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Vector competition analysis of a Leucaena–maize alley cropping system in western Kenya
Forest Ecology and Management 126 (2000) 255±268
Vector competition analysis of a Leucaena±maize alley cropping system in western Kenya Moses Imo, Victor R. Timmer* Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, Ont. M5S 3B3, Canada Received 14 October 1998; accepted 24 February 1999
Abstract The effects of 5-year old Leucaena hedgerows on growth and nutrient uptake of a maize crop were examined for one cropping season in a humid highland area of western Kenya. Three between-alley spacing (2, 4 and 8 m) and two within-alley spacing (1.0 and 0.5 m) treatments plus a treeless control were compared with or without N and P fertilizer addition. Although the total tree biomass increased with increasing tree density, maize productivity decreased with between-alley spacing but not with within-alley spacing. Compared to the sole crop maize, the biomass was lower in the 8 and 2 m alleys, re¯ecting competition effects. Vector competition analysis revealed the greatest competition for N in the 8 m alleys, and for moisture and/or light in 2 m alleys. Maize productivity and nutrient uptake were highest in 4 m alleys, re¯ecting synergistic effects of the trees, presumably due to increased N mineralized from the mulch added as pruning. These results have demonstrated the potential of vector competition analysis to quantify tree±crop nutrient interactions in agroforestry systems. Since both growth and nutritional parameters are compared relative to a standardized reference, a wide range of ecological conditions, tree±crop mixtures, and management alternatives can be quanti®ed and ranked in a systematic manner. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Hedgerow spacing; Tree±crop interactions; Nitrogen mineralization
1. Introduction Research on incorporating trees on farmlands has produced con¯icting results regarding the effects of trees on companion crops. For example, Kang et al. (1985), Maghembe et al. (1986), and Young (1997) have noted the bene®cial attributes of trees on crops because of synergistic effects, while others have reported reduced crop production under trees because *Corresponding author. Tel.: +1-416-978-6774; fax: +1-416978-3834.
of competition effects (Rao et al., 1991; Chamshama et al., 1992). Thus, tree±crop interactions are not constant, and may be in¯uenced by several factors, including species combination and planting densities, climatic characteristics, soil conditions, and management regimes. Systematic methods are, therefore, required to quantify the overall interaction effects in different agroforestry systems. Ong (1995) suggested a simple approach (tree±crop interaction (TCI) equation) in which various interaction factors, such as increased soil fertility and crop productivity, soil conservation, competition, and changes in the micro-
0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 9 9 ) 0 0 0 9 1 - 2
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climate are measured in relation to the performance of a sole crop. Although this approach provides an initial appraisal of individual and total interaction effects (Ong, 1996), it does not help explain the mechanisms associated with the interaction effects. To contribute to a better understanding of the mechanisms involved, we have developed a new diagnostic method (vector competition analysis) for screening alternative strategies for vegetation control in integrated crop management (Imo and Timmer, 1998). In this technique, tree and weed interactions are evaluated in a bivariate model depicting vectors of changing biomass and nutrient uptake relative to competition-free status, while the interpretation of the mechanisms involved are based on vector nutrient diagnosis (Haase and Rose, 1995; Imo and Timmer, 1997). A similar approach has been used by Mead and Mansur (1993) to study the competition for nutrients and moisture between Pinus radiata trees and pasture in an agroforestry situation in New Zealand. The advantage of this graphical diagnostic approach is that vector patterns simplify the interpretation of treatment responses, facilitate multiple site and treatment comparisons, and identify possible mechanisms associated with observed crop responses. The elements of our new diagnostic approach are relevant to agroforestry where both the crop and tree components are of interest. The objectives of this paper were, therefore, two-fold: to describe the development of vector competition analysis for agroforestry applications, and to illustrate its function using data from a ®eld trial testing the effects of Leucaena leucocephala hedgerow spacing and density on a companion maize crop in western Kenya. The growing of L. leucocephala trees together with crops is an option many farmers have considered in this area for a variety of reasons, such as fuelwood and fodder production, soil fertility improvement, and erosion control (Otieno, 1989). This particular experiment, established in 1992, was appropriate because it provided an opportunity to evaluate the effects of varying tree spacing and density on maize at the same time. 2. Vector competition analysis model Vector competition analysis is modeled on regression analysis (Goldberg, 1990) and vector nutrient
diagnosis (Timmer, 1991; Imo and Timmer, 1997) by comparing growth and nutrient uptake of interacting plants in a bivariate graphical model (Imo and Timmer, 1998). In this approach, interspeci®c nutrient interactions are evaluated relative to a reference plant status normalized to 100% (Fig. 1). Crop responses in mixture are then depicted as vectors of changing biomass and nutrient uptake relative to the reference treatment. Such normalization allows the comparison of multiple nutrient elements and management treatments by eliminating inherent differences in plant size and nutrient status. Based on regression approach, growth or nutrient uptake of the crop is plotted on the vertical axis against those of the associated tree component on the horizontal axis (Fig. 1). Diagnostic interpretations of treatment responses on the crop as well as tree are based on vector direction and magnitude observed as an increase (), decrease (ÿ) or no change (0) relative to the reference status. The horizontal and vertical dashed lines drawn across the reference point help to identify competition responses (Box I, Fig. 1) as follows: antagonistic competition (when both species are inhibited), synergistic (when both species are favored), and compensatory (when one species is favored, while the other is inhibited). Vector shifts below the horizontal dashed line (less than 100%) indicate competitive effects of the trees, while shifts above the dashed line exemplify bene®cial effects of the trees. Also, the slope of the vectors indicates the symmetry of the interaction: vector deviations closer to the horizontal dashed line indicate low sensitivity of the crop, while deviations further from the dashed line indicate stronger effects of the trees. Relative differences in biomass (W) and nutrient uptake (U) vectors re¯ect changes in plant nutrient concentration (Imo and Timmer, 1998) or the vector ratio (U/W, Box II in Fig. 1) and help de®ne the nutritional mechanisms involved. Crop nutrient concentration declines when relative uptake is less than relative biomass (i.e. vector ratio U/W < 1), increases when relative uptake is higher than relative biomass (i.e. vector ratio U/W > 1), and does not change when relative uptake is equal to relative biomass (vector ratio U/W 1). Growth dilution is associated with increase in both W and U, but with U/W < 1. Suf®ciency is characterized by increase in both W and U, but when U/W 1. De®ciency response occurs when
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Fig. 1. Vector competition analysis model showing relationships between crop and tree biomass (W) or nutrient content (U) responses relative to a reference treatment standardized to 100%. Vector direction or shifts reflect competition responses (Box I). Box II identifies growth and nutritional interactions for each species, based on the vector ratio U/W. (Modified from Imo and Timmer, 1998).
both W and U increase, and U/W > 1. Excess uptake of nutrients is associated with a decline in both W and U when U/W > 1. Antagonistic dilution occurs when both W and U decline, but when U/W < 1. These
responses are summarized in Box II in Fig. 1. This diagnosis offers a simpli®ed interpretation of complex plant interactions, and provides a systematic base for rationalizing vegetation management prescriptions on
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speci®c sites (Imo and Timmer, 1998). The function and application of this diagnostic technique is illustrated in a study described below. 3. Materials and methods 3.1. Study site This study was conducted at the Moi University farm located on a highland plateau area in Uasin Gishu District, western Kenya (between 008340 N and 308180 E) at an altitude of about 2134 m. This region is considered a medium to high potential agricultural area with a mean annual rainfall of about 1124 mm in two seasonal peaks (April±May and July±August) (Jaetzold and Schmidt, 1983). The mean annual temperature is 17.58C, with a maximum temperature of 26.18C in March and a minimum of 8.48C in August. The topography is characterized by generally level terrain (mean slope 0.1%). The soil was classi®ed as Rhodic Ferralsols: well drained, moderately deep to deep, dark red, and friable clay suitable for maize, wheat and bean production. The soil was generally acidic and of low fertility (pH 5.0 0.16 [H2O], Olsen P was 10.3 1.7 mg kgÿ1, total N at 0.21 0.05%, and exchangeable K, Ca and Mg at 349 10.4, 2.4 1.4 and 0.88 0.14 mg kgÿ1, respectively, and each mean was an average of 20 core samples collected from 0±20 cm depth). 3.2. Experimental treatments Six spacings of L. leucocephala (Lam) de wit, Peruvian variety (Kibwezi provenance) inter-cropped with maize were compared. The trees were planted with non-inoculated seeds sown directly in the ®eld without fertilization in July 1992. The experiment was initially established to assess the effects of tree spacing on pest incidence and severity on beans (Koech, 1995). Each plot size was 6 m 32 m, in a block size of 15 m 100 m (Fig. 2). The trees were planted in a randomized block design with six treatments consisting of three alley widths (2, 4 and 8 m), and two within-alley tree spacings (1.0 m (wide spacing, W) and 0.5 m (close spacing, C)), plus a sole crop control (S), as shown in Fig. 2. These spacings also gave the following trees per hectare densities: 1250 in the
8 m 1.0 m (8W) alley, 2500 in the 8 m 0.5 m (8C) and 4 m 1.0 m (4W) alleys, 5000 in the 4 m 0.5 m (4C) and 2 m 1.0 m (2W) alleys, and 10 000 in the 2 m 0.5 m (2C) alleys. The sole crop control plots were located 20 m from the alley cropping blocks in order to minimize tree effects on these plots. All tree rows were established in the East-West direction so that the inter-crops received maximum sunlight during the day. Hedges of Leucaena were ®rst coppiced to a height of 1.0 m at the age of 10 months when they had attained a height of 1.5 m. Subsequent pruning was done at 1-month intervals during the cropping season in order to reduce shading effects. The prunings were weighed and incorporated into each plot as green leaf manure. At each pruning, samples were taken for immediate biomass determination. Before planting, trenches were dug around each plot to a depth of 0.5 m to minimize belowground inter-plot interference. Maize was planted on 25 May 1997 at a spacing of 90 cm 30 cm. Since N and P are major limiting nutrients in this area, one-half of each plot was fertilized with diammonium phosphate (DAP) fertilizer as recommended by the Kenya Ministry of Agriculture, and the other half left unfertilized (Fig. 2). The fertilizer was broadcast on the soil surface at a rate of 400 kg haÿ1 at the time of planting. 3.3. Sampling and data analysis The dynamics of N availability was studied using Rexyn1 300 (H±OH) cation/anion exchange resins (Fisher Scienti®c), following the methods of preparation and extraction of Krause and Ramlal (1987) and Raison et al. (1987). These resins are a mixture of a strong acid cation exchanger (H ) and a strong base anion exchanger (OHÿ ). 15 ml of the ion resin (10 g fresh wt.) were measured into nylon bags (7.5 cm 7.5 cm) and sealed by sewing. The ion resins were then converted into bicarbonate form using 0.5 M NaHCO3, as described by Sibbesen (1978), rinsed in deionized water, and kept refrigerated until when used in the ®eld. Ten resin bags per plot were installed on 2 June 1997, recovered, and a new set reinstalled on 3 July 1997 and 1 August 1997. The bags were randomly installed 10 cm below ground level by creating a slit with a straight blade, laying the bag ¯at against the soil surface. The bags
M. Imo, V.R. Timmer / Forest Ecology and Management 126 (2000) 255±268 Fig. 2. Field experimental layout. One-half of each plot was either fertilized (shaded (F)) or unfertilized (not shaded (ÿF)). The treatments consisted of three between-alley spacings (8, 4 and 2 m, plus a sole crop), and two within-alley tree spacings (closer spacing (C 0.5 m), and a wider spacing (W 1.0 m)) in three replicates. The sole crop plots (S) were located 20 m from the tree alleys.
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were left buried for a period of 4 weeks, recovered and stored in a cooler until extraction. The bags were rinsed with deionized water, and extracted with 2 M HCl, as described by Yang et al. (1991) for NOÿ 3 and determination using a dual channel Technicon II NH 4 Autoanalyzer System. Four soil core samples per plot were also collected at the start, mid and end of the cropping season, and pooled and air dried for the determination of Olsen P and exchangeable K (after leaching with 1.0 M NH4Ac (pH 7.0)). Six center maize plants per row were harvested randomly (aboveground) between 22 and 24 October 1997 and oven-dried at 708C (Allen, 1974) for biomass determination. Tree samples were obtained at the time of each pruning. Oven-dried plant samples were then bulked and ground for chemical analysis. Ground samples were wet-digested in sulphuric acid±hydrogen peroxide mixture using a block digestor at 3808C (Lowther, 1980), and analyzed for total N, P and K. Total N was determined by the Kjeldahl method (Bremner and Brietenbeck, 1983), P, by the molybdate method, and K, by ¯ame photometry. All data were tested for statistical signi®cance with SAS1 (SAS Institute Inc.) using a split-plot design testing for signi®cant treatment and interaction effects. Treatment means were then compared with the maize crop on the unfertilized 4 m 1.0 m alley as the reference, using the Dunnett test. The effects of tree spacing and density on individual maize plant performance were then evaluated using vector competition analysis (Fig. 1) after expressing responses as percentages of the unfertilized 4 m 1.0 m alley spacing. Total maize productivity in this alley was similar to that of the unfertilized sole crop, and thus was used as reference for comparison with other interacting plants. However, if the objective of the study includes assessing effects of the crop on trees as well, then a sole tree treatment should be included in the analysis and standardized to 100% as the reference. (See for example Imo and Timmer (1998) where competition-free seedling status and unplanted natural vegetation were used as the reference, which enabled testing of the effects of planted trees on neighbouring vegetation as well). Vector nutrient diagnosis (Timmer, 1991; Imo and Timmer, 1997) was used to further elucidate the nutrient mechanisms associated with the observed growth and nutritional responses.
4. Results and discussion 4.1. Soil nutrient availability Generally, resin-extractable N increased with increasing tree density (Fig. 3), probably due to the higher pruning incorporation at higher tree densities. As noted by others working with Leucaena trees, improved soil fertility was attributed to higher mulch production (Kang et al., 1985; Lulandala et al., 1995; Chamshama et al., 1998). In the present study, nitrate levels were consistently higher than NH4±N levels in all the treatments (Fig. 3), presumably because of rapid nitri®cation during the growing season. Fertilizer application increased available N under all treatments, although combined fertilization and alley cropping resulted in higher available N than either alley cropping or fertilization alone. Seasonal patterns of N availability showed increasing NH4±N and NO3± N levels from early to mid-season, and decreased levels later in the growing season (Fig. 3). Increased N availability during the early season may be attributed to rapid mineralization accompanied by low uptake by the young crop, followed by a more rapid uptake as the crop matured later in the season that reduced N availability. Fertilizer application signi®cantly increased soil P availability (Olsen P was 20.8 2.0 mg kgÿ1 with fertilization versus 10.3 1.2 mg kgÿ1 without fertilization), but was not affected by tree spacing and density. Exchangeable K ranged between 333 and 380 mg kgÿ1, and was not affected by either fertilizer addition or tree spacing and planting density. 4.2. Total biomass production Total tree biomass production and nutrient yield increased with closer tree spacing (Fig. 4), re¯ecting the increasing tree population (or density). Leucaena productivity was high (1082±6422 kg haÿ1 per year, depending on tree density; Fig. 4) and comparable to other studies in the humid and sub-humid tropics (Kang, 1993; Lulandala et al., 1995; Chamshama et al., 1998). Total maize productivity in the 4 m alleys was similar to the sole crop (Fig. 4) despite fewer plants in the former (26 667 versus 31 111 maize plants per hectare in 4 m and sole crop plots, respectively), demonstrating that reduced maize density was
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Fig. 3. Concentration of NH4±N and NO3±N in resin bags placed for one season 10 cm below the soil surface in each treatment. The treatments were S, sole crop, between-alley spacing (8, 4 or 2 m), within-alley spacing (C 0.5 m, W 1.0 m), and either unfertilized (ÿF) or fertilized (F). The bars indicate standard errors of the mean, n 9.
offset by increased per plant yield in the 4 m alley. The lower maize yield obtained on the 2 and 8 m alleys (Fig. 4), however, illustrates the competitive effects of the trees on these alleys. Interestingly, within-row spacing did not affect the performance of the maize crop signi®cantly (Fig. 4), suggesting that the observed crop responses were associated more with alley width than within-alley tree spacing. Fertilizer application favoured the productivity of both trees and maize by increasing the total biomass of both components consistently (Fig. 4). The competitive effects of the trees on the 8 m alleys were much smaller with fertilization, indicating reduced competition effects in these alleys due to fertilizer addition (Fig. 4). However, yield differences between sole and mixed cropped maize on the 2 m alleys when fertilized remained high and similar to the unfertilized treatments, suggesting little bene®t of fertilizer addition to these alleys. Relative fertilizer response was highest for maize and lowest for trees at low tree density, suggesting that most of the added fertilizer was absorbed by the crop at low density, and by the trees at higher tree density. Relative response to fertilization
on the 4 m alleys was similar between maize and the Leucaena trees, probably due to lack of competitive effects. Combined tree and crop biomass production (Fig. 4) showed that the 4 m alley system was the most productive, demonstrating a higher capture and use ef®ciency of growth resources in this alley spacing. Total biomass productivity was consistently higher with fertilizer addition in each cropping system (Fig. 4), con®rming suspected N and P limitations in the study site. Total productivity was higher in the unfertilized 4 m alleys compared to the fertilized sole crop, exemplifying that inter-cropping at this tree spacing can substitute effectively for fertilizer addition. 4.3. Growth and nutrient uptake responses In order to examine the mechanisms associated with the effects of Leucaena alleys on the maize inter-crop, maize biomass and nutrient uptake were expressed on per individual plant basis to eliminate the in¯uence of maize density on different treatments. For example, it is possible that maize productivity differences could
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Fig. 4. Tree and maize biomass production under sole crop (S), between-alley spacing (8, 4 or 2 m), within-alley spacing (C 0.5 m, W 1.0 m), and either unfertilized (ÿF) or fertilized (F). The bars are standard errors of the means.
be due to varying maize population because of the replacement factor. Analysis of variance (Table 1) revealed highly signi®cant alley width and fertilization effects on growth, and nutrient uptake and concentration of individual maize plants (p < 0.0001), but no within-alley Leucaena spacing effects on maize performance. Similar comparisons also showed signi®cant alley spacing and fertilizer interactions (p < 0.0001) (except for P and K concentrations), but no alley width and within-alley interactions (Table 1). These results agree with earlier observations on the same experimental site, indicating that the yield of beans was affected only by alley width, but not within-alley spacing, or as a result of their interactions (Koech, 1995). Compared to the sole crop, per plant maize biomass was ÿ20±ÿ33% lower on the 8 and 2 m alleys, but was 12±25% higher on the 4 m alleys without fertilizer
addition (Table 2). These responses further demonstrate competition by Leucaena trees on the 8 and 2 m alleys, but synergistic effects of the trees on the 4 m alleys. Despite enhanced maize productivity from fertilization, maize biomass on the fertilized 8 and 2 m alleys was still lower than that of the fertilized sole crop. The negative effects of the fertilized Leucaena trees were, however, slightly smaller (ÿ10± ÿ26% lower) with than without fertilization (ÿ20± ÿ33% lower). The bene®cial effects of the 4 m alley cropping were further enhanced by fertilizer addition (23±30% higher than the fertilized sole crop), re¯ecting a positive 4 m alley±fertilizer interaction, as previously revealed by the analysis of variance (Table 2). With no fertilization, uptake and concentration of N were reduced on the 8 m alleys (Table 2), probably because of higher competition for this nutrient, but were higher on the 4 m alleys (Table 2), presumably due to increased N availability (Fig. 3). The lack of signi®cant effects on maize nutrient uptake on the 2 m alleys despite reduced growth (Table 2) suggests that the nutrients were probably not limiting crop growth under this spacing. Also, the lack of tree effects on P uptake by the crop indicates that P was probably not involved in the observed tree±crop interactions. Fertilization, however, increased maize N and P uptake and concentrations in all treatments (Table 2), re¯ecting increased nutrient availability, but resulted in a decline in K concentration (Table 2), probably because of growth dilution. Similar comparisons indicated that N and K uptake by the crop were reduced on the 8 m, but were higher on the 4 m alleys without fertilizer addition. These responses were further evaluated by vector competition analysis to elucidate the nature of tree±crop nutrient interactions, as discussed below. 4.4. Vector competition analysis In order to evaluate the possible nutrient mechanisms involved, maize biomass and nutrient uptake and concentration from Table 2 were expressed as percentages of the status of the unfertilized 4 m 1.0 m alley (Table 3) for plotting as vector competition diagrams (Fig. 5). This alley treatment was designated as the reference and normalized to 100%, as discussed previously. Vector direction and size depict the effects of increasing or decreasing tree density on crop per-
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Table 1 Analysis of variance table (p > F) with main effects of between-alley spacing (M), within-alley spacing (S), fertilization (F), and their interactions (M S, S F, M F and M S F) for per plant maize biomass, nutrient uptake and concentrationa Parameter
M
S
F
MS
SF
MF
MSF
Biomass
0.0001 (99.12)
0.315 (10.5)
0.0001 (68.17)
0.467 (0.78)
0.0545 (4.03) 0.0001 (11.83) 0.1781 (1.85)
Nutrient concentration N P K
0.0001 (77.28) 0.2833 (1.33) 0.0117 (4.41)
0.5997 (0.28) 0.001 (140.05) 0.0726 (2.88) 0.2339 (1.48) 0.0047 (5.39) 0.9127 (0.01) 0.0001 (151.94) 0.3454 (1.10) 0.9781 (0.00) 0.2425 (1.48) 0.0235 (5.74) 0.0001 (26.68) 0.1886 (1.77) 0.0283 (5.35) 0.3987 (1.02)
Nutrient content N P K
0.0001 (72.03) 0.2938 (1.14) 0.0001 (88.89) 0.9393 (0.06) 0.0837 (3.22) 0.0001 (16.03) 0.5600 (0.58) 0.0001 (100.32) 0.3138 (1.05) 0.0001 (128.50) 0.4176 (0.90) 0.0544 (4.03) 0.0001 (17.03) 1.2600 (0.30) 0.0001 (97.86) 0.2322 (1.49) 0.0001 (51.24) 0.6803 (0.39) 0.1014 (2.87) 0.0002 (9.56) 2.2000 (0.13)
1.0000 (0.00) 0.8676 (0.14) 0.9184 (0.09)
a
F values are in parentheses; total df 41.
Table 2 Mean maize biomass (g per plant), and nutrient concentration and content under different tree spacings without fertilizationa Treatment
Biomass
Nutrient concentration (%) N
P
K
Unfertilized Sole crop 8 m 1.0 m 8 m 0.5 m 4 m 1.0 m 4 m 0.5 m 2 m 1.0 m 2 m 0.5 m
100b ± 67b (1082) 73b (2153) 125 (2513) 112 (3077) 80b (3282) 73b (4889)
3.04 ± 1.99b (4.3) 2.07b (4.6) 4.27 (4.34) 3.95 (4.54) 3.81b (4.43) 4.23 (4.26)
0.11 0.12 0.09 0.09 0.09 0.12 0.11
Fertilized Sole crop 8 m 1.0 m 8 m 0.5 m 4 m 1.0 m 4 m 0.5 m 2 m 1.0 m 2 m 0.5 m
144b 129b 106b 187b 178b 110b 119b
3.62 ± 3.10 (5.67) 3.45 (5.00) 5.65b (5.20) 5.31b (5.60) 4.22b (5.67) 4.41b (5.60)
0.14b 0.15b 0.15b 0.16b 0.15b 0.14b 0.14b
± (1470) (2581) (2918) (4787) (4991) (6422)
Nutrient content (g per plant)
± (0.12) (0.11) (0.11) (0.11) (0.12) (0.12) ± (0.13) (0.13) (0.13) (0.12) (0.13) (0.13)
3.38 4.03 3.27 3.77 3.71 3.80 3.56
± (2.5) (2.4) (2.63) (2.90) (2.58) (2.54)
3.23b ± 3.00 (3.59) 3.23 (3.00) 2.87 (3.17) 3.29 (3.21) 3.27 (3.00) 3.53 (3.21)
N
P
K
3.05b ± 1.33b (51) 1.51b (93) 5.34 (116) 4.42 (140) 3.05b (142) 3.09b (208)
0.11b ± 0.08b (1.3) 0.07b (2.3) 0.11 (2.8) 0.10 (3.4) 0.09b (3.8) 0.08b (5.7)
3.39b ± 2.70b (28) 2.39b (53) 4.71 (66) 4.16 (89) 3.04b (85) 2.60b (124)
5.21 ± 3.99b (74) 3.67 (134) 10.67b (163) 9.45b (271) 4.64 (283) 5.27 (360)
0.20b ± 0.19b (2.0) 0.16 (3.3) 0.29b (3.5) 0.27b (6.2) 0.16 (6.5) 0.17b (8.2)
4.65b 3.87b 3.43b 5.37b 5.86b 3.60b 4.21b
0.0001 33.57
0.0001 27.78
Summary of analysis of variance for the Dunnett test (unfertilized 4 m 1.0 m as the reference) Source: treatment Probability 0.0001 0.0001 0.0001 0.0021 0.0001 F-ratio 29.23 23.49 10.41 3.61 23.42
± (44) (82) (94) (144) (160) (189)
a
Total tree biomass (kg haÿ1), nutrient concentration (%), and total nutrient yield (kg haÿ1) are given in parentheses. Maize response is significantly different (p < 0.05) from the unfertilized 4 m 1.10 m alley following the Dunnett test. df 26. Note that the tree responses in parentheses were not determined statistically. ±: Tree parameter not determined because the case was that of sole crop. b
formance. For example, in Fig. 5, maize biomass, and N, P and K uptake of the unfertilized 4 m 1.0 m alley (4W) amounting to 125, 5.34, 0.11 and 4.71 g,
respectively (from Table 2), were standardized to 100% to serve as reference points. Vectors 8W which characterize crop performance in the 8 m 1.0 m
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Table 3 Relative maize biomass, nutrient concentration and content under different tree spacingsa Treatment
Relative biomass
Relative concentration N
P
Relative content K
N
P
K
Unfertilized Sole crop 8 m 1.0 m 8 m 0.5 m 4 m 1.0 m 4 m 0.5 m 2 m 1.0 m 2 m 0.5 m
80 ± 54 (43) 58 (86) 100 (100) 90 (122) 64 (131) 58 (195)
71 ± 46 (102) 48 (4) 100 (100) 93 (99) 89 (94) 99 (93)
128 137 107 100 103 133 130
± (105) (98) (100) (100) (104) (105)
90 ± 107 (97) 87 (2) 100 (100) 98 (110) 101 (98) 94 (97)
57 ± 25 (42) 28 (92) 100 (100) 83 (130) 57 (131) 58 (191)
102 ± 73 (48) 62 (93) 100 (100) 93 (126) 85 (133) 76 (207)
72 ± 57 (42) 51 (79) 100 (100) 88 (135) 65 (129) 55 (188)
Fertilized Sole crop 8 m 1.0 m 8 m 0.5 m 4 m 1.0 m 4 m 0.5 m 2 m 1.0 m 2 m 0.5 m
115 ± 103 (58) 85 (103) 150 (116) 142 (190) 88 (199) 96 (256)
85 ± 73 (131) 81 (115) 132 (120) 124 (129) 99 (131) 103 (129)
161 171 168 180 176 163 164
± (120) (114) (109) (116) (117) (114)
86 80 86 76 87 87 94
98 ± 75 (76) 69 (118) 198 (139) 177 (246) 87 (259) 99 (330)
185 177 143 270 250 144 157
99 ± 82 (67) 73 (124) 114 (142) 124 (218) 76 (242) 89 (286)
± (114) (120) (122) (114) (122) (112)
± (74) (130) (137) (215) (237) (311)
a Relative tree responses are given in parentheses. ±: Tree parameter not determined because the case was that of sole crop.
alley (67, 1.33, 0.08 and 2.70 g for respective biomass, and N, P and K contents) extend from the reference to the corresponding points of relative values (54%, 25%, 73% and 57%, respectively, from Table 3). These vectors show that maize biomass, and N, P and K uptake on the 8 m 1.0 m alley were lower than those on the 4 m 1.0 m alley, probably because of higher competitive effects of the trees on the former alleys, as discussed later in Section 4.4. Response vectors for all other treatments were plotted in a similar manner (Fig. 5). Thus, without fertilization maize growth and nutrient uptake were reduced on the 8 m (Shift A in Fig. 1) and 2 m (Shift B, Fig. 1) alleys when compared to the 4 m 1.0 m alley, presumably due to higher competition effects. Also, the higher productivity of both 4 m 1.0 m and 4 m 0.5 m alleys, when compared to the sole crop, illustrates the bene®cial effects of the trees in these alleys. Notably, the largest synergistic effects (Shift C, Fig. 1) were associated with the fertilized 4 m alleys, while the strongest antagonistic effects (Shift A, Fig. 1) were associated with the unfertilized 8 m alleys (Fig. 5). The latter response was the most symmetric of the treatments since its vector was most closely aligned with the diagonal. The 2 m alleys
exhibited compensatory competition (Shift B, Fig. 1) where tree productivity increased while inducing inhibitory effects on the crop (Fig. 5). Although similar in tree density (2500 trees per hectare), maize biomass production and nutrient uptake were higher in the 4 m 1.0 m than 8 m 0.5 m alleys, probably because of higher mulch added as pruning (Fig. 4) and available N (Fig. 3) associated with the former alley treatment. Also, maize productivity was higher in the 4 m 0.5 m than the 2 m 1.0 m alley despite the latter having the same tree density (5000 trees per hectare), possibly because of higher competition effects associated with the more closely spaced Leucaena alleys in the latter. Comparison of relative biomass (W) and nutrient uptake (U) responses also facilitated the interpretation of nutrient interactions involving nutrient dilution, suf®ciency, and accumulation mechanisms, based on the vector ratio U/W, as summarized in Box II (Fig. 1). Thus, with fertilization (Fig. 5) N and P de®ciency response of maize on 4 m alleys was re¯ected by proportionally greater N and P accumulation compared to respective biomass increments (i.e. vector ratio U/W > 1), as con®rmed by vector diagnosis as discussed later (see caption in Fig. 6). The
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Fig. 5. Vector competition diagrams of biomass and nutrient uptake. The treatments were: S, sole crop, between-alley spacing (8, 4 or 2 m), within-alley spacing (C 0.5 m, W 1.0 m), and fertilized (F). The unfertilized 4 m 1.0 m alley response was standardized to 100% for comparison with the other treatments. Only major response vectors are shown. See Table 2 for statistically significant vectors.
lack of signi®cant changes in K uptake by the crop after fertilization (Figs. 5 and 6) suggest that K was not limiting maize growth on this site. With no fertilization, however, maize biomass and nutrient uptake were lower in the 8 and 2 m alleys compared to the 4 m alley and sole crop (Fig. 5), indicating higher competition effects. Comparative vector length indicates that relative N uptake was less than relative biomass in the unfertilized 8 m alleys, signifying N dilution (U/W < 1) as con®rmed by vector nutrient diagnosis in Fig. 6. Since N dilution in this treatment was associated with reduced growth and N uptake (thus, antagonistic dilution (Imo and Timmer, 1998, Figure 1)), competition for N was probably greater than for other growth resources (Box II in Fig. 1). Growth reduction in the 2 m alleys was, however, associated
with excess nutrient uptake (relative nutrient uptake was greater than relative biomass, as shown in Fig. 5 and further con®rmed by vector nutrient diagnosis in Fig. 6). Thus, competition for light and/or moisture was probably greater than the competition for nutrients in the 2 m alleys. Notice, however that crop productivity in the 4 m alleys was higher than the sole crop (Table 2, Fig. 5), indicating synergistic effects of the Leucaena trees on the maize crop under this spacing. Vector diagnosis (Fig. 7) revealed enhanced N uptake and concentration (a typical de®ciency response, Imo and Timmer, 1997), presumably because of improved N mineralization (Fig. 3) and low competition effects (Fig. 5). Results show that crop yield was maximized at 4 m alley widths, lending support to the notion that tree density and spacing can
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Fig. 7. Vector nutrient diagnosis of relative biomass and nutrient status of unfertilized maize grown in the 4 m 1 m alley (4W) and the (treeless) sole crop (S). The vector indicates a primary response to N deficiency (the largest vector is for N) in the 4 m alleys, presumably because of improved N availability and lower competition effects.
Fig. 6. Vector nutrient diagnosis of relative maize biomass and nutrient status for the different alley widths. The dashed diagonals represent fertilized (F) treatments, while the dotted diagonals represent unfertilized (ÿF) treatments. Open symbols represent 1.0 m within-alley spacing, while closed symbols represent 0.5 m within-alley spacing. The unfertilized 4 m 1.0 m crop response was normalized to 100% as the reference. Only vectors associated with major responses are shown. Note that the downward-pointing vector in the 8 m alleys infers antagonistic dilution of N, while the upward-pointing vector infers luxury or excess uptake of P. The vectors in the 4 m alley indicate primary and secondary deficiency responses to added P and N, respectively, since the P vector was larger than the N vector. The vector on the 2 m alley shows excess N uptake.
be optimized to balance facilitation and competition effects in alley cropping designs (Vandermeer, 1998). The growing body of evidence regarding the cost and bene®ts of integrating trees on farmlands over a wide range of ecological conditions requires a reliable quantitative base for analyzing the possible outcomes of tree±crop interactions in these situations. Since these interactions are likely to vary, depending on ecological conditions, species mixture and arrangement, and management regimes, the evaluation of such interactions must be systematic and able to reveal the mechanisms involved and diagnose possible limitations. For soil fertility components of tree±crop interactions, this approach is an improvement over the TCI equation of Ong (1995) as growth responses can be linked to individual nutrient elements, and their role in the overall interaction effects. Nutrient competition is associated with plant interactions that result in reduced growth, nutrient uptake and concentration (Box II in Fig. 1). However, when growth reduction is associated with increased nutrient concentration (when relative nutrient uptake is higher than relative biomass), then competition for moisture and/or light is possibly greater than that for nutrients. Increased growth associated with (i) elevated nutrient levels (both concentration and uptake) re¯ects positive
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fertility effects, and (ii) with increased uptake but decreased nutrient concentration exempli®es improved moisture, light and/or the microclimate favorable for crop growth (Box beneath Fig. 2). 5. Conclusions Although limited to only one season and site, results from this study have shown the potential of vector competition analysis in quantifying tree±crop nutrient interactions in agroforestry. Vector patterns simpli®ed the interpretations of complex interactions occurring between the competing plants, while normalization to a common base (100%) enabled simultaneous comparison between biomass and nutritional responses for different treatments. Ranking of Leucaena alley width revealed that, for this site and season, the 4 m alleys improved crop productivity, while competition was higher in the 8 and 2 m alleys since productivity was lower than that of the sole crop. The higher maize productivity in the 4 m alleys was associated with increased N uptake, presumably from higher N mineralized from added mulch (Fig. 3), re¯ecting improved fertility and lower competition effects. Apparently, the wider 8 m alleys did not have signi®cant effects on soil fertility (Fig. 3), but enhanced competition for N, as revealed by vector competition analysis. The more closely spaced 2 m alleys increased soil fertility (Fig. 3), but probably resulted in higher tree competition for moisture and/or light since vector competition analysis con®rmed that nutrients were not limiting growth. Combined alley cropping and fertilizer supplementation was more productive than either alley cropping or fertilizer addition alone, possibly because of positive fertilizer±P and mulch±N interactions. Acknowledgements We are grateful to Moi University for allowing the use of its ®eld and laboratory facilities. We would also like to thank James L. Kiyiapi, Eric K. Koech and John R. Okalebo for their valuable assistance in the ®eld and during laboratory analysis. Financial support for this research was provided by the Rockefeller Foundation.
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References Allen, S.E., (Ed.), 1974. Chemical Analysis of Ecological Materials, Wiley, New York, NY, USA. Bremner, J.M., Brietenbeck, G.A., 1983. A simple steam distillation method for Kjeldahl analysis of soil and plant materials. Comm. Soil Sci. Plant Anal. 14, 905±913. Chamshama, S.A.O., Monela, G.C., Sekiete, K.E.A., Persson, A., 1992. Suitability of the taungya system at North Kilimanjaro forest plantation. Tanzanian Agrofor. Syst. 17, 1±11. Goldberg, D.E., 1990. Components of resource competition in plant communities. In: Grace, J.B., Tilman, D. (Eds.), Perspectives on Plant Competition, Academic Press, New York, USA. Chamshama, S.A.O., Mugasha, A.G., Klovstad, A., Haveraaen, O., Maliondo, S.M.S., 1998. Growth and yield of maize alley cropped with Leucaena leucocephala and Faidherbia albida in Morogoro. Tanzanian Agrofor. Syst. 40, 215±225. Haase, D.L., Rose, R., 1995. Vector analysis and its use for interpreting plant nutrient shifts in response to silvicultural treatments. For. Sci. 41, 54±66. Imo, M., Timmer, V.R., 1998. Vector competition analysis: a new approach for evaluating vegetation control methods in young black spruce plantations. Can. J. Soil Sci. 78, 3±15. Imo, M., Timmer, V.R., 1997. Vector diagnosis of nutrient dynamics in mesquite seedlings. For. Sci. 43, 268±273. Jaetzold, R., Schmidt, H., 1983. Farm Management Handbook of Kenya, Ministry of Agriculture, Kenya, and the German Agency for Technical co-operation. Kang, B.T., 1993. Alley cropping: past achievement and future directions. Agrofor. Syst. 23, 141±155. Kang, B.T., Grimme, H., Lawson, T., 1985. Alley cropping sequentially cropped maize and cowpeas with Leucaena on a sandy soil in Southern Nigeria. Plant Soil 85, 267±277. Koech, E.K., 1995. Disease incidence and severity on beans in alley systems, Ph.D. dissertation, University of Wales, Bangor, 206 pp. Krause, H.H., Ramlal, D., 1987. In situ nutrient extraction by resin from forested, clear-cut and site prepared soil. Can. J. Soil Sci. 67, 943±952. Maghembe, J.A., Kaoneka, A.R.S., Lulandala, L.L.L., 1986. Intercropping, weeding and spacing effects on growth and nutrient content in Leucaena leucocephala at Morogoro. Tanzanian For. Ecol. Manage. 16, 269±279. Mead, D.J., Mansur, I., 1993. Vector analysis of foliage data to study competition for nutrients and moisture: an agroforestry example. NZ J. For. Sci. 23, 27±39. Lowther, J.R., 1980. Use of a simple sulphuric acid±hydrogen peroxide digest for analysis of Pinus radiata needles. Comm. Soil Sci. Plant Anal. 11, 175±188. Lulandala, L.L.L., Munishi, P.K.T., Malindo, S.M.S., 1995. Effect of spacing and management regime on Leucaena fodder yield and nutrient removals in alley cropping systems in Tanzania. Int. Tree Crops J. 8, 129±137. Ong, C.K., 1996. A framework for quantifying the various effects of tree±crop interactions. In: Ong, C.K., Huxley, P. (Eds.), Tree±Crop Interactions: A Physiological Approach, CABI, UK.
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Ong, C.K., 1995. The dark side of intercropping: manipulation of soil resources. In: Sinoquet, H., Cruz, P. (Eds.), Proc. Int. Conf. on Ecophysiology of Intercropping, 6±11 December 1993, Guadeloupe. Otieno, J.H.O., 1989. Agroforestry in western Kenya, M.Sc. dissertation, University College of North Wales, Bangor. Raison, R.J., Connell, M.H., Khanna, P.K., 1987. Methodology for studying fluxes of soil mineral-N in situ. Soil Biol. Biochem. 19, 521±530. Rao, M.R., Ong, C.K., Pathak, P., Sharma, M.M., 1991. Productivity in annual cropping and agroforestry system on a shallow Alfisol in semi-arid India. Agrofor. Syst. 15, 51±63.
Sibbesen, E., 1978. An investigation of the anion exchange resin method for soil phosphate extraction. Plant Soil 50, 305±321. Timmer, V.R., 1991. Interpretation of seedling analysis and visual symptoms. In: van den Driessche, R. (Ed.), Mineral Nutrition of Conifer Seedlings, CRC Press, Boca Raton, pp. 113±134. Vandermeer, J., 1998. Maximizing crop yield in alley crops. Agrofor. Syst. 40, 199±206. Yang, J.E., Skogley, E.O., Schaff, B.E., 1991. Nutrient flux to mixed-bed ion-exchange resin: temperature effects. Soil Sci. Soc. Am. J. 55, 762±767. Young, A., 1997. Agroforestry for Soil Management, CAB International, Wallingford, 276 pp.
Vector competition analysis of a Leucaena±maize alley cropping system in western Kenya Moses Imo, Victor R. Timmer* Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, Ont. M5S 3B3, Canada Received 14 October 1998; accepted 24 February 1999
Abstract The effects of 5-year old Leucaena hedgerows on growth and nutrient uptake of a maize crop were examined for one cropping season in a humid highland area of western Kenya. Three between-alley spacing (2, 4 and 8 m) and two within-alley spacing (1.0 and 0.5 m) treatments plus a treeless control were compared with or without N and P fertilizer addition. Although the total tree biomass increased with increasing tree density, maize productivity decreased with between-alley spacing but not with within-alley spacing. Compared to the sole crop maize, the biomass was lower in the 8 and 2 m alleys, re¯ecting competition effects. Vector competition analysis revealed the greatest competition for N in the 8 m alleys, and for moisture and/or light in 2 m alleys. Maize productivity and nutrient uptake were highest in 4 m alleys, re¯ecting synergistic effects of the trees, presumably due to increased N mineralized from the mulch added as pruning. These results have demonstrated the potential of vector competition analysis to quantify tree±crop nutrient interactions in agroforestry systems. Since both growth and nutritional parameters are compared relative to a standardized reference, a wide range of ecological conditions, tree±crop mixtures, and management alternatives can be quanti®ed and ranked in a systematic manner. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Hedgerow spacing; Tree±crop interactions; Nitrogen mineralization
1. Introduction Research on incorporating trees on farmlands has produced con¯icting results regarding the effects of trees on companion crops. For example, Kang et al. (1985), Maghembe et al. (1986), and Young (1997) have noted the bene®cial attributes of trees on crops because of synergistic effects, while others have reported reduced crop production under trees because *Corresponding author. Tel.: +1-416-978-6774; fax: +1-416978-3834.
of competition effects (Rao et al., 1991; Chamshama et al., 1992). Thus, tree±crop interactions are not constant, and may be in¯uenced by several factors, including species combination and planting densities, climatic characteristics, soil conditions, and management regimes. Systematic methods are, therefore, required to quantify the overall interaction effects in different agroforestry systems. Ong (1995) suggested a simple approach (tree±crop interaction (TCI) equation) in which various interaction factors, such as increased soil fertility and crop productivity, soil conservation, competition, and changes in the micro-
0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 9 9 ) 0 0 0 9 1 - 2
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climate are measured in relation to the performance of a sole crop. Although this approach provides an initial appraisal of individual and total interaction effects (Ong, 1996), it does not help explain the mechanisms associated with the interaction effects. To contribute to a better understanding of the mechanisms involved, we have developed a new diagnostic method (vector competition analysis) for screening alternative strategies for vegetation control in integrated crop management (Imo and Timmer, 1998). In this technique, tree and weed interactions are evaluated in a bivariate model depicting vectors of changing biomass and nutrient uptake relative to competition-free status, while the interpretation of the mechanisms involved are based on vector nutrient diagnosis (Haase and Rose, 1995; Imo and Timmer, 1997). A similar approach has been used by Mead and Mansur (1993) to study the competition for nutrients and moisture between Pinus radiata trees and pasture in an agroforestry situation in New Zealand. The advantage of this graphical diagnostic approach is that vector patterns simplify the interpretation of treatment responses, facilitate multiple site and treatment comparisons, and identify possible mechanisms associated with observed crop responses. The elements of our new diagnostic approach are relevant to agroforestry where both the crop and tree components are of interest. The objectives of this paper were, therefore, two-fold: to describe the development of vector competition analysis for agroforestry applications, and to illustrate its function using data from a ®eld trial testing the effects of Leucaena leucocephala hedgerow spacing and density on a companion maize crop in western Kenya. The growing of L. leucocephala trees together with crops is an option many farmers have considered in this area for a variety of reasons, such as fuelwood and fodder production, soil fertility improvement, and erosion control (Otieno, 1989). This particular experiment, established in 1992, was appropriate because it provided an opportunity to evaluate the effects of varying tree spacing and density on maize at the same time. 2. Vector competition analysis model Vector competition analysis is modeled on regression analysis (Goldberg, 1990) and vector nutrient
diagnosis (Timmer, 1991; Imo and Timmer, 1997) by comparing growth and nutrient uptake of interacting plants in a bivariate graphical model (Imo and Timmer, 1998). In this approach, interspeci®c nutrient interactions are evaluated relative to a reference plant status normalized to 100% (Fig. 1). Crop responses in mixture are then depicted as vectors of changing biomass and nutrient uptake relative to the reference treatment. Such normalization allows the comparison of multiple nutrient elements and management treatments by eliminating inherent differences in plant size and nutrient status. Based on regression approach, growth or nutrient uptake of the crop is plotted on the vertical axis against those of the associated tree component on the horizontal axis (Fig. 1). Diagnostic interpretations of treatment responses on the crop as well as tree are based on vector direction and magnitude observed as an increase (), decrease (ÿ) or no change (0) relative to the reference status. The horizontal and vertical dashed lines drawn across the reference point help to identify competition responses (Box I, Fig. 1) as follows: antagonistic competition (when both species are inhibited), synergistic (when both species are favored), and compensatory (when one species is favored, while the other is inhibited). Vector shifts below the horizontal dashed line (less than 100%) indicate competitive effects of the trees, while shifts above the dashed line exemplify bene®cial effects of the trees. Also, the slope of the vectors indicates the symmetry of the interaction: vector deviations closer to the horizontal dashed line indicate low sensitivity of the crop, while deviations further from the dashed line indicate stronger effects of the trees. Relative differences in biomass (W) and nutrient uptake (U) vectors re¯ect changes in plant nutrient concentration (Imo and Timmer, 1998) or the vector ratio (U/W, Box II in Fig. 1) and help de®ne the nutritional mechanisms involved. Crop nutrient concentration declines when relative uptake is less than relative biomass (i.e. vector ratio U/W < 1), increases when relative uptake is higher than relative biomass (i.e. vector ratio U/W > 1), and does not change when relative uptake is equal to relative biomass (vector ratio U/W 1). Growth dilution is associated with increase in both W and U, but with U/W < 1. Suf®ciency is characterized by increase in both W and U, but when U/W 1. De®ciency response occurs when
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Fig. 1. Vector competition analysis model showing relationships between crop and tree biomass (W) or nutrient content (U) responses relative to a reference treatment standardized to 100%. Vector direction or shifts reflect competition responses (Box I). Box II identifies growth and nutritional interactions for each species, based on the vector ratio U/W. (Modified from Imo and Timmer, 1998).
both W and U increase, and U/W > 1. Excess uptake of nutrients is associated with a decline in both W and U when U/W > 1. Antagonistic dilution occurs when both W and U decline, but when U/W < 1. These
responses are summarized in Box II in Fig. 1. This diagnosis offers a simpli®ed interpretation of complex plant interactions, and provides a systematic base for rationalizing vegetation management prescriptions on
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speci®c sites (Imo and Timmer, 1998). The function and application of this diagnostic technique is illustrated in a study described below. 3. Materials and methods 3.1. Study site This study was conducted at the Moi University farm located on a highland plateau area in Uasin Gishu District, western Kenya (between 008340 N and 308180 E) at an altitude of about 2134 m. This region is considered a medium to high potential agricultural area with a mean annual rainfall of about 1124 mm in two seasonal peaks (April±May and July±August) (Jaetzold and Schmidt, 1983). The mean annual temperature is 17.58C, with a maximum temperature of 26.18C in March and a minimum of 8.48C in August. The topography is characterized by generally level terrain (mean slope 0.1%). The soil was classi®ed as Rhodic Ferralsols: well drained, moderately deep to deep, dark red, and friable clay suitable for maize, wheat and bean production. The soil was generally acidic and of low fertility (pH 5.0 0.16 [H2O], Olsen P was 10.3 1.7 mg kgÿ1, total N at 0.21 0.05%, and exchangeable K, Ca and Mg at 349 10.4, 2.4 1.4 and 0.88 0.14 mg kgÿ1, respectively, and each mean was an average of 20 core samples collected from 0±20 cm depth). 3.2. Experimental treatments Six spacings of L. leucocephala (Lam) de wit, Peruvian variety (Kibwezi provenance) inter-cropped with maize were compared. The trees were planted with non-inoculated seeds sown directly in the ®eld without fertilization in July 1992. The experiment was initially established to assess the effects of tree spacing on pest incidence and severity on beans (Koech, 1995). Each plot size was 6 m 32 m, in a block size of 15 m 100 m (Fig. 2). The trees were planted in a randomized block design with six treatments consisting of three alley widths (2, 4 and 8 m), and two within-alley tree spacings (1.0 m (wide spacing, W) and 0.5 m (close spacing, C)), plus a sole crop control (S), as shown in Fig. 2. These spacings also gave the following trees per hectare densities: 1250 in the
8 m 1.0 m (8W) alley, 2500 in the 8 m 0.5 m (8C) and 4 m 1.0 m (4W) alleys, 5000 in the 4 m 0.5 m (4C) and 2 m 1.0 m (2W) alleys, and 10 000 in the 2 m 0.5 m (2C) alleys. The sole crop control plots were located 20 m from the alley cropping blocks in order to minimize tree effects on these plots. All tree rows were established in the East-West direction so that the inter-crops received maximum sunlight during the day. Hedges of Leucaena were ®rst coppiced to a height of 1.0 m at the age of 10 months when they had attained a height of 1.5 m. Subsequent pruning was done at 1-month intervals during the cropping season in order to reduce shading effects. The prunings were weighed and incorporated into each plot as green leaf manure. At each pruning, samples were taken for immediate biomass determination. Before planting, trenches were dug around each plot to a depth of 0.5 m to minimize belowground inter-plot interference. Maize was planted on 25 May 1997 at a spacing of 90 cm 30 cm. Since N and P are major limiting nutrients in this area, one-half of each plot was fertilized with diammonium phosphate (DAP) fertilizer as recommended by the Kenya Ministry of Agriculture, and the other half left unfertilized (Fig. 2). The fertilizer was broadcast on the soil surface at a rate of 400 kg haÿ1 at the time of planting. 3.3. Sampling and data analysis The dynamics of N availability was studied using Rexyn1 300 (H±OH) cation/anion exchange resins (Fisher Scienti®c), following the methods of preparation and extraction of Krause and Ramlal (1987) and Raison et al. (1987). These resins are a mixture of a strong acid cation exchanger (H ) and a strong base anion exchanger (OHÿ ). 15 ml of the ion resin (10 g fresh wt.) were measured into nylon bags (7.5 cm 7.5 cm) and sealed by sewing. The ion resins were then converted into bicarbonate form using 0.5 M NaHCO3, as described by Sibbesen (1978), rinsed in deionized water, and kept refrigerated until when used in the ®eld. Ten resin bags per plot were installed on 2 June 1997, recovered, and a new set reinstalled on 3 July 1997 and 1 August 1997. The bags were randomly installed 10 cm below ground level by creating a slit with a straight blade, laying the bag ¯at against the soil surface. The bags
M. Imo, V.R. Timmer / Forest Ecology and Management 126 (2000) 255±268 Fig. 2. Field experimental layout. One-half of each plot was either fertilized (shaded (F)) or unfertilized (not shaded (ÿF)). The treatments consisted of three between-alley spacings (8, 4 and 2 m, plus a sole crop), and two within-alley tree spacings (closer spacing (C 0.5 m), and a wider spacing (W 1.0 m)) in three replicates. The sole crop plots (S) were located 20 m from the tree alleys.
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were left buried for a period of 4 weeks, recovered and stored in a cooler until extraction. The bags were rinsed with deionized water, and extracted with 2 M HCl, as described by Yang et al. (1991) for NOÿ 3 and determination using a dual channel Technicon II NH 4 Autoanalyzer System. Four soil core samples per plot were also collected at the start, mid and end of the cropping season, and pooled and air dried for the determination of Olsen P and exchangeable K (after leaching with 1.0 M NH4Ac (pH 7.0)). Six center maize plants per row were harvested randomly (aboveground) between 22 and 24 October 1997 and oven-dried at 708C (Allen, 1974) for biomass determination. Tree samples were obtained at the time of each pruning. Oven-dried plant samples were then bulked and ground for chemical analysis. Ground samples were wet-digested in sulphuric acid±hydrogen peroxide mixture using a block digestor at 3808C (Lowther, 1980), and analyzed for total N, P and K. Total N was determined by the Kjeldahl method (Bremner and Brietenbeck, 1983), P, by the molybdate method, and K, by ¯ame photometry. All data were tested for statistical signi®cance with SAS1 (SAS Institute Inc.) using a split-plot design testing for signi®cant treatment and interaction effects. Treatment means were then compared with the maize crop on the unfertilized 4 m 1.0 m alley as the reference, using the Dunnett test. The effects of tree spacing and density on individual maize plant performance were then evaluated using vector competition analysis (Fig. 1) after expressing responses as percentages of the unfertilized 4 m 1.0 m alley spacing. Total maize productivity in this alley was similar to that of the unfertilized sole crop, and thus was used as reference for comparison with other interacting plants. However, if the objective of the study includes assessing effects of the crop on trees as well, then a sole tree treatment should be included in the analysis and standardized to 100% as the reference. (See for example Imo and Timmer (1998) where competition-free seedling status and unplanted natural vegetation were used as the reference, which enabled testing of the effects of planted trees on neighbouring vegetation as well). Vector nutrient diagnosis (Timmer, 1991; Imo and Timmer, 1997) was used to further elucidate the nutrient mechanisms associated with the observed growth and nutritional responses.
4. Results and discussion 4.1. Soil nutrient availability Generally, resin-extractable N increased with increasing tree density (Fig. 3), probably due to the higher pruning incorporation at higher tree densities. As noted by others working with Leucaena trees, improved soil fertility was attributed to higher mulch production (Kang et al., 1985; Lulandala et al., 1995; Chamshama et al., 1998). In the present study, nitrate levels were consistently higher than NH4±N levels in all the treatments (Fig. 3), presumably because of rapid nitri®cation during the growing season. Fertilizer application increased available N under all treatments, although combined fertilization and alley cropping resulted in higher available N than either alley cropping or fertilization alone. Seasonal patterns of N availability showed increasing NH4±N and NO3± N levels from early to mid-season, and decreased levels later in the growing season (Fig. 3). Increased N availability during the early season may be attributed to rapid mineralization accompanied by low uptake by the young crop, followed by a more rapid uptake as the crop matured later in the season that reduced N availability. Fertilizer application signi®cantly increased soil P availability (Olsen P was 20.8 2.0 mg kgÿ1 with fertilization versus 10.3 1.2 mg kgÿ1 without fertilization), but was not affected by tree spacing and density. Exchangeable K ranged between 333 and 380 mg kgÿ1, and was not affected by either fertilizer addition or tree spacing and planting density. 4.2. Total biomass production Total tree biomass production and nutrient yield increased with closer tree spacing (Fig. 4), re¯ecting the increasing tree population (or density). Leucaena productivity was high (1082±6422 kg haÿ1 per year, depending on tree density; Fig. 4) and comparable to other studies in the humid and sub-humid tropics (Kang, 1993; Lulandala et al., 1995; Chamshama et al., 1998). Total maize productivity in the 4 m alleys was similar to the sole crop (Fig. 4) despite fewer plants in the former (26 667 versus 31 111 maize plants per hectare in 4 m and sole crop plots, respectively), demonstrating that reduced maize density was
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Fig. 3. Concentration of NH4±N and NO3±N in resin bags placed for one season 10 cm below the soil surface in each treatment. The treatments were S, sole crop, between-alley spacing (8, 4 or 2 m), within-alley spacing (C 0.5 m, W 1.0 m), and either unfertilized (ÿF) or fertilized (F). The bars indicate standard errors of the mean, n 9.
offset by increased per plant yield in the 4 m alley. The lower maize yield obtained on the 2 and 8 m alleys (Fig. 4), however, illustrates the competitive effects of the trees on these alleys. Interestingly, within-row spacing did not affect the performance of the maize crop signi®cantly (Fig. 4), suggesting that the observed crop responses were associated more with alley width than within-alley tree spacing. Fertilizer application favoured the productivity of both trees and maize by increasing the total biomass of both components consistently (Fig. 4). The competitive effects of the trees on the 8 m alleys were much smaller with fertilization, indicating reduced competition effects in these alleys due to fertilizer addition (Fig. 4). However, yield differences between sole and mixed cropped maize on the 2 m alleys when fertilized remained high and similar to the unfertilized treatments, suggesting little bene®t of fertilizer addition to these alleys. Relative fertilizer response was highest for maize and lowest for trees at low tree density, suggesting that most of the added fertilizer was absorbed by the crop at low density, and by the trees at higher tree density. Relative response to fertilization
on the 4 m alleys was similar between maize and the Leucaena trees, probably due to lack of competitive effects. Combined tree and crop biomass production (Fig. 4) showed that the 4 m alley system was the most productive, demonstrating a higher capture and use ef®ciency of growth resources in this alley spacing. Total biomass productivity was consistently higher with fertilizer addition in each cropping system (Fig. 4), con®rming suspected N and P limitations in the study site. Total productivity was higher in the unfertilized 4 m alleys compared to the fertilized sole crop, exemplifying that inter-cropping at this tree spacing can substitute effectively for fertilizer addition. 4.3. Growth and nutrient uptake responses In order to examine the mechanisms associated with the effects of Leucaena alleys on the maize inter-crop, maize biomass and nutrient uptake were expressed on per individual plant basis to eliminate the in¯uence of maize density on different treatments. For example, it is possible that maize productivity differences could
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Fig. 4. Tree and maize biomass production under sole crop (S), between-alley spacing (8, 4 or 2 m), within-alley spacing (C 0.5 m, W 1.0 m), and either unfertilized (ÿF) or fertilized (F). The bars are standard errors of the means.
be due to varying maize population because of the replacement factor. Analysis of variance (Table 1) revealed highly signi®cant alley width and fertilization effects on growth, and nutrient uptake and concentration of individual maize plants (p < 0.0001), but no within-alley Leucaena spacing effects on maize performance. Similar comparisons also showed signi®cant alley spacing and fertilizer interactions (p < 0.0001) (except for P and K concentrations), but no alley width and within-alley interactions (Table 1). These results agree with earlier observations on the same experimental site, indicating that the yield of beans was affected only by alley width, but not within-alley spacing, or as a result of their interactions (Koech, 1995). Compared to the sole crop, per plant maize biomass was ÿ20±ÿ33% lower on the 8 and 2 m alleys, but was 12±25% higher on the 4 m alleys without fertilizer
addition (Table 2). These responses further demonstrate competition by Leucaena trees on the 8 and 2 m alleys, but synergistic effects of the trees on the 4 m alleys. Despite enhanced maize productivity from fertilization, maize biomass on the fertilized 8 and 2 m alleys was still lower than that of the fertilized sole crop. The negative effects of the fertilized Leucaena trees were, however, slightly smaller (ÿ10± ÿ26% lower) with than without fertilization (ÿ20± ÿ33% lower). The bene®cial effects of the 4 m alley cropping were further enhanced by fertilizer addition (23±30% higher than the fertilized sole crop), re¯ecting a positive 4 m alley±fertilizer interaction, as previously revealed by the analysis of variance (Table 2). With no fertilization, uptake and concentration of N were reduced on the 8 m alleys (Table 2), probably because of higher competition for this nutrient, but were higher on the 4 m alleys (Table 2), presumably due to increased N availability (Fig. 3). The lack of signi®cant effects on maize nutrient uptake on the 2 m alleys despite reduced growth (Table 2) suggests that the nutrients were probably not limiting crop growth under this spacing. Also, the lack of tree effects on P uptake by the crop indicates that P was probably not involved in the observed tree±crop interactions. Fertilization, however, increased maize N and P uptake and concentrations in all treatments (Table 2), re¯ecting increased nutrient availability, but resulted in a decline in K concentration (Table 2), probably because of growth dilution. Similar comparisons indicated that N and K uptake by the crop were reduced on the 8 m, but were higher on the 4 m alleys without fertilizer addition. These responses were further evaluated by vector competition analysis to elucidate the nature of tree±crop nutrient interactions, as discussed below. 4.4. Vector competition analysis In order to evaluate the possible nutrient mechanisms involved, maize biomass and nutrient uptake and concentration from Table 2 were expressed as percentages of the status of the unfertilized 4 m 1.0 m alley (Table 3) for plotting as vector competition diagrams (Fig. 5). This alley treatment was designated as the reference and normalized to 100%, as discussed previously. Vector direction and size depict the effects of increasing or decreasing tree density on crop per-
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Table 1 Analysis of variance table (p > F) with main effects of between-alley spacing (M), within-alley spacing (S), fertilization (F), and their interactions (M S, S F, M F and M S F) for per plant maize biomass, nutrient uptake and concentrationa Parameter
M
S
F
MS
SF
MF
MSF
Biomass
0.0001 (99.12)
0.315 (10.5)
0.0001 (68.17)
0.467 (0.78)
0.0545 (4.03) 0.0001 (11.83) 0.1781 (1.85)
Nutrient concentration N P K
0.0001 (77.28) 0.2833 (1.33) 0.0117 (4.41)
0.5997 (0.28) 0.001 (140.05) 0.0726 (2.88) 0.2339 (1.48) 0.0047 (5.39) 0.9127 (0.01) 0.0001 (151.94) 0.3454 (1.10) 0.9781 (0.00) 0.2425 (1.48) 0.0235 (5.74) 0.0001 (26.68) 0.1886 (1.77) 0.0283 (5.35) 0.3987 (1.02)
Nutrient content N P K
0.0001 (72.03) 0.2938 (1.14) 0.0001 (88.89) 0.9393 (0.06) 0.0837 (3.22) 0.0001 (16.03) 0.5600 (0.58) 0.0001 (100.32) 0.3138 (1.05) 0.0001 (128.50) 0.4176 (0.90) 0.0544 (4.03) 0.0001 (17.03) 1.2600 (0.30) 0.0001 (97.86) 0.2322 (1.49) 0.0001 (51.24) 0.6803 (0.39) 0.1014 (2.87) 0.0002 (9.56) 2.2000 (0.13)
1.0000 (0.00) 0.8676 (0.14) 0.9184 (0.09)
a
F values are in parentheses; total df 41.
Table 2 Mean maize biomass (g per plant), and nutrient concentration and content under different tree spacings without fertilizationa Treatment
Biomass
Nutrient concentration (%) N
P
K
Unfertilized Sole crop 8 m 1.0 m 8 m 0.5 m 4 m 1.0 m 4 m 0.5 m 2 m 1.0 m 2 m 0.5 m
100b ± 67b (1082) 73b (2153) 125 (2513) 112 (3077) 80b (3282) 73b (4889)
3.04 ± 1.99b (4.3) 2.07b (4.6) 4.27 (4.34) 3.95 (4.54) 3.81b (4.43) 4.23 (4.26)
0.11 0.12 0.09 0.09 0.09 0.12 0.11
Fertilized Sole crop 8 m 1.0 m 8 m 0.5 m 4 m 1.0 m 4 m 0.5 m 2 m 1.0 m 2 m 0.5 m
144b 129b 106b 187b 178b 110b 119b
3.62 ± 3.10 (5.67) 3.45 (5.00) 5.65b (5.20) 5.31b (5.60) 4.22b (5.67) 4.41b (5.60)
0.14b 0.15b 0.15b 0.16b 0.15b 0.14b 0.14b
± (1470) (2581) (2918) (4787) (4991) (6422)
Nutrient content (g per plant)
± (0.12) (0.11) (0.11) (0.11) (0.12) (0.12) ± (0.13) (0.13) (0.13) (0.12) (0.13) (0.13)
3.38 4.03 3.27 3.77 3.71 3.80 3.56
± (2.5) (2.4) (2.63) (2.90) (2.58) (2.54)
3.23b ± 3.00 (3.59) 3.23 (3.00) 2.87 (3.17) 3.29 (3.21) 3.27 (3.00) 3.53 (3.21)
N
P
K
3.05b ± 1.33b (51) 1.51b (93) 5.34 (116) 4.42 (140) 3.05b (142) 3.09b (208)
0.11b ± 0.08b (1.3) 0.07b (2.3) 0.11 (2.8) 0.10 (3.4) 0.09b (3.8) 0.08b (5.7)
3.39b ± 2.70b (28) 2.39b (53) 4.71 (66) 4.16 (89) 3.04b (85) 2.60b (124)
5.21 ± 3.99b (74) 3.67 (134) 10.67b (163) 9.45b (271) 4.64 (283) 5.27 (360)
0.20b ± 0.19b (2.0) 0.16 (3.3) 0.29b (3.5) 0.27b (6.2) 0.16 (6.5) 0.17b (8.2)
4.65b 3.87b 3.43b 5.37b 5.86b 3.60b 4.21b
0.0001 33.57
0.0001 27.78
Summary of analysis of variance for the Dunnett test (unfertilized 4 m 1.0 m as the reference) Source: treatment Probability 0.0001 0.0001 0.0001 0.0021 0.0001 F-ratio 29.23 23.49 10.41 3.61 23.42
± (44) (82) (94) (144) (160) (189)
a
Total tree biomass (kg haÿ1), nutrient concentration (%), and total nutrient yield (kg haÿ1) are given in parentheses. Maize response is significantly different (p < 0.05) from the unfertilized 4 m 1.10 m alley following the Dunnett test. df 26. Note that the tree responses in parentheses were not determined statistically. ±: Tree parameter not determined because the case was that of sole crop. b
formance. For example, in Fig. 5, maize biomass, and N, P and K uptake of the unfertilized 4 m 1.0 m alley (4W) amounting to 125, 5.34, 0.11 and 4.71 g,
respectively (from Table 2), were standardized to 100% to serve as reference points. Vectors 8W which characterize crop performance in the 8 m 1.0 m
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Table 3 Relative maize biomass, nutrient concentration and content under different tree spacingsa Treatment
Relative biomass
Relative concentration N
P
Relative content K
N
P
K
Unfertilized Sole crop 8 m 1.0 m 8 m 0.5 m 4 m 1.0 m 4 m 0.5 m 2 m 1.0 m 2 m 0.5 m
80 ± 54 (43) 58 (86) 100 (100) 90 (122) 64 (131) 58 (195)
71 ± 46 (102) 48 (4) 100 (100) 93 (99) 89 (94) 99 (93)
128 137 107 100 103 133 130
± (105) (98) (100) (100) (104) (105)
90 ± 107 (97) 87 (2) 100 (100) 98 (110) 101 (98) 94 (97)
57 ± 25 (42) 28 (92) 100 (100) 83 (130) 57 (131) 58 (191)
102 ± 73 (48) 62 (93) 100 (100) 93 (126) 85 (133) 76 (207)
72 ± 57 (42) 51 (79) 100 (100) 88 (135) 65 (129) 55 (188)
Fertilized Sole crop 8 m 1.0 m 8 m 0.5 m 4 m 1.0 m 4 m 0.5 m 2 m 1.0 m 2 m 0.5 m
115 ± 103 (58) 85 (103) 150 (116) 142 (190) 88 (199) 96 (256)
85 ± 73 (131) 81 (115) 132 (120) 124 (129) 99 (131) 103 (129)
161 171 168 180 176 163 164
± (120) (114) (109) (116) (117) (114)
86 80 86 76 87 87 94
98 ± 75 (76) 69 (118) 198 (139) 177 (246) 87 (259) 99 (330)
185 177 143 270 250 144 157
99 ± 82 (67) 73 (124) 114 (142) 124 (218) 76 (242) 89 (286)
± (114) (120) (122) (114) (122) (112)
± (74) (130) (137) (215) (237) (311)
a Relative tree responses are given in parentheses. ±: Tree parameter not determined because the case was that of sole crop.
alley (67, 1.33, 0.08 and 2.70 g for respective biomass, and N, P and K contents) extend from the reference to the corresponding points of relative values (54%, 25%, 73% and 57%, respectively, from Table 3). These vectors show that maize biomass, and N, P and K uptake on the 8 m 1.0 m alley were lower than those on the 4 m 1.0 m alley, probably because of higher competitive effects of the trees on the former alleys, as discussed later in Section 4.4. Response vectors for all other treatments were plotted in a similar manner (Fig. 5). Thus, without fertilization maize growth and nutrient uptake were reduced on the 8 m (Shift A in Fig. 1) and 2 m (Shift B, Fig. 1) alleys when compared to the 4 m 1.0 m alley, presumably due to higher competition effects. Also, the higher productivity of both 4 m 1.0 m and 4 m 0.5 m alleys, when compared to the sole crop, illustrates the bene®cial effects of the trees in these alleys. Notably, the largest synergistic effects (Shift C, Fig. 1) were associated with the fertilized 4 m alleys, while the strongest antagonistic effects (Shift A, Fig. 1) were associated with the unfertilized 8 m alleys (Fig. 5). The latter response was the most symmetric of the treatments since its vector was most closely aligned with the diagonal. The 2 m alleys
exhibited compensatory competition (Shift B, Fig. 1) where tree productivity increased while inducing inhibitory effects on the crop (Fig. 5). Although similar in tree density (2500 trees per hectare), maize biomass production and nutrient uptake were higher in the 4 m 1.0 m than 8 m 0.5 m alleys, probably because of higher mulch added as pruning (Fig. 4) and available N (Fig. 3) associated with the former alley treatment. Also, maize productivity was higher in the 4 m 0.5 m than the 2 m 1.0 m alley despite the latter having the same tree density (5000 trees per hectare), possibly because of higher competition effects associated with the more closely spaced Leucaena alleys in the latter. Comparison of relative biomass (W) and nutrient uptake (U) responses also facilitated the interpretation of nutrient interactions involving nutrient dilution, suf®ciency, and accumulation mechanisms, based on the vector ratio U/W, as summarized in Box II (Fig. 1). Thus, with fertilization (Fig. 5) N and P de®ciency response of maize on 4 m alleys was re¯ected by proportionally greater N and P accumulation compared to respective biomass increments (i.e. vector ratio U/W > 1), as con®rmed by vector diagnosis as discussed later (see caption in Fig. 6). The
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265
Fig. 5. Vector competition diagrams of biomass and nutrient uptake. The treatments were: S, sole crop, between-alley spacing (8, 4 or 2 m), within-alley spacing (C 0.5 m, W 1.0 m), and fertilized (F). The unfertilized 4 m 1.0 m alley response was standardized to 100% for comparison with the other treatments. Only major response vectors are shown. See Table 2 for statistically significant vectors.
lack of signi®cant changes in K uptake by the crop after fertilization (Figs. 5 and 6) suggest that K was not limiting maize growth on this site. With no fertilization, however, maize biomass and nutrient uptake were lower in the 8 and 2 m alleys compared to the 4 m alley and sole crop (Fig. 5), indicating higher competition effects. Comparative vector length indicates that relative N uptake was less than relative biomass in the unfertilized 8 m alleys, signifying N dilution (U/W < 1) as con®rmed by vector nutrient diagnosis in Fig. 6. Since N dilution in this treatment was associated with reduced growth and N uptake (thus, antagonistic dilution (Imo and Timmer, 1998, Figure 1)), competition for N was probably greater than for other growth resources (Box II in Fig. 1). Growth reduction in the 2 m alleys was, however, associated
with excess nutrient uptake (relative nutrient uptake was greater than relative biomass, as shown in Fig. 5 and further con®rmed by vector nutrient diagnosis in Fig. 6). Thus, competition for light and/or moisture was probably greater than the competition for nutrients in the 2 m alleys. Notice, however that crop productivity in the 4 m alleys was higher than the sole crop (Table 2, Fig. 5), indicating synergistic effects of the Leucaena trees on the maize crop under this spacing. Vector diagnosis (Fig. 7) revealed enhanced N uptake and concentration (a typical de®ciency response, Imo and Timmer, 1997), presumably because of improved N mineralization (Fig. 3) and low competition effects (Fig. 5). Results show that crop yield was maximized at 4 m alley widths, lending support to the notion that tree density and spacing can
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Fig. 7. Vector nutrient diagnosis of relative biomass and nutrient status of unfertilized maize grown in the 4 m 1 m alley (4W) and the (treeless) sole crop (S). The vector indicates a primary response to N deficiency (the largest vector is for N) in the 4 m alleys, presumably because of improved N availability and lower competition effects.
Fig. 6. Vector nutrient diagnosis of relative maize biomass and nutrient status for the different alley widths. The dashed diagonals represent fertilized (F) treatments, while the dotted diagonals represent unfertilized (ÿF) treatments. Open symbols represent 1.0 m within-alley spacing, while closed symbols represent 0.5 m within-alley spacing. The unfertilized 4 m 1.0 m crop response was normalized to 100% as the reference. Only vectors associated with major responses are shown. Note that the downward-pointing vector in the 8 m alleys infers antagonistic dilution of N, while the upward-pointing vector infers luxury or excess uptake of P. The vectors in the 4 m alley indicate primary and secondary deficiency responses to added P and N, respectively, since the P vector was larger than the N vector. The vector on the 2 m alley shows excess N uptake.
be optimized to balance facilitation and competition effects in alley cropping designs (Vandermeer, 1998). The growing body of evidence regarding the cost and bene®ts of integrating trees on farmlands over a wide range of ecological conditions requires a reliable quantitative base for analyzing the possible outcomes of tree±crop interactions in these situations. Since these interactions are likely to vary, depending on ecological conditions, species mixture and arrangement, and management regimes, the evaluation of such interactions must be systematic and able to reveal the mechanisms involved and diagnose possible limitations. For soil fertility components of tree±crop interactions, this approach is an improvement over the TCI equation of Ong (1995) as growth responses can be linked to individual nutrient elements, and their role in the overall interaction effects. Nutrient competition is associated with plant interactions that result in reduced growth, nutrient uptake and concentration (Box II in Fig. 1). However, when growth reduction is associated with increased nutrient concentration (when relative nutrient uptake is higher than relative biomass), then competition for moisture and/or light is possibly greater than that for nutrients. Increased growth associated with (i) elevated nutrient levels (both concentration and uptake) re¯ects positive
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fertility effects, and (ii) with increased uptake but decreased nutrient concentration exempli®es improved moisture, light and/or the microclimate favorable for crop growth (Box beneath Fig. 2). 5. Conclusions Although limited to only one season and site, results from this study have shown the potential of vector competition analysis in quantifying tree±crop nutrient interactions in agroforestry. Vector patterns simpli®ed the interpretations of complex interactions occurring between the competing plants, while normalization to a common base (100%) enabled simultaneous comparison between biomass and nutritional responses for different treatments. Ranking of Leucaena alley width revealed that, for this site and season, the 4 m alleys improved crop productivity, while competition was higher in the 8 and 2 m alleys since productivity was lower than that of the sole crop. The higher maize productivity in the 4 m alleys was associated with increased N uptake, presumably from higher N mineralized from added mulch (Fig. 3), re¯ecting improved fertility and lower competition effects. Apparently, the wider 8 m alleys did not have signi®cant effects on soil fertility (Fig. 3), but enhanced competition for N, as revealed by vector competition analysis. The more closely spaced 2 m alleys increased soil fertility (Fig. 3), but probably resulted in higher tree competition for moisture and/or light since vector competition analysis con®rmed that nutrients were not limiting growth. Combined alley cropping and fertilizer supplementation was more productive than either alley cropping or fertilizer addition alone, possibly because of positive fertilizer±P and mulch±N interactions. Acknowledgements We are grateful to Moi University for allowing the use of its ®eld and laboratory facilities. We would also like to thank James L. Kiyiapi, Eric K. Koech and John R. Okalebo for their valuable assistance in the ®eld and during laboratory analysis. Financial support for this research was provided by the Rockefeller Foundation.
267
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