannahs. Agrofor. Syst. 45, 109±129. Ong, C.K., Black, C.R., Marshall, F.M., Corlett, J.E., 1996. Principles of resource capture and utilisation of light and water. In: Ong, C.K., Huxley, P.A. (Eds.), Tree±Crop Interactions in Agroforestry Systems: a Physiological Approach. CAB International, pp. 73±158. Ong, C.K., Corlett, J.E., Singh, R.P., Black, C.R., 1991. Above and below ground interactions in agroforestry systems. For. Ecol. Manage. 45, 45±57. Ong, C.K., Deans, J.D., Wilson, J., Mutua, J., Khan, A.A.H., Lawson, E.M., 1999. Exploring below ground complementarity in agroforestry using sap flow and root fractal techniques. Agrofor. Syst. 44, 87±103. Ong, C.K., Khan, A.A.H., Black, C.R., Lott, J.E., Howard, S.B., Wallace, J.S., Jackson, N.A., Smith, M., 2000. Productivity, climate and water use in Grevillea robusta-based agroforestry systems on hillslopes in semi-arid Kenya. Agric. Ecosyst. Environ., in press. Ong, C.K., Odongo, J.C.W., Marshall, F., Black, C.R., 1992. Water use of agroforestry systems in semi-arid India. In: Calder, I.R., Hall, R.L., Adlard, P.G. (Eds.), Growth and Water Use of Plantations. Wiley, Chichester, UK, pp. 347±358. Onyewotu, L.O.Z., Ogigirigi, M.A., Stigter, C.J., 1994. A study of the competitive effects of Eucalyptus camendulensis shelterbelts and an adjacent millet (Pennisetum typhoides) crop. Agric. Ecosyst. Environ. 51, 281±286. Rao, M.R., Ong, C.K., Pathak, P., Sharma, M.M., 1991. Productivity of annual cropping systems on a shallow alfisol in semi-arid India. Agrofor. Syst. 15, 51±63. Rao, M.R., Sharma, M.M., Ong, C.K., 1990. A study of the potential of hedgerow intercropping in semi-arid India using a two-way systematic design. Agrofor. Syst. 11, 243±258. Rockstrom, J., 1997. On-farm agrohydrological analysis of the Sahelian yield crisis: rainfall partitioning, soil nutrients and water use efficiency of pearl millet. PhD Thesis, University of Stockholm, Sweden, 210 pp.
J.E. Lott et al. / Forest Ecology and Management 139 (2000) 187±201 Schroth, G., 1995. Tree root characteristics as criteria for species selection and system design in agroforestry. Agrofor. Syst. 30, 125±143. Smith, D.M., Jackson, N.A., Roberts, J.M., Ong, C.K., 1999. Root distributions in a Grevillea robusta-maize agroforestry system in semi-arid Kenya. Plant and Soil 211, 191±205. Stewart, M., Bromley, T., 1994. Use of Melia volkensii in a semiarid agroforestry system in Kenya. Comm. For. Rev. 20, 128± 131. Van Noordwijk, M., Lusiana, B., 1998. WaNuLCAS: Background
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of a model of water, nutrient and light capture in agroforestry systems. Working Document, ICRAF, 86 pp. Van Noordwijk, M., Purnomosidhi, P., 1995. Root architecture in relation to tree±crop±soil interactions and shoot pruning in agroforestry. Agrofor. Syst. 30, 161±173. Vandenbelt, R.J., Williams, J.H., 1992. The effect of soil surface temperature on the growth of millet in relation to the effect of Faidherbia albida trees. Agric. For. Meteorol. 60, 93±100. Willey, R.W., 1985. Evaluation and presentation of intercropping advantages. Exper. Agric. 21, 119±133.
Long-term productivity of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya II. Crop growth and system performance J.E. Lotta,b,1, S.B. Howarda,b,2, C.K. Ongb, C.R. Blacka,* a
School of Biological Sciences, University of Nottingham, Sutton Bonington Campus, Loughborough, LE12 5RD, UK b International Centre for Research in Agroforestry (ICRAF), PO Box 30677, ICRAF House, Gigiri, Nairobi, Kenya Received 30 March 1999; received in revised form 10 December 1999; accepted 14 December 1999
Abstract Maize and cowpea were grown as sole stands or in agroforestry systems containing grevillea trees (Grevillea robusta A. Cunn.). Crop and system performance were examined over a 4.5-year-period (nine growing seasons) commencing in October 1991; failure of the rains caused the loss of one cropping season. A rotation of cowpea (Vigna unguiculata L. Walp.) and maize (Zea mays L.) was grown during the ®rst ®ve seasons after planting the trees, while maize was grown continuously during the ®nal four seasons. Sole maize was also grown under spectrally neutral shade netting which reduced incident radiation by 25, 50 or 75% to establish the relative importance of shade and below-ground competition for water and nutrients in determining the performance of understorey crops. The above-ground biomass and grain yield of understorey crops were not signi®cantly affected by the presence of grevillea during the ®rst four seasons, but were greatly reduced in subsequent seasons as the trees became increasingly dominant; maize yields reached 50% of the sole crop values only once during the ®nal four seasons, when rainfall was unusually high. The hypothesis that competition for water was the primary limiting factor for understorey crops was supported by the observation that above-ground biomass and grain yield were greater in the shade net treatments than in agroforestry maize, demonstrating that shade was not solely responsible for the substantial yield losses in the latter treatment. Performance ratios (ratio of values for the agroforestry system relative to sole stands) for total above-ground and trunk biomass in grevillea were initially low, re¯ecting the impact of competition with associated crops during tree establishment, but increased to unity within 2.5 years. Performance ratios for the understorey crops exhibited the reverse trend, initially being close to unity but approaching zero for three of the ®nal four seasons. Performance ratios were never close to unity for both trees and crops during the same season, indicating that there was always competition for available resources irrespective of crop species or tree size. The implications for agroforestry system design and future research are discussed. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Agroforestry; Cowpea; Grevillea robusta; Kenya; Maize; Productivity; Semi-arid
* Corresponding author. Tel.: 44-115-9516337; fax: 44-1159516334 E-mail address: [email protected] (C.R. Black). 1 Present address: British Waterways, Llanthony Warehouse, Gloucester, GL1 2EJ, UK. 2 Present address: WWF-UK, Panda House, Weyside Park, Catteshall Lane, Godalming, Surrey, GU7 1XR, UK.
1. Introduction Agroforestry has attracted considerable interest in recent years because of its potential to maintain or increase agricultural productivity in areas where high energy input, large-scale agriculture is impractical
0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 2 6 7 - X
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(Kidd and Pimental, 1992). Cannell et al. (1996) proposed that agroforestry may increase productivity provided the trees capture resources that are underutilised by annual crops. This hypothesis is supported by studies at Hyderabad, India which demonstrated that substantial improvements in productivity resulting from an increase in the utilisation of annual rainfall from 40 to 80% were achieved in agroforestry systems containing perennial pigeonpea (Cajanus cajan L.) and groundnut (Arachis hypogaea L.; Ong et al., 1996). However, experiments at Machakos, Kenya involving root trenching and tree species such as Leucaena leucocephala (Govindarajan et al., 1996) demonstrated that alley cropping may adversely affect crop productivity in semi-arid environments because increased competition for water outweighs potential soil fertility bene®ts resulting from applications of tree mulch, nitrogen ®xation or increased root turnover. Other studies have shown that regular pruning of the trees in alley cropping systems encourages the proliferation of ®ne roots in the surface soil horizons, so decreasing spatial niche separation between the tree and crop roots and hence the potential for complementarity in the use of below-ground resources (Van Noordwijk and Purnomosidhi, 1995). It is therefore important to select for trees with appropriate root architecture in order to achieve spatial complementarity and avoid major crop yield losses. The tree species examined in the present study, Grevillea robusta (grevillea), offers promise for use in agroforestry systems because observations of its root distribution (Jonsson et al., 1988; Mwihomeke, 1993) and measurements of water uptake (Lott et al., 1996; Howard et al., 1997) indicate that it is capable of extracting substantial quantities of water from beneath the crop rooting zone. Its widespread popularity with farmers in East Africa also suggests that adverse effects on associated crops are limited. Several mechanisms may enable agroforestry systems to use available water more effectively than sole crops and improve microclimatic conditions for understorey crops (Ong et al., 2000). These include: reductions in soil evaporation resulting from shading by the tree canopy and reduced air movement through the understorey environment; alterations in microclimatic conditions arising from reductions in air temperature, wind speed and saturation de®cit which
decrease crop water use (Monteith et al., 1991); and reductions in tissue temperature which bene®t the phenology and productivity of understorey crops by minimising exposure to heat stress (Monteith et al., 1991; Vandenbelt and Williams, 1992; Jonsson et al., 1999). The key question is whether these potential bene®ts may be outweighed by the detrimental effects of competition for light, water and nutrients between the trees and crops. The objective of the present study was to quantify the changing impact on understorey crops as grevillea trees established and matured. In Part I, it was shown that treegrowth in the agroforestry system was adversely affected by competition with the crops during the establishment period, and that tree height, leaf area and leaf, branch and trunk biomass were signi®cantly reduced relative to sole stands. Although tree height in the agroforestry treatment regained parity with sole grevillea within 2.5 years, total above-ground and trunk biomass and leaf area did not recover fully within the 4.5-year observation period. As the economic value of the trees was permanently affected, the viability of this grevillea-based system hinges on the extent to which the presence of trees affected crop productivity and overall system performance. This paper examines the changing impact of grevillea on crop and system performance as the trees matured. 2. Materials and methods 2.1. Experimental design Location, climate, soil characteristics, experimental design and the planting arrangement and management of grevillea (Grevillea robusta A. Cunn.) trees within the Complementarity In Resource Use on Sloping land (CIRUS) trial were described in Part I; details of the wide range of physical, biological and physiological measurements made are given by Ong et al. (2000). Relevant details of crop management, treatments imposed and growth analysis procedures are outlined below. 2.2. Treatments The three treatments reported here comprised sole crops (Cg) of maize (Zea mays L.) or cowpea (Vigna
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unguiculata L. Walp.), a sole tree treatment (Td) and an agroforestry treatment (CTd). Tree and crop spacings were identical in the monocrop and agroforestry treatments; grevillea was grown in a 3 m4 m dispersed arrangement (833 trees haÿ1), while maize and cowpea were grown at spacings of 0.3 m1.0 m (33 333 plants haÿ1) and 0.15 m50.0 m (13 333 plants haÿ1), respectively (Ong et al., 2000). 2.3. Growing seasons Rainfall distribution in Machakos District is bimodal, with the short rains extending from October to the end of December and the long rains from March to the end of May. These periods correspond to the short growing season (S) extending from ca. 1 October to 28/29 February, the long growing season (L) between ca. 1 March and 31 July and the dry season (D) between ca. 1 August and 30 September. Each season is de®ned according to the year in which it occurred; thus the 1993/1994 short growing season is identi®ed as S93/94. 2.4. Shading treatments The productivity of understorey crops in agroforestry systems is affected by shading and competition for water and nutrients. To distinguish between the effects of shading and below-ground competition with grevillea, areas of sole maize were covered during all seasons between the 1994 long (L94) and 1995/1996 short growing seasons (S95/96) with spectrally neutral shade netting (Lowes of Dundee, UK) to remove 25, 50 or 75% of the incident radiation. The netting was attached to 3 m4 m steel frames, equivalent in size to individual cells (de®ned as the 12 m2 of ground area enclosed by four adjacent trees) in the sole grevillea and agroforestry treatments (Lott et al., 2000a).
189
Equivalent unshaded control areas were demarcated in the sole crop plots at the beginning of the season to allow comparison of shaded and unshaded sole stands. The shade nets treatments were de®ned as Cg 25, 50 and 75% depending on the proportion of incident radiation removed; the unshaded control cells are referred to as Cg 0%. 2.5. Growth analysis Cowpea and maize were grown on a rotational basis during the ®rst ®ve seasons (S91/92±S93/94). However, this was replaced by continuous maize cultivation during the ®nal four seasons (L94±S95/96) to avoid infection of cowpea by root-rot (Fusarium udum) and re¯ect local farming practice. Growth analysis procedures for the ®rst ®ve seasons are described by Howard (1997) and for the ®nal four seasons by Lott (1998), but are brie¯y outlined here for clarity. 2.5.1. Cowpea and maize: S91/92±S93/94 seasons Measurements during the ®rst two cropping seasons were con®ned to ®nal harvest, when fresh and dry weights were determined on a row-wise basis for the pods and haulms of cowpea (S91/92) and the stover, cobs and grain of maize (L92). Poor rainfall (<25% of the seasonal average; Table 1) caused complete failure of the maize during the L93 season; detailed comparison of performance of the same crop during equivalent cropping seasons was therefore possible only for S92/93 and S93/94, and hence only for cowpea. As the planting density for cowpea was four times greater than for maize, regular destructive analyses were possible. During S92/93, all 36 plants from one quarter of one cell in each replicate agroforestry plot (Fig. 1a) and an equivalent area of the sole cowpea plots were harvested at ca. 10-day-intervals. Leaf,
Table 1 Rainfall (mm) in CIRUS during the long and short growing seasons and annual total rainfall for the period between planting (October 1991) and completion of the crop growth measurements reported here (March 1996) Time period
Meana
1991
1992
1993
1994
1995
Long growing season (1 March±31 July ) Short growing season (1 October±28/29 February) Annual total (1 January±30 December)
359 350 782
229 404 653
261 773 675
112 381 799
199 628 810
302 337 666
a
Data for 9-year-period for Machakos Maruba Dam Station.
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Fig. 1. (a and b) sampling areas for destructive analyses of cowpea growth and development in the dispersed agroforestry (CTd) treatment during the 1992/1993 (a) 1993/1994 (b) short growing seasons; (c), measurement locations for maize growth and development, meristem and soil temperatures and total shortwave radiation in the dispersed agroforestry (CTd) treatment during the 1994/1995 and 1995/1996 short growing seasons.
stem and pod fresh weights were recorded for six randomly selected plants from each plot before recombining these with the remainder of the sample to determine total fresh and dry weights. Leaf area was estimated using the dry weight:area ratio of disks cut from 40 randomly selected leaves. Measurements at ®nal harvest were made on a row-wise basis for each plot after separating the plants into pods and haulms.
A more detailed sampling strategy was adopted during S93/94 to analyse the spatial variation in crop growth at different locations relative to trees in the agroforestry treatment. Three plants were harvested at 12 locations within one cell (Fig. 1b) of each replicate agroforestry plot on six occasions; plants were also sampled at six locations in the sole crop plots. Leaf, stem and pod fresh and dry weights and leaf area,
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determined using the punched disk method described above, were measured for each plant. At maturity, all cells in the agroforestry plots not previously examined were sampled at the same locations. The remaining plants were harvested on a per cell (agroforestry treatment) or row-wise basis (sole crops). 2.5.2. Maize: L94±S95/96 seasons As the lower planting density of maize precluded routine destructive analysis, allometric methods were developed to allow regular non-destructive estimation of biomass and leaf area. These estimates were based on regression equations established between allometric parameters and destructive measurements involving limited numbers of plants. Allometric relationships for determining leaf area and above-ground biomass were established from weekly non-destructive and destructive measurements of ®ve plants sampled from each of the four replicate agroforestry and sole crop plots; plants were chosen to re¯ect the full range of sizes present. Parameters measured during non-destructive analysis were the smallest and largest basal stem diameters to permit calculation of the ellipsoidal cross-sectional area, height to the tip of the youngest leaf, height to the top of the canopy and the number of green leaves; destructive measurements included above-ground fresh and dry weights and leaf area. Linear regression analysis was used to establish equations describing the relationship between the non-destructively and destructively determined values, with the non-destructive variables being combined to form appropriate allometric parameters (AP) chosen on the basis of previous work (B. McIntyre, personal communication) and goodness of ®t. Two allometric parameters were used, APa for leaf area and leaf dry weight estimation: APa hLN e
(1)
and APb for stem dry weight estimation: APb he x
(2)
where he, LN and CSAm, respectively, denote height to the tip of the youngest fully expanded leaf, leaf number and the mean basal stem cross-sectional area. Total above-ground dry biomass was calculated as the sum of the values for leaf and stem biomass. The allometric relationships obtained were validated
Fig. 2. Relationship between allometric and measured values for maize plants grown as sole crops (Cg) or in the dispersed agroforestry system (CTd) during the 1995 long and 1995/1996 short growing seasons: (a) total leaf area for Cg (r20.85) and CTd maize (r20.72); (b) above-ground biomass for Cg (r20.86) and CTd maize (r20.89).
against destructive harvests carried out at thinning (ca. 25 DAS) and on at least one other occasion during each season. Fig. 2 illustrates the reliability of the estimates for leaf area (Cg: r20.85; CTd, r20.72) and above-ground biomass (Cg: r20.86; CTd, r20.89). The in¯uence of distance and direction from trees on crop performance was examined using nondestructive measurements made at ca. 7-day-intervals between thinning and ®nal harvest in the agroforestry and sole maize treatments. Sixteen sampling locations were chosen in the former treatment to represent the full range of interactions between trees and crops (Fig. 1c); each position was replicated around seven
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trees. Four plants were also examined at each sampling date in all replicate plots of the shade net (Cg 25, 50 and 75%) and unshaded sole crop treatments (Cg 0%) at positions equivalent to those in the agroforestry (CTd) treatment. These measurements were used to determine leaf area and biomass using the allometric procedures described above. At maturity, the plants used for non-destructive growth analysis were harvested and the fresh weights of leaves, stems and cobs were determined separately prior to drying at 708C. The remaining plants in all treatments were harvested on a row-wise basis and fresh and dry weights determined as described above. The timing of germination, ¯oral initiation, anthesis, silking and physiological maturity (Fischer and Palmer, 1984) were determined to establish treatment effects on the duration of the vegetative, reproductive and grain-®lling periods. 2.6. System productivity When the combined yield of the tree and crop components exceeds that of equivalent sole stands, agroforestry systems may be described as over-yielding and demonstrating complementarity of resource use (Ong et al., 1996). Of the various approaches developed to establish whether over-yielding is occurring in agroforestry and intercropping systems, the most common is the land equivalent ratio (LER; Willey, 1985; Rao et al., 1990, 1991). LER may be de®ned as the land area required under sole cropping to produce the yield achieved by intercropping, and corresponds to the sum of the performance ratios for the tree and crop components. Performance ratios were calculated for both components as: PRt
Ma Ms
(3)
where PRt represents the performance ratio for grevillea, and Ma and Ms denote the above-ground biomass of trees (t haÿ1) in the agroforestry and sole tree treatments. To calculate performance ratios for trunk biomass (PRtt), the appropriate values for the agroforestry and sole tree stands were substituted for Ma and Ms in Eq. (2). Performance ratios for total above-ground biomass (PRb) and grain yield (PRg) in maize and cowpea were calculated in an analogous manner.
2.7. Statistical analysis The results were subjected to analysis of variance using GENSTAT 5 to identify signi®cant treatment effects. 3. Results and discussion 3.1. Crop growth and final yields Fig. 3 shows the timecourses for leaf area index (LAI) and above-ground dry biomass (AGB) for cowpea in the agroforestry and sole crop systems during S92/93 and S93/94 as determined by destructive growth analyses. There was no signi®cant treatment effect during S92/93, when LAI and AGB reached maximum values of ca. 60 and 75 days after sowing (DAS; Fig. 3a and b). However, both variables were consistently lower in the agroforestry treatment than in sole cowpea during S93/94 (p<0.05), although the maximum values occurred at approximately the same time as in S92/93. LAI values for sole cowpea (Fig. 3a) were consistently lower between 35±65 DAS during S93/94, re¯ecting the much lower seasonal rainfall (381 mm versus 773 mm; Table 1), but above-ground biomass was less affected (Fig. 3b). The differences in maximum LAI and AGB between treatments were signi®cantly greater (p<0.01) during S93/94, re¯ecting the increasing dominance of the trees after ®ve cropping seasons and the lower rainfall. Fig. 4 shows allometric measurements of LAI and AGB for maize in the sole and agroforestry treatments during S94/95 and S95/96. The timecourses extend to maturity for biomass but only to anthesis for leaf area because the allometric approach for determining leaf area could not be used after appreciable foliar senescence began. The performance of maize in the agroforestry treatment differed greatly between seasons, with the reductions in LAI and AGB relative to sole maize being much smaller during the very wet S94/95 season than during the near-average S95/96 season (628 mm versus 317 mm; Table 1); development was also delayed by ca. 30 days in the agroforestry treatment maize during S95/96. This effect was not attributable to slower accumulation of thermal time under the trees (Lott, 1998), and may have originated from severe water stress resulting from a combination of
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Fig. 3. Timecourses for leaf area index and above-ground biomass of cowpea grown as sole crops (Cg) or in the dispersed agroforestry system (CTd) during the 1992/1993 and 1993/1994 short growing seasons (S92/93 and S93/94). Double standard errors of the mean are shown.
low rainfall and competition with the trees for available soil moisture. This conclusion is supported by the observation that the timing of ¯owering was similar in the agroforestry and sole maize stands during S94/95, when water stress was alleviated by high rainfall. Final yield data for maize and cowpea are shown for all cropping seasons in Fig. 5. Cowpea was grown during S91/92, S92/93 and S93/94 and maize during all other seasons, although poor rains caused crop failure during L93. Grain yield did not differ signi®cantly between treatments during the ®rst three seasons despite rapid tree growth (Lott et al., 2000a) and highly variable seasonal rainfall (Table 1). Signi®cant treatment effects ®rst became apparent during S93/94, when above-ground biomass and grain yield were greater in sole cowpea than in the agroforestry treatment (p<0.001). It is conceivable that the unusually
high rainfall during the preceding S92/93 season (773 mm versus long-term average of 350 mm; Table 1) may have precluded any competitive impact of the relatively large trees within the agroforestry system. The results for the ®nal four cropping seasons (L94± S95/96) clearly demonstrate the adverse effect of the established grevillea trees on the growth and productivity of maize. Above-ground biomass and grain yield were greatly reduced in the agroforestry treatment and almost no grain yield was obtained in three of the four seasons; yield reached ca. 50% of that for sole maize only when rainfall was much above average during S94/95 (628 mm; Table 1). These results suggest that water availability was the primary limitation for maize growth, a conclusion supported by the shade net experiments which simulated the shading effect of
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Fig. 4. Timecourses for leaf area index and above-ground biomass of maize growing as sole crops (Cg) or in the dispersed agroforestry system (CTd) during the 1994/1995 and 1995/1996 short growing seasons (S94/95 and S95/96). Double standard errors of the mean are shown.
the trees in the absence of competition for water and nutrients. Above-ground biomass and grain yield were consistently greater (p<0.05) in all shade net treatments than in the agroforestry treatment with exception of grain yield in the Cg 50% treatment during the very wet S94/95 season (Fig. 5). Above-ground biomass and grain yield in the Cg 25 and 50% treatments were not signi®cantly different from unshaded sole maize during the driest seasons (L94 and S95/96), but were signi®cantly reduced during seasons of high rainfall (S94/95 and L95; p<0.05), demonstrating the existence of an interaction between the effects
of shade and water availability. Shade was therefore not solely responsible for the substantial reduction in maize growth in the agroforestry system, as the mean intensity of shade imposed by grevillea on the understorey maize was intermediate between the Cg 25 and 50% shade net treatments. Harvest index was less affected than biomass or grain yield in the agroforestry treatment except during S95/96 (Fig. 5). Crop productivity is often assessed in terms of the quantity of grain obtained at ®nal harvest because economic value accrues mainly from grain production. However, farmers in the dry tropics frequently
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Fig. 5. Above-ground biomass, grain yield and harvest index for maize and cowpea crops during the short and long growing seasons and annual totals for the period between the 1991/1992 and 1995/1996 short growing seasons (S91/92±S95/96). Values are shown for crops grown as sole stands (Cg), in the dispersed agroforestry treatment (CTd), or as sole stands under 0, 25 or 50% artificial shade (Cg 0, 25 and 50%). Cowpea was grown during the 1991/1992, 1992/1993 and 1993/1994 short growing seasons and maize in all other seasons. Single standard errors of the mean are shown. Letters denote significant treatment differences (p<0.05).
attribute considerable value to maize stover by using it for animal fodder, barriers against erosion (trash lines), or as mulch (Okoba et al., 1998). Indeed, a market for maize stover exists in many dryland areas of Kenya, India and West Africa (E. Franzel, personal communication), resulting in a net export of crop
residues from the farm. It is therefore important for farmers that neither total above-ground biomass nor grain yield are compromised when crops are grown in agroforestry systems. The observation that aboveground biomass and grain yield in the Cg 25% shade treatment were generally comparable to unshaded sole
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maize (Fig. 5) indicates that the intensity of shade cast by overstorey grevillea trees was not in itself detrimental within this dryland environment, and hence that system productivity may be increased by the introduction of appropriate tree species. Belsky (1994) also used arti®cial shade treatments in the savanna of eastern Kenya to demonstrate that shading by Acacia nilotica and Adamsonia digitata did not adversely affect the understorey vegetation. However, it is clear that limitations imposed by below-ground competition, particularly for water, must be minimised if farmers are to adopt the technology. This conclusion is in marked contrast with earlier observations of root distribution (Jonsson et al., 1988; Mwihomeke, 1993) in eastern Africa and measurements of water uptake by 2±3-year-old trees at Machakos (Lott et al., 1996; Howard et al., 1997) which suggested that grevillea may extract a substantial proportion of its water requirements from beneath the crop rooting zone and therefore offers considerable promise of complementarity with associated crops. However, the potential of grevillea to exploit deep moisture reserves would have been severely constrained by the limited soil depth above the bedrock in the present trial (typically 0.5±1.4 m; Howard, 1997; Ong et al., 2000). Measurements showed that the roots of 4±6year-old trees were present throughout the soil pro®le and there was no spatial separation of the rooting zones of grevillea and maize (Smith et al., 1999); competition for water in the surface horizons was therefore unavoidable and complementarity of water use would have been possible only if alternative sources had been available 3.2. Complementarity of resource use and system productivity The overall height of the histograms shown in Fig. 6 represents the LER values for total above-ground biomass (Fig. 6a) and the main economic components (trunks and grain; Fig. 6b) for each cropping season. The stippled and solid areas within each column indicate the performance ratios for the tree and crop components within the agroforestry system. Missing values are apparent for the crop component during S91/92 and L92, when total above-ground biomass was not recorded (Fig. 6a), and L93, when poor rainfall caused crop failure (Fig. 6a and b). The
Fig. 6. Seasonal land equivalent ratios (overall column height) for the dispersed agroforestry (CTd) treatment calculated for (a) total above-ground biomass and (b) the main economic products, grain yield and trunk biomass. Single standard errors of the mean are shown. The stippled and solid areas within each column represent the performance ratios for the tree and crop components of the CTd system.
LER values were signi®cantly above or not signi®cantly different from unity in all except these instances, indicating either that a larger proportion of the available resources was captured, or that the captured resources were used more effectively for dry matter production. This situation occurs when there is niche differentiation between the system components or green area duration is extended (Loomis and Conner, 1992), and provides evidence of complementarity. The performance ratios of grevillea expressed either as total above-ground biomass (PRt) or trunk biomass (PRtt) were low (0.14 and 0.15, respectively) during
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the ®rst cropping season (S91/92), but subsequently increased steadily to reach unity by L94 (Fig. 6a); similar values were obtained for all subsequent seasons. These results indicate that seasonal performance was poorer when grevillea was grown in the agroforestry treatment than in sole stands during the ®rst 2 years, but thereafter was comparable in both treatments, supporting the conclusions of Lott et al. (2000a). Performance ratios for above-ground crop biomass (PRb) and grain yield (PRg) exhibited the reverse trend, as the values were close to unity during the ®rst three cropping seasons but approached zero during three of the ®nal four seasons (Fig. 6), although performance improved during the very wet S94/95 season (PRg0.39; PRb0.46). However, it should be remembered that cowpea was grown during three of the four seasons between S91/92 and S93/94 and that maize was grown continuously thereafter. The decline in crop performance in the agroforestry treatment may therefore have been at least partly attributable to the change in crop species from a C3 legume to a C4 cereal rather than changing tree/crop interactions resulting from tree growth. Performance ratios never approached unity for both the tree and crop components during the same season, demonstrating that there was always competition for the same resource pool irrespective of crop species or tree size. The observed LER values are consistent with alley cropping studies in India and Kenya (Ong et al., 1991, 1992; Rao et al., 1990, 1991; Jama et al., 1995), in which resource capture was dominated by the tree component because of the relatively high planting densities used. In their comparison of agroforestry and savanna systems, Ong and Leakey (1999) concluded that agroforestry research has typically focussed on fast growing tree species planted at close spacings and high densities, with the result that the trees captured most of the available resources and the potential microclimatic bene®ts for understorey crops were negated by reductions in soil moisture resulting from increased interception losses and water use by the trees. Results from other experiments at Machakos were used to show that, when seasonal rainfall was below 250 mm, maize yields were linearly correlated with the quantity of water transpired by ®ve tree species including grevillea: crop failure occurred when tree transpiration exceeded 100 mm. Independent assessments of transpiration based on a light
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interception model (McIntyre et al., 1996) suggested that only 50% of the seasonal rainfall was used for transpiration and the remainder was lost as soil evaporation in both the sole crop and agroforestry systems. Further evaluation of water use by the trees and crops during the latter stages of the CIRUS trial is reported elsewhere (Lott et al., 2000b). 3.3. Implications for future research The overstorey agroforestry system reported here demonstrated important and changing interactions between the tree and crop components which favoured crop growth during tree establishment, but increasingly favoured the tree component as the system matured. The nature of these interactions and their in¯uence on system performance are examined elsewhere (Lott, 1998). However, it is apparent that the extent to which these interactions in¯uence the economic potential of the system may depend on the size of the interface zone between trees and crops and the proportion of farmland affected. It is also important to emphasise that current understanding of resource capture in agroforestry systems is based largely on well-managed small plots, often situated on research stations, in which 30±50% of rainfall is used for transpiration. For example, plot level studies at Machakos reported rainfall utilisation values for maize and cowpea of 40±45% (McIntyre et al., 1996), suggesting that the opportunities for agroforestry were limited. However, such levels of rainfall utilisation are rarely achieved either in farmers' ®elds or at the landscape level in sub-Saharan Africa because of the prevailing low input agricultural practices, suggesting that there is ample scope for increasing water use by incorporating trees into existing landuse systems. For instance, Rockstrom (1997) showed that only 6±16% of the rainfall on a watershed in Niger was used for transpiration by pearl millet, the remainder being lost by soil evaporation (40%) or deep drainage (33±40%). Further studies are required to establish the mechanisms underpinning the success of recently developed indigenous agroforestry systems. An interesting example is the indigenous simultaneous agroforestry systems based on Melia volkensii (Gurke) developed by farmers in semi-arid Kenya, which have minimal effects on crop yields (Stewart and Bromley, 1994). The widely dispersed trees (10±
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15 m spacing) are regularly pruned to minimise competition and provide high quality timber and other products including durable termite resistant poles, beehives, fodder and fuelwood (Kidundo, 1997). Clearly, there are important lessons to be learnt from such systems; these are currently being examined by ICRAF in association with the Kenyan Forestry Research Institute (KEFRI) and the UK Institute of Terrestrial Ecology (ITE). Ideally, planting densities in agroforestry systems should be manipulated to maximise productivity and economic returns. However, as agroforestry generally involves long payback periods, crop performance must be maintained to the greatest possible extent. The crop component is therefore likely to continue to be planted at optimal or near-optimal densities for sole crops and the trees at sub-optimal densities, even though this may not provide the most productive utilisation of resources within the system. There is a considerable need for research aimed at optimising system densities for the tree and crop components and establishing the extent to which these should be adjusted, perhaps by reducing tree populations, as the system matures. Pruning also represents a viable management option to limit competition and encourage temporal complementarity as pruning prior to the cropping season reduces water use by the trees during crop establishment and may maintain favourable conditions for crop growth throughout the season (Jones and Sinclair, 1995). In areas of bimodal rainfall such as Machakos, pruning would most appropriately be carried out immediately prior to the season of most reliable rainfall to minimise competition at a time when crop yields are potentially greatest; during the less reliable rainy season, crop growth may be limited by erratic or inadequate rainfall, to which the trees are more resistant owing to their more extensive root systems (Schroth, 1995). Regular pruning of the tree canopy provides a compromise between improving crop growth and limiting tree productivity. However, there is scope to limit detrimental effects on tree growth while still providing a favourable environment for understorey crops; for example, it is conceivable that tree canopies pruned to give differing shapes may transpire at similar rates but exhibit differing aboveground interactions with understorey crops. Alternatively, Brenner (1991) and Onyewotu et al. (1994)
demonstrated that root pruning in the upper 1 m of the soil pro®le may be used to limit competition with associated crops. However, such practices are justi®able only when the productive potential of the tree is high and crop yield losses are severe, although root pruning has been adopted by farmers in Bangladesh (Hocking, 1998). There is a need for further ®eld studies of water use and productivity under various root and shoot pruning regimes. Previous selection programmes for grevillea provenances for use in agroforestry have focussed largely on tree height and growth rate (Kallinganire and Hall, 1993), while ICRAF's own programme has concentrated on height, trunk diameter, straightness, bole form, wood density and crown diameter (Esegu and Odoul, 1992). However, Howard (1997) suggested that characteristics which improve complementarity may represent more appropriate selection criteria, while Harwood and Owino (1992) proposed that a sparse and narrow crown and deep rooting habit were desirable traits. ICRAF is currently testing a simple technique for assessing root architecture and root functioning based on allometric approaches and fractal branching principles which may be used to predict the severity of root competition (Ong et al., 1999). Although attributes which improve complementarity are desirable, it is essential that the search for such characteristics does not compromise the economic potential of the system; thus tree species with limited economic value are of little bene®t to resource-poor farmers irrespective of the extent of complementarity with understorey crops. The crop component of agroforestry systems is almost inevitably the species and cultivar best suited to the prevailing climatic conditions when grown as a sole crop. This choice might nevertheless be poorly adapted to the understorey microclimatic conditions experienced by crops in agroforestry systems, particularly in semi-arid environments. For example, any delay in development resulting from modi®cation of the thermal environment and increased soil water de®cits in the crop rooting zone may make it necessary to adopt shorter duration varieties or species, especially in areas such as Machakos where the period between consecutive cropping seasons is short. The increased incidence of water stress resulting from below-ground competition also suggests that cultivars which emerge and establish rapidly and have deep and
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extensive root systems may compete more effectively for available water and nutrients with the established root systems of trees. However, such attributes may not be ideal during tree establishment when competition from associated crops may irreversibly alter the productive potential of the trees (Lott et al., 2000a). A sequence of crop species or cultivars may therefore be preferable for different stages during the life cycle of speci®c agroforestry systems. By characterising the understorey environment at each stage, it may be possible to identify crops or cultivars grown as sole stands in areas where climatic conditions are comparable to those within agroforestry systems which might be suitable for use in speci®c systems. 4. Conclusions Long-term studies of overstorey agroforestry systems in semi-arid environments are rare owing to the substantial ®nancial, labour and time investments involved. The present study has shown that the nature and extent of the interactions between the tree and crop components change greatly as the system matures, and that the intensity of these interactions depends on the prevailing environmental conditions, particularly seasonal rainfall. Although it is impossible to provide recommendations with multi-site applicability for optimising the design of agroforestry systems in semi-arid regions from a single study, the work has highlighted areas where further research is needed. As experimental research and subsequent technology transfer to farmers are hampered by the long lead times required for agroforestry systems to establish and mature, the development of simulation models providing quantitative assessments of the impact of speci®c tree/crop combinations and management strategies on system function and productivity may offer a rapid and cost-effective alternative for screening potential agroforestry systems. The output from such models would permit future ®eld trials to focus on systems identi®ed as showing promise. Validation of such models requires substantial experimental datasets providing detailed information on resource capture and productivity over extended periods, such as that reported here. Recent tests of the HyPAR model developed by Lawson et al. (1996) using the present dataset are described by Lott et al. (1998). However,
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continued development of HyPAR and other models such as WaNuLuCAS (Van Noordwijk and Lusiana, 1998) using experimental datasets from a range of sites is essential to establish the likely impact of local conditions on the success of speci®c agroforestry systems. As several appropriate datasets already exist, there is a need for these to be made available in suitable formats to support the development of reliable and robust simulation models. Acknowledgements This publication is an output from a research project funded by the Department for International Development of the United Kingdom. However, the Department for International Development can accept no responsibility for any information provided or views expressed. This work was funded from project R5810 of the Forestry Research Programme. Additional technical and material support was provided by ICRAF, The University of Nottingham, The Royal Society, the UK Natural Environment Research Council and the Swedish International Development Agency. We wish to thank Ahmed Khan, Nick Jackson, Raphael Maweu and ®eld staff at Machakos Research Station for their support, and Jackie Humphreys for excellent secretarial assistance. References Belsky, A.J., 1994. Influence of trees on savanna productivity: test of shade, nutrients and tree-grass competition. Ecology 75, 922±932. Brenner, A.J., 1991. Tree±Crop Interactions within a Sahelian Windbreak System. PhD Thesis, University of Edinburgh, UK, 284 pp. Cannell, M.G.R., Van Noordwijk, M., Ong, C.K., 1996. The central agroforestry hypothesis: the trees must acquire resources that the crop would not otherwise acquire. Agrofor. Syst. 34, 27±31. Esegu, O.F.I., Odoul, P.A., 1992. Baseline selection of Grevillea robusta in Western Kenya. In: Harwood, C.E. (Ed.), Grevillea robusta in Agroforestry Systems. ICRAF, Nairobi, pp. 183± 188. Fischer, K.S., Palmer, A.F.E., 1984. Tropical maize. In: Goldsworthy, P.R., Fisher, N.M. (Eds.), The Physiology of Tropical Field Crops. Wiley, Chichester, UK, pp. 213±248. Govindarajan, M., Rao, M.R., Mathuva, M.N., Nair, R.K., 1996. Soil-water and root dynamics under hedgerow intercropping in semi arid Kenya. Agron. J. 88, 513±520.
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J.E. Lott et al. / Forest Ecology and Management 139 (2000) 187±201
Harwood, C.E., Owino, F., 1992. Design of a genetic improvement strategy for Grevillea robusta. In: Harwood, C.E. (Ed.), Proceedings of an International Grevillea robusta in Agroforestry Systems. ICRAF, Nairobi, pp. 141±150. Hocking, D., 1998. Trees in wetland rice fields: an exciting project in Bangladesh. Agrofor. Today 10, 4±6. Howard, S.B., 1997. Resource Capture and Productivity of Agroforestry Systems in Kenya. PhD Thesis, University of Nottingham, UK, 259 pp. Howard, S.B., Ong, C.K., Black, C.R., Khan, A.A.H., 1997. Using sap-flow gauges to quantify water uptake by tree roots from beneath the crop rooting zone in agroforestry systems. Agrofor. Syst. 35, 15±29. Jama, B., Nair, P.K.R., Rao, M.R., 1995. Productivity of hedgerow shrubs and maize under alley cropping and block planting systems in semiarid Kenya. Agrofor. Syst. 31, 257±274. Jones, M., Sinclair, F., 1995. Differences in root system responses of two semi-arid tree species to crown pruning. Agrofor. For. 7, 24±27. Jonsson, K., Fidjeland, L., Maghembe, J.A., Hogberg, P., 1988. The vertical distribution of fine roots of five tree species and maize in Morogoro, Tanzania. Agrofor. Syst. 6, 63±69. Jonsson, K., Ong, C.K., Odongo, J.C.W., 1999. Influence of scattered nere and karite trees on microclimate. Exper. Agric. 35, 39±53. Kallinganire, A., Hall, J.B., 1993. Growth and biomass production of young Grevillea robusta provenances in Rwanda. For. Ecol. Manage. 62, 73±84. Kidd, C.V., Pimental, D., 1992. Integrated Resource Management: Agroforestry for Development. Academic Press, San Diego, USA. Kidundo, M., 1997. Melia volkensii Ð propagating the tree of knowledge. Agrofor. Today 9, 21±22. Lawson, G.J., Cannell, M.G.R., Mobbs, D.C., Crout, N.M.J., Muetzelfeldt, R., Wallace, J.S., Gregory, P.J., Thomas, T.H., Jagtap, S., Ludlow, A.R., Cobbina, J., Arah, J., Macdonald, K.J., Taylor, J., Sharp, L., Wiggins, G., Flynn, L., Livesley, S, Willis, R.W., Friend, A.D., 1996. Agroforestry Modelling and Research Coordination Annual Report: June 1995±May 1996. Institute of Terrestrial Ecology, Penicuik, UK, 178 pp. Loomis, R.S., Conner, D.J., 1992. Crop Ecology: Productivity and Management in Agricultural Systems. Cambridge University Press, Cambridge, UK, 600 pp. Lott, J.E., 1998. Resource Capture and Use in Semi-arid Overstorey Agroforestry Systems. PhD Thesis, University of Nottingham, UK, 259 pp. Lott, J.E., Black, C.R., Ong, C.K., 1998. Comparison of Output from HyPAR with Observed Maize Yields in Kenya. Report prepared for the UK Department of International Development, Mounted on Website http://www.ac.uk/ite/edin/agro/ampnews7pdf, 45 pp. Lott, J.E., Howard, S.B., Ong, C.K., Black, C.R., 2000a. Long term productivity of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya: I. Tree growth. For. Ecol. Manage. 139, 175±186. Lott, J.E., Khan, A.A.H., Ong, C.K., Black, C.R., 1996. Sap flow measurements of lateral tree roots in agroforestry systems. Tree Physiol. 16, 995±1001.
Lott, J.E., Ong, C.K., Black, C.R., 2000b. Long term water use of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya. For. Ecol. Manage., submitted for publication. McIntyre, B.D., Riha, S., Ong, C.K., 1996. Light interception and evapotranspiration in hedgerow agroforestry systems. Agric. For. Meteorol. 81, 31±40. Monteith, J.L., Ong, C.K., Corlett, J.E., 1991. Microclimatic interactions in agroforestry systems. For. Ecol. Manage. 45, 31± 44. Mwihomeke, S.T., 1993. A comparative study of the rooting depth of Grevillea robusta interplanted with sugar cane along contour strips. In: Harwood, C.E. (Ed.), Grevillea robusta in Agroforestry Systems. ICRAF, Nairobi, pp. 117±124. Okoba, B., Twomlow, S., Mugo, C., 1998. Evaluation of indigenous soil and water conservation technologies for runoff and soil loss control in semi-arid Mbeere District, Kenya. In: Proceedings of DFID Land Management Workshop on Modern Methods From Traditional Soil and Water Conservation Technologies, 13±15 January 1998. Kabale, Uganda, pp. 135±156. Ong, C.K., Leakey, R.R.B., 1999. Why tree±crop interactions in agroforestry appear at odds with tree±grass interactions in tropical savannahs. Agrofor. Syst. 45, 109±129. Ong, C.K., Black, C.R., Marshall, F.M., Corlett, J.E., 1996. Principles of resource capture and utilisation of light and water. In: Ong, C.K., Huxley, P.A. (Eds.), Tree±Crop Interactions in Agroforestry Systems: a Physiological Approach. CAB International, pp. 73±158. Ong, C.K., Corlett, J.E., Singh, R.P., Black, C.R., 1991. Above and below ground interactions in agroforestry systems. For. Ecol. Manage. 45, 45±57. Ong, C.K., Deans, J.D., Wilson, J., Mutua, J., Khan, A.A.H., Lawson, E.M., 1999. Exploring below ground complementarity in agroforestry using sap flow and root fractal techniques. Agrofor. Syst. 44, 87±103. Ong, C.K., Khan, A.A.H., Black, C.R., Lott, J.E., Howard, S.B., Wallace, J.S., Jackson, N.A., Smith, M., 2000. Productivity, climate and water use in Grevillea robusta-based agroforestry systems on hillslopes in semi-arid Kenya. Agric. Ecosyst. Environ., in press. Ong, C.K., Odongo, J.C.W., Marshall, F., Black, C.R., 1992. Water use of agroforestry systems in semi-arid India. In: Calder, I.R., Hall, R.L., Adlard, P.G. (Eds.), Growth and Water Use of Plantations. Wiley, Chichester, UK, pp. 347±358. Onyewotu, L.O.Z., Ogigirigi, M.A., Stigter, C.J., 1994. A study of the competitive effects of Eucalyptus camendulensis shelterbelts and an adjacent millet (Pennisetum typhoides) crop. Agric. Ecosyst. Environ. 51, 281±286. Rao, M.R., Ong, C.K., Pathak, P., Sharma, M.M., 1991. Productivity of annual cropping systems on a shallow alfisol in semi-arid India. Agrofor. Syst. 15, 51±63. Rao, M.R., Sharma, M.M., Ong, C.K., 1990. A study of the potential of hedgerow intercropping in semi-arid India using a two-way systematic design. Agrofor. Syst. 11, 243±258. Rockstrom, J., 1997. On-farm agrohydrological analysis of the Sahelian yield crisis: rainfall partitioning, soil nutrients and water use efficiency of pearl millet. PhD Thesis, University of Stockholm, Sweden, 210 pp.
J.E. Lott et al. / Forest Ecology and Management 139 (2000) 187±201 Schroth, G., 1995. Tree root characteristics as criteria for species selection and system design in agroforestry. Agrofor. Syst. 30, 125±143. Smith, D.M., Jackson, N.A., Roberts, J.M., Ong, C.K., 1999. Root distributions in a Grevillea robusta-maize agroforestry system in semi-arid Kenya. Plant and Soil 211, 191±205. Stewart, M., Bromley, T., 1994. Use of Melia volkensii in a semiarid agroforestry system in Kenya. Comm. For. Rev. 20, 128± 131. Van Noordwijk, M., Lusiana, B., 1998. WaNuLCAS: Background
201
of a model of water, nutrient and light capture in agroforestry systems. Working Document, ICRAF, 86 pp. Van Noordwijk, M., Purnomosidhi, P., 1995. Root architecture in relation to tree±crop±soil interactions and shoot pruning in agroforestry. Agrofor. Syst. 30, 161±173. Vandenbelt, R.J., Williams, J.H., 1992. The effect of soil surface temperature on the growth of millet in relation to the effect of Faidherbia albida trees. Agric. For. Meteorol. 60, 93±100. Willey, R.W., 1985. Evaluation and presentation of intercropping advantages. Exper. Agric. 21, 119±133.