Defining competition vectors in a temperate alley croppingsystem in the midwestern USA

2. Competition for water

S. JOSE1, A. R. GILLESPIE1, *, J. R. SEIFERT1 and D. J. BIEHLE2

1 Department of Forestry and Natural Resources, Purdue University, West Lafayette, IN 47907-1159, USA; 2 Southeast Purdue Agriculture Center, Butlerville, IN 47223, USA (*Author forcorrespondence: E-mail: andyg@fnr.purdue.edu)

Key words: belowground competition, leaf area, root biomass, sap flow, soil moisture

Abstract. A study was carried out in an alley cropping system in the Eastern Maize Belt of theUnited States with the objective of quantifying the competition for water between trees blackwalnut (Juglans nigra L.) and red oak (Quercus rubra L.) and maize (Zea mays L.). Spatialand temporal variation in soil moisture, tree and maize rooting patterns, tree and maize wateruptake, and maize leaf area expansion were determined in three treatments; ‘barrier’ (polyeth-ylene root barriers separating maize and tree roots), ‘trench’ (trenching without a polyethylenebarrier), and ‘no barrier’ (control). Significant temporal variation in soil moisture was observedin both black walnut and red oak alley cropping systems. The ‘barrier’ and ‘trench’ treatmentsresulted in higher soil moisture in the ‘maize alley’ and lower soil moisture in the ‘tree row’than the ‘no barrier’ treatment. Uptake of water by maize was higher in the ‘barrier’ than the‘no barrier’ treatment. However, tree water uptake was higher in the ‘no barrier’ treatment thanin the ‘barrier’ treatment because tree roots utilized water from the ‘maize alley’. Maize leafarea was negatively impacted by water stress in the ‘no barrier’ treatment. Quantification ofrooting patterns revealed that maize and tree roots were concentrated in the top 30 cm soillayer where water fluctuations were greatest. The ‘barrier’ and ‘trench’ treatments successfullyeliminated the belowground root competition between trees and maize and resulted in greaterleaf area and higher grain yields than the ‘no barrier’ treatment. Thus, competition for waterrather than competition for light seems to be critical in defining the productivity and sustain-ability of this alley cropping system.

Introduction

Alley cropping systems combining crops with timber producing trees, espe-cially black walnut (Juglans nigra L.) and red oak (Quercus rubra L.), havebeen established on several sites in the midwestern US having differentpatterns of precipitation and water availability (Garrett and Kurtz, 1983;Noweg and Kurtz, 1987; Garrett and Buck, 1997). As suggested by Howardet al. (1997), the degree of competition, and ultimately crop yield, in suchmulti-species systems is dependent on the partitioning of resources such aslight, water, and nutrients. In a biological context, the success of such acomplex system depends primarily on minimizing the negative interactionsbetween trees and crops – both above (mainly for light) and belowground(for water and nutrients). Although competition for soil moisture has been

Agroforestry Systems 48: 41–59, 2000. 2000 Kluwer Academic Publishers. Printed in the Netherlands.

reported as the major factor limiting success of alley cropping systems in thesemi-arid tropics (Singh et al., 1989; Ong et al., 1991), such results have notbeen reported from temperate alley cropping systems.

Our work with light and photosynthesis has shown little impact of treealleys on maize photosynthesis (Gillespie et al., this issue). This observationhas led us to further investigate the belowground processes and resourcesthat define system productivity. The present study was carried out with theobjective of quantifying the competition for water between trees (black walnutand red oak) and maize (Zea mays L.) in an alley cropping system in themidwestern United States. The specific questions addressed by our study were:

1. How does soil moisture vary spatially and temporally in an alley croppingsystem with and without root competition between trees and crops?

2. How do tree and maize rooting patterns differ spatially? 3. How is water uptake by maize and trees impacted by belowground com-

petition? 4. How is maize growth (measured as leaf area expansion) affected by com-

petition for water?

Materials and methods

Study area

The study was carried out in an alley cropping system with maize and twofine hardwood species (black walnut, and red oak), established in 1985, atthe Purdue University Southeastern Agriculture Center in Indiana, USA(39°3

′ N, 85°30′ W). The area is characterized by a Parke Silt Loam soil (UlticHapludalf), a well drained loess over loamy glacial till, underlain by sand andgravel. The experimental site included a randomized complete block designwith 12 plots for each of the tree species. Six rows of maize were planted inthe alleys created by tree rows (inter-row tree spacing of 8.5 m and intra-rowspacing of 2.4 m) at a density of 65,000 plants per hectare. In the spring of1995, three treatments were applied randomly to each of the four blocks toseparate belowground root interactions. These included: (1) Root barrier wherea polyethylene barrier was inserted after trenching on both sides of the treerows, preventing tree root growth into the maize alley, (2) Trench alone, and(3) Control where tree and maize roots intermingled. Detailed descriptionsof the study site, treatments, and cultural practices for our alley croppingsystem have been given in Gillespie et al. (this issue).

Daily precipitation data during the study period were collected from aweather station adjacent to the experimental plots. Figure 1 shows the dailyprecipitation events during the growing season in 1996.

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Soil moisture

Soil moisture was measured bi-weekly from June through September, 1996using a Troxler neutron soil moisture gauge (Model 3222, Troxler ElectronicsInc., NC, USA) in two plots for each treatment. Aluminum access tubes wereinstalled to a depth of 1 m at five different distances from the tree row of0, 1.1, 2.3, 3.5, and 4.3 m, where 0 m represents the tree row and 4.3 mrepresents the middle of the alley. Measurements were made at depths of 15,45, and 75 cm and expressed as moisture at 0–30, 30–60, and 60–90 cm,respectively. Neutron probe counts were converted to gravimetric soil moisturebased on a calibration performed using gravimetric soil samples.

Rooting pattern

Spatial variation in rooting pattern was investigated for trees on May 22, 1996,prior to planting maize. The biomass of fine tree roots (< 2 mm in diameter)was quantified at distances of 0, 1.1, 2.3, 3.5, and 4.3 m where 0 m repre-sents the tree row (0.1 m from the tree base) and 4.3 m represents the middleof the alley. Root samples were taken from soil cores (4.4 cm in diameter)

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Figure 1. Daily precipitation between May and September, 1996 at the Southeastern PurdueAgriculture Center, Butlerville, IN, USA.

collected to a depth of 90 cm using a hydraulic soil coring apparatus mountedon a tractor. In 1995, cores were collected on July 27 (65 days after plantingmaize) to determine the rooting pattern of maize. Following collection, rootswere separated from the soil using an automated washing system and thensorted into tree roots and maize roots (specifically for 1995) using visualcharacteristics. Decayed roots were excluded. Tree roots were sorted intofine (< 2 mm in diameter) and coarse (> 2 mm in diameter). However, onlyfine root biomass was used for further analysis. Roots were dried in an ovenfor 48 hours at 65 °C to determine dry weight.

Water uptake

Water uptake was quantified using Dynamax sap flow gauges (Dynamax Inc.,Houston, TX, USA) which measure sap flow using the stem heat balancetechnique (Sakuratani, 1981; Steinberg et al., 1989). In recent years, thismethod has successfully been used in estimating sap flow of larger stems(Steinberg et al., 1990; Cienciala and Lindroth, 1995).

Sap flow was measured simultaneously in the ‘barrier’ and ‘no-barrier’treatments (The ‘trench’ treatment was excluded from this measurement dueto a limited number of sap flow gauges). One plot was selected from each ofthe two treatments and in each treatment, maize sap flow was measured ontwo plants in the first and third rows. Measurement of sap flow for trees waslimited to one black walnut tree per treatment. Dynamax trunk gauges wereused to measure sap flow of these trees which were selected to have similardiameter, height, and leaf area (Table 1). Weekly water uptake was calcu-lated as the sum of daily total water uptake (from 6:00 am until 8:00 pm) inboth trees and maize.

Maize leaf area expansion

Since leaf elongation is among the plant processes most sensitive to moisturestress (Hsiao, 1973; Taiz and Zeigler, 1991), leaf area expansion was moni-tored throughout the growing season in 1996. For this purpose, 10 plants (fiveeach from two plots) were selected in the first and third rows in each treat-

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Table 1. Characteristics of black walnut trees used for sap flow measurements in the ‘barrier’and ‘no barrier’ treatments of a midwestern USA alley cropping trial.

Parameter Barrier No barrier

DBH (cm) 9.9 9.6 Height (m) 8.2 8.4 Leaf area (m2)a 37.1 33.9

a Leaf area was calculated from allometric equations developed using stem cross sectional areaat 1.37 m and total canopy leaf area (Jose and Gillespie, unpublished data).

ment. Leaf length (LL) and maximum width (WL) were measured for all leaveson each plant and leaf area (AL) of individual leaves was calculated using thefollowing formula (McKee, 1964):

AL = 0.73 (LL × WL)

Whole plant leaf area was calculated by summing leaf area of individualleaves. Data were fitted to a sigmoidal curve to model leaf area developmentwithin each treatment.

Data analysis

Soil moisture data were analyzed by soil layer using ANOVA following a splitblock design where distance was considered as a subunit of the main block(Little and Hills, 1978). Although data were collected for five different dis-tances from the tree base, they were grouped together as ‘tree row’ (distance0 and 1.1 m) and ‘maize alley’ (distances 2.3, 3.5, and 4.3 m) for analysis.Main effects and interactions were tested for significance using the appro-priate error terms. Tree fine root and maize root biomass data also wereanalyzed within a split block design. Since root data were not normally dis-tributed, data were log-transformed before ANOVA. If significant treatmenteffects were revealed at α = 0.05, then Tukey’s range test was used for meanseparation.

Water uptake data by corn were analyzed using a paired t-test (no statis-tical analysis was done on tree data because of limited sample size). Leaf areaexpansion was modeled using the following sigmoidal function:

AL = {(a – d) / (1 + (x/c)b)} + d

where ‘x’ is the time in days after planting, and a, b, c, and d are regressioncoefficients.

Results

Soil moisture

Soil moisture varied significantly as distance increased from the tree row tothe middle of the alley (Figures 2 and 3). Variation in soil moisture was moreprominent in the upper soil layer (0–30 cm) in both the black walnut (P =0.0352) and red oak (P = 0.0009) systems. For example, early in the growingseason, soil moisture was as high as 0.21 kg kg–1 (averaged for collection datesacross all distances and for all treatments) in the upper soil layer in the blackwalnut system. This decreased to 0.11 kg kg–1 (47.3% decrease) towards theend of the season. This decrease was 19.9% and 24.5%, respectively, at

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Figure 2. Spatial and temporal variation in gravimetric soil water content among differenttreatments in the black walnut alley cropping system in 1996 in Butlerville, IN, USA.

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Figure 3. Spatial and temporal variation in gravimetric soil water content among differenttreatments in the red oak alley cropping system in 1996 in Butlerville, IN, USA.

30–60 cm and 60–90 cm depths. In the red oak system, moisture decreased41.9% in the surface layer with similar decreases in deeper layers. As evidentfrom the moisture patterns in the upper horizon, two distinct dry periods wereobserved in 1996. Soil moisture decreased consistently from June 4 throughJuly 13. During the next dry period (July 25 through August 30), moisturedepletion was much greater. Again, these variations were prominent only inthe 0–30 cm layer of soil. In other layers, moisture depletion was gradualand consistent throughout the growing season.

The ‘trench’ treatment resulted in a higher moisture content within the‘maize alley’ for both systems (Figures 2 and 3, and Table 2). Although ‘maizealley’ moisture content was higher due to trenching, lower moisture contentwas observed within the ‘tree row’. In the red oak system, a greater increasewas observed for ‘maize alley’ moisture content within the 0–30 cm layer and60–90 cm layer compared to the black walnut system. The moisture increasewithin the 30–60 cm layer was similar to that for the black walnut system.The percent reductions in ‘tree row’ moisture content were also higher withinthe red oak system than in the black walnut system. Spatial variation in soilmoisture due to the ‘barrier’ treatment was similar in nature to that oftrenching.

Rooting pattern

Spatial distribution of fine root biomass for black walnut and red oak is shownin Figure 4. Tree fine root distribution was significantly affected by differenttreatments for both black walnut (P = 0.0297) and red oak (P = 0.0015). Fineroot biomass within the ‘no barrier’ treatment represented the natural systemwithout any mechanical hindrance to root growth. In both black walnut and

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Table 2. Mean gravimetric soil water content for different treatments within the ‘tree row’ and‘corn alley’ during the growing season in black walnut and red oak alley cropping systems.

Depth Treatment Gravimetric water kg kg–1

Black walnut Red oak

Tree row Corn alley Tree row Corn alley

0–30 cm No barrier 0.15 0.15 0.16 0.18 Trench 0.14 0.16 0.13 0.19Barrier 0.13 0.18 0.13 0.20

30–60 cm No barrier 0.22 0.23 0.23 0.23 Trench 0.19 0.24 0.20 0.26Barrier 0.21 0.24 0.19 0.26

60–90 cm No barrier 0.23 0.25 0.24 0.25 Trench 0.22 0.26 0.20 0.26Barrier 0.22 0.25 0.18 0.27

red oak, fine root biomass decreased as distance (P < 0.0001 for both species)and depth (P < 0.0001 for both species) increased (Figure 4). However, when‘trench’ or ‘barrier’ treatments were applied, fine root biomass increased ata distance of 1.1 m probably due to a proliferation of new growth from prunedroots. This increase was seen only within the 0–30 cm soil layer for blackwalnut. However, red oak fine root biomass increased even within the 30–60cm and 60–90 cm soil layers. For the ‘barrier’ treatment, non-maize fine root

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Figure 4. Variation in tree fine root biomass (< 2 mm in diameter) with distance from the treerow for black walnut and red oak alley cropping systems under different treatments; trench andbarrier treatments were applied 1.2 m from the tree row. Errors bars represent one standard errorof the mean.

biomass observed beyond the barrier was negligible. Live fine roots beyondthe barrier may be those of any weed species growing within the ‘maize alley’since no attempt was made to separate weed roots. Within the ‘trench’ treat-ment, tree fine roots were observed even beyond the trench line (one yearafter treatment). Although only trace quantities were detected at distances of3.5 and 4.3 m (which may be a mixture of tree roots and weed roots), a sub-stantially higher quantity was observed at 2.3 m, probably as a result of rootregrowth into the ‘maize alley’.

Rooting patterns for maize did not exhibit any significant differences fordifferent treatments (Figure 5). In general, root biomass was the highest forthe upper soil layer and decreased with depth. On average, 59.5% of the maizeroots were observed in the 0–30 cm layer, 27.8% in the 30–60 cm layer, and12.7% in the 60–90 cm layer.

Water uptake

Water uptake by both trees and maize was affected by the ‘barrier’ and ‘nobarrier’ treatments. On average, water uptake by black walnut trees per weekwas 206.9 kg in the ‘barrier’ treatment which varied from the ‘no barrier’treatment with an uptake of 238.3 kg of water per week. Although trees inboth treatments had similar water uptake during the early growing season(from June 9 through July 20), noticeable changes occurred later in thegrowing season (Figure 6). Tree water uptake for the ‘barrier’ treatment starteddecreasing during mid-season and continued to decrease until the end of themeasurement period in comparison to the ‘no barrier’ treatment. During this

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Figure 5. Corn root biomass by depth for different treatments in the black walnut and red oakalley cropping systems at 65 days after planting in Butlerville, IN, USA.

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Figure 6. Weekly water uptake for black walnut and corn (data are shown for row 1, i.e. therow closest to the tree row, and row 3, the row farthest from the tree row) in the ‘barrier’ and‘no barrier’ treatments in Butlerville, IN, USA.

period (from July 21 through September 6), average weekly water uptake bytrees was 152.2 kg for the ‘barrier’ treatment whereas it was 211.2 kg for the‘no barrier’ treatment. This pattern was further evident in the diurnal patternsof tree water uptake (Figure 7). During the early season, trees in both treat-ments were transpiring as much as 6–7 kg of water per hour at mid-day. Laterin the season, the trees growing in the ‘no barrier’ treatment could maintain

a peak flow rate of 6–7 kg of water per hour. However, peak flow ratedecreased to 2.9 kg per hour for the trees growing in the ‘barrier’ treatment.

Maize plants started showing significant treatment differences in wateruptake during the fifth week (Figure 8). In general, seasonal water uptake wassignificantly higher for plants in the ‘barrier’ treatment than for plants in the‘no barrier’ treatment. In the ‘barrier’ treatment, average weekly water uptakewas 4.75 kg plant–1 for the first row which was lower than the third row witha water uptake of 5.39 kg plant–1 (P = 0.01). The ‘no barrier’ plants had amuch lower uptake of water within the first row (3.02 kg plant–1) and thirdrow (4.05 kg plant–1), which differed significantly within the treatment (P =0.002) as well as from the ‘barrier’ treatment (P < 0.001). As evident fromthe diurnal pattern of water uptake (Figure 7), maize plants in the ‘barrier’and ‘no barrier’ treatments showed marked differences in water uptake asearly as July 8 (47 days after planting) that continued until the end of theseason.

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Figure 7. Water uptake for black walnut and corn (by row) during one day from July andSeptember, 1996 in the ‘barrier’ and ‘no barrier’ treatments in an alley cropping system inButlerville, IN, USA.

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Figure 8. Corn leaf area expansion for different treatments in the black walnut and red oakalley cropping systems in Butlerville, IN, USA.

Leaf area expansion

Leaf area expansion followed a sigmoidal pattern and was modelled as afunction of time as shown in Figure 8. This model accurately predicted leafarea at any stage of plant development within each treatment (Figure 9).Although pattern of leaf area expansion was similar between treatments earlyin the growing season, noticeable changes occurred within a month of planting.In both black walnut and red oak systems, maize plants within the ‘barrier’treatment had higher leaf area than the ‘trench’ and ‘no barrier’ treatments

(Figure 8). For example, average maize leaf area at 102 days after plantingwithin the ‘barrier’ treatment was 0.62 and 0.57 m2 plant–1 for the black walnutand red oak systems, respectively. These values were reduced to 0.51 (forthe black walnut system) and 0.48 m2 plant–1 (for the red oak system) withinthe ‘no barrier’ treatment.

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Figure 9. Relationship between measured and predicted leaf area of corn plants within blackwalnut and red oak alley cropping systems in Butlerville, IN, USA.

Discussion

Although not reported previously in temperate alley cropping systems, ourresults indicate severe competition for water between trees and maize, espe-cially following a late season drought typical for parts of the midwestern US.Temporal variation in soil moisture, at least in the top 0–30 cm layer, followedlocal precipitation patterns (Figures 1, 2, and 3) showing two distinct dryperiods during the 1996 growing season. Moisture depletion in deeper layers,however, was not correlated with rainfall events. Moisture declined gradu-ally in both the 30–60 cm and 60–90 cm layers indicating that the small,late-season rainfall events were inadequate to percolate down to these deeperlayers given uptake by plants. Similar results have been reported from amaize-leucaena (Leucaena leucocephala (Lam.) de Wit.) alley cropping trialin Kenya where soil moisture decreased gradually during the growing seasondue to rainfall that was inadequate to saturate the deeper horizons(Govindarajan et al., 1996).

Cutting tree roots within the ‘barrier’ or ‘trench’ treatments resulted inhigher soil moisture content within the ‘maize alleys’. In the ‘no barrier’ treat-ment, tree roots had access to the maize alley where they competed for waterand nutrients with maize roots. Since maize roots were uniformly distributedwithin the alley without significant differences between treatments, it isreasonable to conclude that additional moisture depletion within the ‘nobarrier’ treatment was due to competition from tree roots. In a similar trialusing belowground root barriers in a temperate alley cropping system in Ohio,USA, Ssekabembe et al. (1994) showed that soil moisture was higher withinthe alley in the ‘barrier’ treatment than the ‘no barrier’ treatment where blacklocust (Robinia pseudoacacia L.) hedgerows depleted soil moisture.

Tree root sampling (one year after applying treatments) revealed a highdensity of tree fine roots within the ‘maize alley’ in the ‘no barrier’ treatment.As expected, tree roots (live or intact) were absent within the ‘maize alley’in the ‘barrier’ treatment. Although not as high as in the ‘no barrier’ treat-ment, tree roots were present within the ‘maize alley’ in the ‘trench’ treat-ment, especially at 2.3 m from the tree row, indicating root regrowth into thealley. When tree roots were pruned in either the ‘trench’ or ‘barrier’ treat-ments, a higher root biomass was observed at 1.1 m than at 0 m. This impliesan increased root proliferation along the disturbed soil profile where rootswere cut. Soil moisture depletion at this distance was also higher, especiallyfor black walnut (within 30–60 cm and 60–90 cm layers) probably as a resultof increased water absorption by the higher tree root biomass. Thus, activi-ties that disturb root systems and change rooting profiles will change spatialwater uptake patterns accordingly.

Soil moisture, in general, was depleted more quickly within the ‘tree row’area than in the ‘maize alley’ in the ‘trench’ and ‘barrier’ treatments. Treeroots were absorbing moisture from a limited volume of soil in the ‘barrier’treatment. In the ‘trench’ treatment most of the functional roots were still

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present within the ‘tree row’. When tree roots were not restricted in theirgrowth (i.e. the ‘no barrier’ treatment), tree water uptake was much higher incomparison to the trees with restricted root growth (i.e. the ‘barrier’ treat-ment). Late in the growing season (July 21 through September 6), the treeswithin the ‘no barrier’ treatment were transpiring 38.8% more water per weekthan the trees within the ‘barrier’ treatment. Thus, in addition to soil moisturedepletion, this is direct evidence that trees were successfully competing withmaize for the available soil water.

It is generally found that fine root abundance of trees is much lower thanannual crop species (Atkinson, 1980; Gillespie, 1989). Although studiescomparing tree and crop fine root distribution patterns under similar condi-tions are rare in the temperate region, a few studies have been reported fromthe tropics. While examining the fine root distribution patterns of five treespecies (two-year-old) and maize (mature), Jonsson et al. (1988) observed thattrees had roughly twice the biomass as that of maize. Having a higher fineroot biomass may favor trees when they are mixed with crops as in alleycropping systems. This was the reason for reduced crop yield in a leucaena-millet (Pennisetum glaucum (L.) R. Br.) alley cropping system in the semi-arid tropics (Ong et al., 1991). Using belowground root barriers, these authorsconcluded that competition for water by tree roots was responsible for thenegative interactions and thus for crop failure. The higher tree water uptake,combined with the lower ‘maize alley’ soil moisture in the ‘no barrier’ treat-ment in our study, is an indication of competition for water by trees. This isthe likely reason for decreased water uptake in maize when grown withoutisolation from tree roots.

The ultimate result of soil moisture depletion under maize is an impact onplant size. Maize plants in the ‘barrier’ treatment had on average 21.1% higherleaf area (in the black walnut system at 102 days after planting) than plantsin the ‘no barrier’ treatment (Figure 8). Since leaf elongation is the firstprocess inhibited by water stress (Hsiao, 1973; Taiz and Zeigler, 1991), thissuggests that plants in the ‘no barrier’ treatment were experiencing waterstress. And, in fact, maize plants were absorbing much less water wheninteracting with trees belowground (Figures 7 and 8). Water stress has beenfound to cause large reductions in internode length, plant height (NeSmith andRitchie, 1992a; Robertson, 1994), and leaf area (NeSmith and Ritchie, 1992b;Gavloski et al., 1992) in maize. In our study, soil moisture was the majorfactor that differed between treatments within the ‘maize alley’ (Figures 3 and4). Hence, the differential leaf area development was a result of the varia-tions in soil moisture between treatments which resulted from differential treewater use.

As reported previously, grain yield was significantly lower in the ‘nobarrier’ treatment than in the ‘barrier’ and ‘trench’ treatments (up to 39.5%and 33.4% reductions in the black walnut system and red oak system, respec-tively) which further indicates the importance of competition for water(Gillespie et al., in press). Similar results have been reported from the semi-

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arid tropics where a 74.2% reduction in sorghum (Sorghum bicolor Moench.)grain yield resulted from competition for moisture from four-year-old leucaenatrees (Singh et al., 1989). In our study, plants were shorter in the ‘no barrier’treatment with smaller leaves. Reduction in leaf area will result in an overalldecrease in photosynthate production even though photosynthetic rates appearlittle affected (Gillespie et al., in press). Further, moisture stress can lead topreferential carbon allocation to plant roots rather than shoots as an adapta-tion to low water availability (Sharp and Davies, 1979; Klepper, 1991). Asthe ‘no barrier’ treatment had similar root biomass but significantly lower leafarea (and thus shoot biomass) in comparison to the ‘barrier’ and ‘trench’ treat-ments, a lower shoot:root ratio can be expected than within the other treat-ments. For example, a shoot (grain+stover):root ratio of 6.1 was found forthe ‘no barrier’ maize plants whereas it was 9.2 for the ‘barrier’ plants. Similarresults were observed by Govindarajan et al. (1996) in a leucaena-maize alleycropping trial in Kenya. In our study, the effect of competition for water bytrees was further exacerbated by the late season drought experienced mainlyby the ‘no barrier’ plants during the critical periods of flowering and grainfilling. Water deficits during or shortly after anthesis are considered to beamong the most devastating to grain yield in maize (Begg and Turner, 1976).In a field study in Michigan, yield reductions in excess of 90% were observedwhen water deficit spanned the interval from just prior to tassel emergenceto the beginning of grain fill (NeSmith and Ritchie, 1992a). The variations ingrain yield by row in our system (Gillespie et al., in press) indicate that, withinthe ‘no barrier’ treatment, competition for moisture was the highest at the firstrow of maize (close to the tree row) and lowest at the third row (farthestfrom the tree row).

Since preventing or reducing root interaction between trees and maizethrough root zone treatments resulted in a similar yield (normal monocul-ture) in all rows, we conclude that shading is not a major factor at this stageof system development. Further, competition for nutrients can be expected tobe minimal since all treatments were fertilized annually. In allied research,we have ruled out the possibility of allelopathic interactions between blackwalnut and maize (Jose and Gillespie, 1998). Soil concentrations of juglone,the alleochemical, were low enough to have no impact on maize growth. Thus,it appears that water remains as the key factor in defining the productivity andsustainability of our alley cropping systems in the midwestern US. Further,with annual pruning and periodic thinning to create high-value timber, lightmay never become a significant limiting factor in this system. This might notbe the case where trees were managed for other objectives, such as nut pro-duction. In practice, to manage this system to reduce competition for waterand maintain crop yields at high levels, farmers have a number of traditionalpractices that could be utilized, including tillage with different implementsor even the injection of fertilizer. The critical depth will depend on speciesand soil as shown by our root distributions by treatment.

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Acknowledgements

This research was funded by the North Central Sustainable AgricultureResearch and Education program of the United States Department ofAgriculture under cooperative agreement no. 92-COOP-1-7266. The authorsexpress their sincere thanks to J. Kissel, E. Arth, T. Stall, and S. Bell fortheir help at various stages of this project. Scientific Contribution No. 15713of the Purdue University Agricultural Research Program.

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