Water use in a Grevillea robusta–maize overstorey agroforestry system in semi-arid Kenya

Forest Ecology and Management 180 (2003) 45–59

Water use in a Grevillea robusta–maize overstorey agroforestry system in semi-arid Kenya J.E. Lotta,b, A.A.H. Khanb, C.R. Blacka,*, C.K. Ongb,1 a

School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough LE12 5RD, UK International Centre for Research in Agroforestry (ICRAF), PO Box 30677, ICRAF House, Gigiri, Nairobi, Kenya

b

Received 28 November 2001; received in revised form 4 October 2002; accepted 1 November 2002

Abstract Novel approaches involving a combination of sap flow measurements of transpiration and allometric estimates of biomass production were used to determine seasonal water use by trees and crops in agroforestry systems. The results were used to test the hypothesis that agroforestry may improve productivity by capturing a greater proportion of annual rainfall than annual crops. Grevillea robusta A. Cunn., which is reputed to have a deep rooting habit, was grown in semi-arid Kenya either as sole stands or in combination with maize (Zea mays L.). Water use by individual trees and maize plants was determined using constant temperature heat balance gauges and scaled to provide stand-level estimates of transpiration based on linear relationships (r 2 > 0:70) between sap flow and leaf area across a range of tree ages and environmental conditions. Maximum stand-level transpiration rates for grevillea ranged from 2.6 to 4.0 mm per day, consistent with previous studies in similar environments. Biomass production by grevillea was closely correlated with stand-level transpiration (r 2 > 0:69
0:74), suggesting that nondestructive estimates of biomass increments can be used to provide reliable estimates of seasonal transpiration. Cumulative water use by grevillea over the 4.5-year observation period was comparable in the sole tree and agroforestry treatments, reaching a maximum utilisation of annual rainfall of 64–68% 3–4 years after planting. Approximately 25% of the water transpired by the trees was used during the dry season, indicating that they were able to utilise off-season rainfall, comprising 16% of the total annual rainfall, and residual water remaining in the soil profile after the cropping period. During the 1995 long rains, when 221 mm of rain was received, transpiration by sole maize was <50% of precipitation, compared to ca. 85% by the trees in the sole grevillea and agroforestry treatments. These results confirm that agroforestry systems may greatly increase rainfall utilisation compared to annual cropping systems. However, careful consideration of the tradeoffs between the loss of crop production and the additional value provided by tree products is essential. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Agroforestry; Grevillea robusta; Kenya; Maize; Sap flow; Water use

1. Introduction *

Corresponding author. Tel.: þ44-115-9516337; fax: þ44-115-9516334. E-mail addresses: [email protected] (C.R. Black), [email protected] (C.K. Ong). 1 Fax: þ254-2-521001.

Although biomass production is frequently constrained by limited water availability in annual cropping systems in the semi-arid tropics, residual water often remains in the soil after harvest, and off-season rainfall may go unused (Ong and Black, 1994; Black

0378-1127/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-1127(02)00603-5

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and Ong, 2000). Rockstrom (1997) reported that only 6–16% of the rainfall received on a watershed in Niger was used for transpiration by pearl millet, and that most of the remainder was lost by evaporation (40%) or deep drainage (33–40%). Similarly, sorghum/ pigeonpea intercrops grown on the alfisols of the Deccan plateau in India used only 41% of the annual rainfall, while the remainder was lost as runoff (26%) or by soil evaporation and deep drainage (33%; Ong et al., 1992); 20% or 152 mm of the annual precipitation was received outside the cropping season. There is therefore considerable scope to develop improved agricultural technologies to exploit these untapped reserves. The hypothesis that agroforestry may improve productivity by increasing the proportion of annual rainfall captured has gained support in recent years. Ong et al. (1992) reported that hedgerow plantings of sole leucaena extracted a larger proportion of the available soil moisture than sole crops or intercrops of sorghum and pigeonpea. Widely spaced alley crops (4.4 m between hedges) extracted even more water than sole leucaena, indicating that the agroforestry system was most effective in utilising available soil moisture. Similarly, measurements using the heat balance approach showed that annual water use by perennial pigeonpea grown in an agroforestry system with groundnut was 887 mm, or 84% of the annual rainfall, double that for the most productive intercropping system (Ong and Black, 1994). Almost half of the total water use (416 mm) occurred between January and June, when only 211 mm of rain was received, indicating that 205 mm was extracted from soil reserves. However, the high demand for water imposed by fast growing exotic trees may be a cause for concern to farmers in many tropical countries, particularly in semi-arid regions. For instance, Calder et al. (1997) reported that eucalypt plantations in southern India not only used all of the rainfall which infiltrated the soil but also extracted a further 100 mm of water for each 1 m depth of soil penetrated by the roots This is a matter for serious concern as the roots of eucalypt trees may reach depths of 8 m within 3 years of planting (Calder et al., 1997). In the Sahel, the roots of mature acacia trees may reach water tables located at depths of up to 30 m (Deans et al., 1995). It is therefore essential to consider the implications of

increased water use in agroforestry systems for medium and longer-term water budgets. Particular attention should be paid to the source of water used by trees, the rate of water depletion below the crop rooting zone, and the prospects for deep recharge during periods of high rainfall (Smith et al., 1997a). Measurements of stand-level water use by established trees are technically difficult because their deep rooting systems preclude the soil water balance approaches often used to determine water use by sole crops or annual intercropping systems, in which the roots are confined mainly to the upper 50–100 cm of the soil profile (Ong and Black, 1994). Although sap flow techniques have been widely used to determine transpiration by individual trees and crop plants (Ong et al., 2000), the values obtained must be scaled to obtain community level estimates of water use. Their complexity and labour-intensive nature also limits the number of plants which can be measured and hence the number of treatments that can be examined simultaneously. Continuous measurements of water use over extended periods as trees mature are therefore not feasible using sap flow technology alone. Scaling methods to estimate water use during periods when sap flow is not measured should ideally maintain complete independence of measurements of transpiration by the trees from other components of the water balance (Hatton and Vertessey, 1990). This has been achieved for trees growing in humid environments (e.g. Allen et al., 1999), where water supplies are rarely limiting, thereby enabling scaling procedures to focus on key driving variables for transpiration such as solar energy supply and atmospheric demand. However, in semi-arid environments, the impact of soil water availability on water use by trees cannot be ignored. Previous studies of the impact of limited water supplies on water use (Hatton and Wu, 1995; Hall and Allen, 1997) have either included parameters describing the soil water deficit, thereby failing to maintain the independence of water balance components, or have used scaling methods based on biomass production. The latter approach was adopted in the present study by regressing sap flow against tree biomass production for periods when sap flow was measured. The regression equations obtained were then used in conjunction with regular estimates of tree biomass to determine water use during periods when sap flow was not measured.

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The purpose of the present study was to develop and validate techniques for making long-term measurements of water use by trees and crops, and to apply these to test the hypothesis that the success of agroforestry in semi-arid regions depends on the ability of trees to capture resources which the crops cannot due to spatial or temporal complementarity, or a combination of both (Cannell et al., 1996). Specific objectives were: (i) to derive stand-level estimates of transpiration by trees and crops from regular measurements of sap flow and biomass production and; (ii) determine the extent of complementarity of water use in terms of utilisation of residual water left after harvesting annual crops and off-season rainfall.

2. Materials and methods 2.1. Site description and experimental design The work reported here formed part of ICRAF’s Complementarity In Resource Use on Sloping land (CIRUS) programme, which examined the changing patterns of resource capture and tree and crop growth as the trees established and matured in Grevillea robusta-based agroforestry systems in semi-arid Kenya. The trial was located at Machakos Research Station, Kenya (18330 S, 37880 E, altitude 1560 m) on a moderately steep southwest facing slope (18–22%) with no previous cropping history before being cleared of scrub dominated by Acacia species in July 1991. The soil was a well drained, shallow to moderately deep (0–2.5 m) sandy clay loam overlying petroplinthite (murram) and was not nutrient-limiting for plant growth. Its physical and chemical properties are described by Wallace et al. (1995) and Ong et al. (2000). Air temperature and daytime saturation deficit are relatively low for the latitude owing to the altitude of the site. A detailed climatic description is provided by Huxley et al. (1989). Rainfall, atmospheric saturation vapour pressure deficit, air temperature, incident short-wave radiation and net radiation were recorded at 30 min intervals using an automatic weather station (model WS01, Delta T Devices, Cambridge, UK). The trial comprised a balanced incomplete block design containing five treatments with four replicates; plot size was 20 m  20 m. Results are presented here

47

for three treatments; sole crops (Cg) of maize (Zea mays L. Katumani composite), sole stands of grevillea (Td) planted in a dispersed arrangement with a 3 m  4 m spacing, giving 35 trees per plot (833 trees ha
1), and a dispersed arrangement of trees with understorey maize (CTd) planted at the same spacing and density as sole maize. Individual plots were separated by grass walkways to facilitate access and minimise erosion. Vetiver grass (Vetiveria zizanoides L. Nash) strips were contour-planted across the centre of each plot to control erosion. The walkways and vetiver strips were cut at approximately 14-day intervals to avoid competition with adjacent trees and crops. The trees were managed to produce single stems and maintain a uniform canopy structure by cutting back the longer branches at the first pruning (599 days after planting; 6 June 1993) and removing the lower branches (basal pruning) as the trees grew taller at both subsequent prunings (861 and 1254 days after planting; 23 February 1994 and 23 March 1995, respectively). The plots were weeded before planting each crop and as required during the growing season. No fertilisers or mulch were applied. Rainfall distribution at Machakos is bimodal; the short rains extend from October to late December and the long rains from March to late May. These periods correspond to the short growing season (S), extending from ca. 1 October to 28/29 February and the long growing season (L) between ca. 1 March and 31 July. The rainy seasons are separated by a brief dry season (D) between the short and long rains and a long dry season between the long and short rains. Each season is defined according to the year in which it occurred; thus S94/95 denotes the 1994/1995 short growing season. Rainfall is typically greatest in March, April and November, with little being received between July and September. Mean rainfall during the short and long growing seasons between 1963 and 1971 was 414 and 359 mm, respectively, with an annual average of 782 mm (Ong et al., 2000). Potential evaporation ranges from 95 to 165 mm per month, providing an annual total of ca. 1450 mm (Huxley et al., 1989). 2.2. Measurement of standing biomass and yield Leaf area and canopy biomass in grevillea were calculated using allometric procedures based on mea-

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surements of trunk cross-sectional area immediately below the first branch; Lott et al. (2000c) have previously demonstrated the reliability of these procedures for grevillea trees grown as poles. Allometric relationships established from parallel destructive and non-destructive measurements were used to determine leaf area and above-ground biomass for maize (Lott et al., 2000b); yield was measured destructively at crop maturity. 2.3. Sap flow measurements Sap flow through the stems of grevillea trees and maize plants was measured using constant temperature heat balance gauges identical to those described by Khan and Ong (1995). Sap flow was measured for three replicate trees in the Td and CTd treatments during four annual cycles (October 1992–September 1996), commencing 1 year after tree planting. Gauges were left in place for a maximum of two weeks before being transferred to other trees to avoid girdling of the bark by the heater coil as trunk diameter increased. 2.3.1. Calibration of sap flow gauges for grevillea Sap flow measurements were corrected for errors arising from differences in trunk diameter as described by Lott et al. (1996) Jn ¼

Jm 2 ð0:0001d Þ
ð0:0031dÞ þ 1:0552

(1)

where Jm and Jn represent measured and corrected sap flow (g h
1) and d denotes trunk diameter (mm). As this calibration is limited to trunks with a maximum diameter of ca. 9 cm, the gauges were installed immediately below the canopy to ensure trunk diameter remained within the calibration range. This approach also avoided thermal interference from the soil, which may adversely affect the reliability of heat balance measurements (Khan and Ong, 1995). Thermal interference from solar heating was minimised by covering the trunk with reflective foil. 2.3.2. Calibration of sap flow gauges for maize Heat balance gauges were calibrated for maize by making paired sap flow and gravimetric measurements of transpiration over 3-day periods under field conditions using ca. 45 day old pot-grown plants (n ¼ 3); a

Fig. 1. Calibration of heat balance gauges for maize using paired measurements of sap flow and gravimetric weight loss for 45-dayold pot-grown plants. Linear regression: y ¼ 0:98x; r 2 ¼ 0:86.

sensitive top-pan balance was used to record weight loss while making simultaneous sap flow measurements (Lott, 1998). As a close correlation (r 2 ¼ 0:86; n ¼ 295) was obtained between the gravimetric and heat balance estimates of transpiration (Fig. 1), the linear regression obtained (Eq. (2)) was used to derive corrected sap flow values (Jz) from the heat balance measurements (Jm): Jz ¼ 0:9771ðJm Þ

(2)

Sampling frequency and continuity of measurements for maize were limited by plant age and size, as the gauges could not be used until five leaves had emerged and/or stem diameter exceeded 18 mm; sap flow measurements (n ¼ 3) were therefore confined to the period between ca. 40 and 90 days after sowing (DAS). Use of the heat balance approach with rapidly growing maize plants was highly labour-intensive, as gauges had to be transferred between plants at three day intervals to avoid girdling of the rapidly expanding stem by the heater coil; priority was therefore given to the trees, for which the heat balance approach was better suited. 2.4. Estimating stand-level transpiration from sap flow measurements A major constraint of sap flow techniques is that measurements are confined to a limited number of plants or trees, while valid treatment comparisons at

J.E. Lott et al. / Forest Ecology and Management 180 (2003) 45–59

the community level require stand-level estimates of water use. Several studies have approached the problem by adopting scaling methods based on tree density (Hatton and Vertessey, 1990; Wullschleger et al., 1998), crown size (Ladefoged, 1963), trunk basal area (Cermak and Kucera, 1987), total trunk cross-sectional area (Allen and Grime, 1995), estimated crown leaf area (Allen et al., 1999; Hall and Allen, 1997; Smith et al., 1997b) or sapwood area (Thorburn et al., 1993). The success or failure of these scaling methods proved strongly dependent on the composition of the stand; in the examples cited above, this ranged from monoculture plantations to mixed forests, regular to irregular tree spacings, and open to closed canopy structures. The main difference between trees in the present study was in their size, as they were regularly spaced, of uniform age, and provided a discontinuous canopy (Ong et al., 2000). Allen and Grime (1995) and Smith and Allen (1996) suggested that the most appropriate methods for such stands were those based on relationships between sap flow and leaf area for individual trees and stand-level leaf area index (LAI) values, provided frequent estimates of leaf area are available. As leaf area was regularly determined in CIRUS, water use by individual trees could be scaled to obtain stand-level transpiration using T¼

n X Ji L i¼1

Ai

(3)

where T represents the stand-level transpiration (mm h
1) and Ji, Ai and L, respectively, denote the measured sap flow (kg h
1) for the ith tree, its estimated leaf area, and the LAI of the stand. Sap flow values for individual cereal plants have been scaled to estimate water use per unit land area on the basis of plant population (Ham et al., 1990; Ozier-Lafontaine et al., 1997). However, these studies concluded that inter-plant variation may introduce substantial errors into stand-level estimates obtained using this approach, and that the leaf area of individual plants and stand LAI were more appropriate scaling factors. Eq. (3) was therefore used to scale sap flow values for individual plants to obtain stand-level estimates of water use in a manner analogous to that used for grevillea.

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Fig. 2. Relationship between cumulative sap flow and aboveground biomass increments for the sole (Td) and dispersed (CTd) agroforestry treatments. Regression equations are: Td, y ¼ 589x þ 2:1; r 2 ¼ 0:74; CTd, y ¼ 525x þ 5:7; r2 ¼ 0:69.

2.5. Estimating transpiration during periods when sap flow was not measured Fig. 2 shows the relationship between total cumulative sap flow in grevillea and the corresponding total above-ground standing biomass increment for each measurement period between January 1993 and May 1996. During this period, standing biomass increased from 0.9 to 20 t ha
1 in Td grevillea and from 0.2 to 17 t ha
1 in CTd grevillea, and individual measurement periods spanned a wide range of environmental conditions associated with both wet and dry seasons. The highly significant r2 values for the regression equations (P < 0:001) indicate that allometric estimates of incremental tree growth can be used to provide reliable estimates of water use during periods when sap flow is not measured. The reliability of stand-level estimates of water use based on relationships between sap flow and leaf area depends on two assumptions being satisfied (Allen and Grime, 1995; Hatton and Wu, 1995). These are that: (i) sap flow and leaf area are closely correlated for all trees in the stand and; (ii) all trees respond similar to the prevailing environmental conditions. The first assumption was tested by comparing the correlation coefficients for linear regressions (r2)

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between mean sap flow rates and median leaf area for all replicate trees during each measurement period, a method adopted previously by Allen and Grime (1995). Approximately 70% of the correlations obtained during the 20-month study period were high (r 2 > 0:70; Lott, 1998), although lower than reported by Hatton et al. (1998). The least consistent correlations were generally obtained for CTd grevillea during the short growing season (r 2 ¼ 0:44
0:93) when periods of rapid canopy expansion following heavy rainfall were succeeded by phases of low rainfall (Lott, 1998; Ong et al., 2000). Close linear correlations between sap flow and leaf area were also observed the 1994 long dry season (August–September; r 2 ¼ 0:79
0:99) despite the minimal rainfall received (22.5 mm; Table 1), albeit at lower flow rates than during the rainy season. Thus, despite occasional short-term lapses, good linear correlations between

sap flow and leaf area were maintained across a range of environmental conditions and tree ages, satisfying the first requirement for scaling sap flow on the basis of the leaf areas of individual trees and stand LAI. Failure to satisfy the second assumption would be highlighted by poor correlations between sap flow rates for replicate trees, indicating that individual trees were responding differently to the prevailing aboveground environmental conditions due to variation in soil water availability or other factors. However, ca. 70% of the correlations between replicate trees were high (r 2 > 0:7; Lott, 1998). Thus, both assumptions required for scaling the sap flow values for individual trees to provide stand-level estimates of water use using sap flow/leaf area relationships were satisfied for much of the time, although their accuracy could be compromised in the short term, particularly during periods of rapid soil drying.

Table 1 Number of days in the season, rainfall, stand-level transpiration for grevillea in the sole (Td) and agroforestry (CTd) treatments, and percentage of rainfall transpired by grevillea during each season and annual cyclea Season

Days

Rainfall (mm)

Transpiration (mm) Td

CTd

Td

CTd

S91/92 D92A L92 D92B 1991/1992

102 49 136 72

357.0 5.0 256.0 1.0 619.0

0.4 0.2 4.4 2.7 7.7

0.5 18.0 5.6 1320.0 4.9

0.1 4.0 1.7 272.8 1.2

S92/93 D93A L93 þ D93B 1992/1993

100 47 214

606.0 30.2 11.8 99.2 185.5 137.3 Rains failed; entire period included in D93A 705.2 215.7 149.1

5.0 187.0

2.0 138.4

30.6

21.1

S93/94 D94A L94 D94B 1993/1994

96 44 123 84

165.4 132.6 157.2 59.2 514.4

119.3 56.3 102.4 52.0 330.0

118.6 62.0 115.2 54.8 350.6

72.1 42.5 65.1 87.8 64.1

71.7 46.8 73.3 92.6 68.2

S94/95 D95A L95 D95B 1994/1995

123 20 112 115

546.8 105.0 221.0 22.5 895.3

244.6 43.3 182.2 70.0 540.1

262.6 55.7 191.3 65.3 574.9

44.7 41.2 82.4 311.1 60.3

48.0 53.0 86.6 290.2 64.2

S95/96

108

238.1

150.5

139.4

63.2

58.5

a

1.7 0.9 14.3 13.2 30.1

Fraction of rainfall used (%)

S, L and D denote the short and long cropping seasons and the dry season, respectively. Seasons are identified by the year in which they occurred, i.e. S91/92 denotes the 1991–1992 short growing season. A and B denote the dry seasons between the short and long, and long and short cropping seasons.

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2.6. Statistical analysis The results were subjected to analysis of variance using GENSTAT 5 to establish significant treatment effects.

3. Results and discussion 3.1. Transpiration by grevillea 3.1.1. Diurnal timecourses Diurnal mean sap flow values for trees in the Td and CTd stands expressed per unit leaf area or as stand-level totals and the timecourses for net radiation and atmospheric saturation vapour pressure deficit, the primary atmospheric driving forces for transpiration, are shown in Figs. 3 and 4. The results represent 3-day periods in 1994 following intense (113 mm during preceding 14 days; Fig. 3) or limited rainfall (9 mm during preceding 3 months; Fig. 4). During both periods, sap flow tracked the timecourses of net radiation closely, although the values were much lower and peaked earlier after the period of low rainfall, suggesting that transpiration was limited by limited water availability. Green and Clothier (1988) and Hatton and Vertessey (1990) observed that the timecourses for sap flow lagged behind net radiation by up to 1.5 h in well-watered trees, an effect which Hatton and Vertessey (1990) attributed to time-lags between changes in transpiration and sap flow at the measurement height (1.5 m). However, the potential for lags between changes in transpiration and measured sap flow was limited in the present study as the heat balance gauges were located immediately below the canopy. The timecourses for sap flow substantially preceded those for saturation deficit, indicating that this was not the primary driving variable for transpiration. 3.1.2. Long-term water use Fig. 5 shows daily stand-level values for water use by Td and CTd grevillea calculated from the sap flow measurements made over two annual cycles (October 1994–September 1996). In both years, water use peaked towards the end of the short growing season when leaf area was greatest (Lott et al., 2000a), but then declined during the latter stages of the annual

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cycle as a consequence of canopy pruning at 1254 days after planting during 1994/1995 and decreasing soil moisture availability in 1995/1996. Water use was greater for Td than for CTd grevillea following the onset of the rains at the beginning of each experimental year (P < 0:001). Maximum daily water use was also greater during S94/95 than during S95/96 (P < 0:01), reaching ca. 4.0 mm per day in both treatments during the former season compared to a maximum of 2.6 mm per day during the latter. The greater maximum values during S94/95 reflect the combination of much higher rainfall (Table 1) and larger leaf area (Lott et al., 2000a). During the 1995 dry season (D95), mean water use was 0.6 mm per day in both treatments, substantially lower than during the short growing season. In a review of water use by forest trees, Rutter (1968) concluded that transpiration rates of 1–2 mm per day were typical of trees experiencing conditions similar to those in CIRUS (i.e. moderate to severe water stress and annual precipitation of 500–800 mm). In more recent studies using sap flow gauges, maximum values of 0.7–2.0 mm per day were recorded for desert (Miller and Poole, 1979) and savannah shrubs (Allen and Grime, 1995), while Allen et al. (1999) reported mean values of 5.0 and 2.4 mm per day for two poplar varieties during wet periods in the UK, declining to 1.9 and 1.6 mm per day during dry periods. Values obtained in the present study are therefore comparable to those reported for a range of tree species grown in water-limited and seasonally water-sufficient environments. Fig. 6 shows accumulated transpiration by Td and CTd grevillea over a period of 1650 days (4.5 years) from planting. The values were consistently greater for Td than for CTd grevillea and total water use throughout the observation period was ca. 5% greater in the sole stand. Both treatments exhibited exponential increases in transpiration until ca. 930 DAP (midway through the 1994 long growing season); no consistent correlation with seasonal variation in rainfall and environmental conditions was apparent during this period. However, after 930 DAP, transpiration declined in both treatments during the long rains and the ensuing dry season, before increasing sharply following the onset of the rains in October of each year. These observations suggest that the trees were initially able to use residual soil moisture during periods of low rainfall to maintain transpiration, but

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Fig. 3. Diurnal timecourses for: (a) mean daytime saturation deficit and net radiation; (b) and (c) mean sap flow rates for grevillea in the sole (Td) and dispersed (CTd) agroforestry treatments. Values are expressed on a unit leaf area (b) and stand-level basis (c) for the period between 3 and 5 November 1995. Measurements were made after intense rainfall when 113 mm of rain was received during the preceding 14 days. Double standard errors of the mean are shown.

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Fig. 4. Diurnal timecourses for: (a) mean daytime saturation deficit and net radiation; (b) and (c) mean sap flow rates for grevillea in the sole (Td) and dispersed (CTd) agroforestry treatments. Values are expressed on a unit leaf area (b) and stand-level basis (c) for the period between 17 and 19 August 1995. Measurements were made after a period when only 9 mm of rain was received during the preceding 3 months and no rain fell during the preceding 14 days. Double standard errors of the mean are shown.

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Fig. 5. Daily total stand-level transpiration rates by grevillea in the sole (Td) and dispersed (CTd) agroforestry treatments during: (a) the 1994/1995; (b) 1995/1996 annual cycles. Solid, stippled and open bars respectively denote the short and long cropping seasons and the dry season. The tree canopies were pruned at 1254 days after planting by removing the basal branches.

between 1991/1992 and 1994/1995. Total water use by Td and CTd grevillea during these off-season periods was 420 and 380 mm, respectively. These values are 38 and 35%, respectively, of the corresponding annual total water use, suggesting the trees were highly effective in capturing off-season rainfall, and that this source of water was important for their continued growth. Closer examination of the proportion of rainfall transpired indicates that water use by the trees exceeded rainfall in three of the seven dry seasons examined, suggesting that they were able to exploit residual and/or deep soil moisture reserves. Two of these three periods preceded a prolonged dry period between March and November 1993, when soil moisture reserves were severely depleted (Ong et al., 2000), while the third occurred after unusually high rainfall during the first 9 months of the 1994/1995 annual cycle replenished soil moisture reserves. Table 1 clearly shows that transpiration was much greater in Td grevillea up to the end of the 1992/1993 annual cycle (P < 0:01), but was then comparable in both treatments. This equalisation of water use occurred after the first pruning at 599 DAP, when the tree canopies were reduced to a similar size in both treatments (Lott et al., 2000a). Transpiration by the trees in the Td and CTd treatments accounted for approximately two thirds of the annual rainfall after 1992/1993, indicating that only one third was available for transpiration by understorey crops, evaporation and runoff. 3.2. Transpiration by maize

were unable to do so during the later stages of the experimental period due to their increasing size and transpirational demand, coupled with incomplete recharge of deep moisture reserves during the rainy seasons. This conclusion is supported by the calculations of the percentage of rainfall transpired by the trees during off-season periods presented in Table 1, which shows total accumulated transpiration for specific cropping periods and dry seasons, rainfall, and the fraction of rainfall transpired by the trees. The cropping seasons correspond to the periods when crops were present and so differ in duration. Rainfall during the off-season periods was regarded as being unavailable for crop growth. Off-season rainfall comprised ca. 425 mm or 16% of the total rainfall during the four annual cycles

3.2.1. Diurnal timecourses Fig. 7 shows diurnal timecourses for sap flow in maize grown as an unshaded sole crop (Cg 0%), under 50% artificial shade (Cg 50%), or in the CTd agroforestry treatment at distances of 50 and 212 cm from the nearest tree at ca. 50 DAS during the L95 season. Sap flow varied diurnally and the values for Cg 50% maize were approximately half those for Cg 0% maize (P < 0:01). Maximum values for Cg 0% maize were less than 6.5% of the corresponding values for CTd grevillea (P < 0:001; Lott, 1998). Transpiration was almost completely suppressed in CTd maize at both locations, and maximum sap flow rates were less than 15% of those for unshaded sole maize. This effect was reflected by the severe reduction in the yield of CTd

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Fig. 6. Cumulative stand-level transpiration by grevillea in the sole (Td) and dispersed (CTd) agroforestry treatments during the 4.5-year period from planting. Solid, stippled and open bars respectively denote the short and long cropping seasons and the dry season. The tree canopies were pruned at 599, 861 and 1254 days after planting by removing the basal branches.

maize during a season of near-average rainfall (Lott et al., 2000b). 3.2.2. Stand-level water use Daily stand-level transpiration by unshaded sole maize (Cg 0%) during L95 (Fig. 8c) tracked the corresponding timecourses for solar radiation and daytime saturation deficit (Fig. 8a and b) between 41 and 75 DAS, but no clear correlation was apparent thereafter, probably because the decreased frequency and intensity of rainfall limited soil water availability (Fig. 8a). This hypothesis is supported by the relatively low grain yields and harvest indices obtained (Lott et al., 2000b), suggesting that the luxuriant vegetative growth and rapid transpiration which occurred during the early stages of the season could not be sustained during the grain set and grain filling period, thereby limiting yield. Water use by Cg 0% maize ranged between 0.5 and 3.5 mm per day (Fig. 8c), reflecting the substantial day-to-day variation in climatic conditions (Fig. 8a and b), and was greater than in the Cg 50% shade treatment (P < 0:01). In previous studies where sap flow approaches were used to provide stand-level

estimates of transpiration, Ham et al. (1990) obtained values of 3.0–4.3 mm per day for sole stands of cotton under well watered conditions, while Wallace et al. (1991) reported seasonal means of 1.9 and 2.3 mm per day for intercropped maize grown under irrigation in Mauritius; daily values ranged from 0.5 to 4.0 mm per day. These values agree closely with those obtained for sole maize in the present study. Water use by CTd maize was greater 50 cm from the trees than at 212 cm (Fig. 8c; P < 0:01), reflecting the greater soil moisture content close to trees following major rain events caused by the interception of precipitation by their canopies and subsequent focusing of water around the trunk by stem flow (Ong et al., 2000). However, irrespective of distance from the trees, water use by CTd maize (0.04–0.18 mm per day) was much lower than that of sole maize (P < 0:001). Daily mean transpiration by Td and CTd grevillea was generally lower than in Cg 0% maize during the corresponding period, ranging between 0.8 and 1.6 mm per day (Fig. 8d); values were initially greater for CTd grevillea, but tended to be slightly greater in the Td treatment towards the end of the observation period. Transpiration declined gradually in both treatments,

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Fig. 7. Diurnal timecourses of sap flow in maize at 50 DAS during the 1995 long growing season: Cg 0% and Cg 50% denote unshaded sole maize and sole maize grown under 50% artificial shade; CTd-50 and CTd-212 represent maize plants at distances of 50 and 212 cm from trees in the CTd treatment. Double standard errors of the mean are shown.

reflecting the decreasing quantity and frequency of rainfall. 3.2.3. Cumulative transpiration Fig. 9 shows cumulative transpiration over the same period for unshaded sole maize, sole grevillea and combined water use by the trees and maize in the CTd agroforestry system. The values for Cg 0% maize were greater than in either of the treatments containing grevillea (P < 0:01). The 47-day measurement period corresponded to the period when transpiration by sole

maize was close to its seasonal maximum due to its large green leaf area (Lott et al., 2000b), the substantial quantity of residual soil water remaining from the previous rainy season, and high rainfall during the early part of the L95 season (Fig. 8a). The plants were either small or senescent before and after the observation period, and so would have contributed little to seasonal total water use. The cumulative total water use by Cg 0% maize of 82 mm, or 37% of the seasonal rainfall, was substantially lower than the seasonal total of 140 mm recorded at Machakos during a season when rainfall was 220 mm, i.e. 64% of the seasonal rainfall (McIntyre et al., 1996); in their study, the remaining 36% was lost as soil evaporation Jackson and Wallace (1999) and Jackson et al. (1999) used a combination of minilysimeter and modelling approaches to quantify soil evaporation in the agroforestry system reported here and concluded that 40–50% of the annual rainfall was lost by evaporation from the soil and tree canopy, comparable to the values reported by McIntyre et al. (1996). Grevillea was not subject to the limitations on transpiration before or after the measurement period experienced by maize. Thus, although accumulated water use by the trees was only ca. 60% of that for unshaded sole maize during the period shown in Fig. 9, cumulative transpiration over the entire cropping season (182 and 191 mm for Td and CTd grevillea, respectively) greatly exceeded that for sole maize. The trees transpired approximately 85% of the seasonal rainfall (Table 1), comparable to the value reported for perennial pigeonpea–groundnut agroforestry systems grown under unimodal rainfall conditions in India (Ong and Black, 1994). The proportion of rainfall transpired by the trees increased rapidly during successive annual cycles, from ca. 1–5% in 1991/1992 to 21–31% in 1992/1993 and 60–68% in 1993/1994 and 1994/1995. During the final four cropping seasons, transpiration by CTd grevillea exceeded 70% of the seasonal rainfall in all except the unusually wet S94/95 season, although the absolute quantity of water extracted by the trees was greatest during this season (ca. 263 and 245 mm, respectively, in the CTd and Td treatments). These observations clearly demonstrate that the poor growth of CTd maize in all except the S94/95 season (Lott et al., 2000b) resulted primarily from severe competition for water with grevillea.

Fig. 8. Daily values for: (a) daily total rainfall and solar radiation; (b) mean daytime saturation deficit; (c) transpiration by unshaded sole maize (Cg 0%), sole maize grown under 50% artificial shade (Cg 50%), and maize in the CTd agroforestry treatment at distances of 50 (CTd50) and 212 cm (CTd-212) from the nearest tree; (d) transpiration of grevillea in the sole (Td) and dispersed (CTd) agroforestry treatments. Measurements were made during the 1995 long growing season between 41 and 88 days after sowing maize and 1282–1329 days after planting grevillea. Double standard errors of the mean are shown for transpiration except when smaller than the symbols.

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although grevillea is highly effective at utilising scarce water resources in semi-arid environments, careful consideration must be given to the distribution of the trees among crops if mutually detrimental effects on tree establishment and crop growth yield are to be avoided.

Acknowledgements

Fig. 9. Cumulative stand-level transpiration for unshaded sole maize (Cg 0%), sole grevillea (Td), and maize plus grevillea in the dispersed agroforestry treatment (CTd) between 41 and 88 days after sowing maize and 1282–1329 days after planting grevillea during the 1995 long growing season.

4. Conclusions The present study supports the hypothesis that trees may improve agricultural productivity by increasing the proportion of annual rainfall captured, or by accessing residual or deep water reserves within the soil profile. Rainfall outside the cropping seasons comprises ca. 16% of the annual precipitation at Machakos. Thus, as the trees continued to grow and transpire during the off-season periods, they were able to utilise rainfall that would have been lost from productive use in annual systems and exploit residual soil moisture. However, as the lateral roots of grevillea may extract substantial quantities of water from the surface soil horizons (Lott et al., 1996), the trees are likely not only to compete with crops during the cropping season but also to reduce the moisture content of the surface soil horizons during the dry season preceding the cropping period. Such effects adversely affected crop yields, which were greatly reduced relative to equivalent sole crops during most cropping seasons after the trees had established (Lott et al., 2000b). However, the reverse situation applied during the first two annual cycles, when transpiration by CTd grevillea was suppressed relative to Td trees, suggesting that the presence of crops adversely affected tree establishment and growth (Lott et al., 2000a). Thus,

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, University of Nottingham, Royal Society, UK Natural Environment Research Council and the Swedish International Development Agency. We thank Ahmed Khan, Nick Jackson, Mark Smith, Raphael Maweu and field staff at Machakos Research Station for invaluable support.

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