Competition in tree row agroforestry systems. 3. Soil water distribution and dynamics

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Plant and Soil 264: 129139, 2004. 2004 Kluwer Academic Publishers. Printed in the Netherlands. 129Competition in tree row agroforestry systems. 3. Soil water distributionand dynamicsS.J. Livesley1,4, P.J. Gregory2 & R.J. Buresh31Forest Science Centre (DSE), Water Street, Creswick, Victoria 3363, Australia. 2Soil Science Department, TheUniversity of Reading, Whiteknights, P.O. Box 233, Reading, RG6 6DW, U.K. 3IRRI, DAPO Box 777, MetroManila, The Philippines. 4Corresponding authorReceived 24 September 1999. Accepted in revised form 10 December 2003Key words: agroforestry, Grevillea robusta, maize, Senna spectabilis, soil water content, water balanceAbstractThe purpose of this study was to test the hypothesis that soil water content would vary spatially with distance froma tree row and that the effect would differ according to tree species. A field study was conducted on a kaoliniticOxisol in the sub-humid highlands of western Kenya to compare soil water distribution and dynamics in a maizemonoculture with that under maize (Zea mays L.) intercropped with a 3-year-old tree row of Grevillea robustaA. Cunn. Ex R. Br. (grevillea) and hedgerow of Senna spectabilis DC. (senna). Soil water content was measuredat weekly intervals during one cropping season using a neutron probe. Measurements were made from 20 cm to adepth of 225 cm at distances of 75, 150, 300 and 525 cm from the tree rows. The amount of water stored was greaterunder the sole maize crop than the agroforestry systems, especially the grevillea-maize system. Stored soil waterin the grevillea-maize system increased with increasing distance from the tree row but in the senna-maize system,it decreased between 75 and 300 cm from the hedgerow. Soil water content increased least and more slowly earlyin the season in the grevillea-maize system, and drying was also evident as the frequency of rain declined. Soilwater content at the end of the cropping season was similar to that at the start of the season in the grevillea-maizesystem, but about 50 and 80 mm greater in the senna-maize and sole maize systems, respectively. The seasonalwater balance showed there was 140 mm of drainage from the sole maize system. A similar amount was lost fromthe agroforestry systems (about 160 mm in the grevillea-maize system and 145 mm in the senna-maize system)through drainage or tree uptake. The possible benefits of reduced soil evaporation and crop transpiration close toa tree row were not evident in the grevillea-maize system, but appeared to greatly compensate for water uptakelosses in the senna-maize system. Grevillea, managed as a tree row, reduced stored soil water to a greater extentthan senna, managed as a hedgerow.IntroductionAnnual crops often exploit only a small fraction ofthe available rainfall and stored soil water reserves.The integration of perennial trees within a farmingsystem can increase the amount of water transpiredand increase overall biomass productivity (Ong et al.,1992; Wallace et al., 1995). This may be achieveddirectly when trees exploit the rainfall and stored wa-ter reserves outside the cropping seasons and/or whena greater proportion of the rainfall within a croppingFAX No: +61-3-5321-4166.E-mail: stephen.livesley@dse.vic.gov.auseason is transpired rather than evaporated, run-off ordrained to below the rooting zone (Ong et al., 1992). Itmay also be achieved indirectly when modification ofmicroclimatic conditions by trees increases the tran-spiration efficiency of the crop, the unit production ofbiomass per unit water transpired (Brenner, 1996).In semi-arid and dry sub-humid areas, evapora-tion from the soil surface can account for 3060%of the annual rainfall (Cooper et al., 1983; Wallace,1991). Trees can reduce these losses and conserve soilwater by providing shade, reducing wind speed andincreasing infiltration with mulch layers and improvedsoil structure (Torquebiau and Kwesiga, 1996; Young,1301997). The loss of water through run-off is importantin areas with sloping land and less permeable soil.The presence of contour hedgerows can increase therate and amount of water infiltration thereby reducingsuch losses (Kiepe, 1995; Agus et al., 1997). Treesand hedgerows, though, may intercept rainfall withtheir foliage so that it fails to reach the soil. Wallaceet al., (1995) found that interception loss was about14% of the rainfall for 34-year-old Grevillea robustatrees grown at Machakos, Kenya.The loss of water through drainage has rarely beenquantified in tropical agroecosystems, but may rep-resent as much as one third of the water balance insemi-arid areas (Ong et al., 1991). When drainagelosses occur there are also possible leaching lossesof nutrients. The root system of a tree can theoretic-ally intercept and take up mineral nutrients and waterthat percolates below the crop rooting zone, therebyreducing such losses. The ability of a trees root sys-tem to take up resources from depth depends on thedistribution of the tree and crop roots, soil hydraulicproperties and rainfall regime (van Noordwijk et al.,1996). The loss of water through drainage may bethe hydrological parameter most easily modified bythe introduction of trees (Wallace et al., 1995) so theloss of nutrients through leaching may also be reduced(Shepherd et al., 1996).The overall productivity of a farming system willonly increase with the introduction of trees if thenegative effect of competition with the crop is com-pensated for by the beneficial effects of trees on theirshared environment and the additional harvestableproducts the trees provide. In agroforestry systemsof semi-arid regions, competition for water betweentrees and crops is the principal determinant of cropproductivity (Singh et al., 1989; Breman and Kessler,1995). In areas with > 1000 mm a1 of rainfall,competition for water is rarely reported or even in-vestigated (Young, 1997). Nonetheless, measuring thewater balance of agroforestry systems in sub-humidand humid areas is important because many processesinvolved in the cycling of nutrients (organic matter de-composition, denitrification and leaching) are greatlyinfluenced by soil hydrological conditions. Further-more, the restricted availability of soil water affectsmany plant functions, including transpiration, photo-synthesis and the movement of nutrient ions and theiruptake by root systems (Schulze, 1991).The objective of this study was to test the hy-pothesis that soil water content would vary spatiallywith distance from a tree row, and that the effectwould differ according to tree species and manage-ment. The aim of the work reported in this paper wasto compare the temporal and spatial distributions ofsoil water below tree row agroforestry systems and amaize monoculture. Soil water content was measuredregularly with a neutron probe and a water balanceestimated. This study complimented other investiga-tions into fine root distribution and dynamics (Livesleyet al., 2000) and soil nitrogen uptake, loss and avail-ability (Livesley et al., 2002) in these agroforestrysystems.Materials and methodsThe research was conducted in western Kenya(3434 E, 006 N, altitude 1330 m) on a fine kaol-initic, isohyperthemic Kandiudalfic Eutrudox. In theupper 15 cm, 73% was clay and 8% sand. Furtherdetails of the site, design and management are given inLivesley et al. (2000) and an extensive physical, chem-ical and hydraulic characterisation of the soil at thissite can be found in Livesley (1999). The experimentwas established in April 1993 with nursery-grownseedlings planted as a single row of Grevillea ro-busta (grevillea) with a 1.0 m spacing between treesand a single row of Senna spectabilis (senna) witha 0.5 cm spacing between trees. Maize (hybrid 512)was cropped biannually within 6 m on both sides ofthe trees during the long rains (March to July) andthe short rains (September to January). Grevillea wasmanaged for timber production with lower branchesremoved annually to reduce shading while senna wasmanaged as a hedgerow that was pruned to leave onlya 20 cm high stump before each cropping season. Allprunings and leaf fall were collected and removedfrom the site. Maize (hybrid 512) was cropped bian-nually within 6 m on both sides of the tree rows duringthe long rains (March to July) and the short rains(September to January). A sole maize plot (6 6 m)was established 12 m from the grevillea-maize systemand managed in the same way as the maize in theagroforestry systems.This paper reports a study during long rains grow-ing season of 1996, three years after the tree rowswere established. Maize was sown on 11 March andharvested on 8 July, 117 days after planting (DAP).Rainfall was measured every 30 min at an on-site met-eorological station, and daily using ten bucket gaugesrandomly distributed throughout the field site. Poten-tial evapotranspiration from a sole maize crop (Eo)131was calculated using the PenmanMonteith equation(Monteith and Unsworth, 1990) with stomatal resist-ance set at zero and allowance for the effect of theheight of the crop on wind speed as it increased from5 cm at 7 DAP to 200 cm at 100 DAP.Soil water contentSoil water content was measured using a neutronprobe. Aluminium access tubes were installed in linesperpendicular to the grevillea tree row and sennahedgerow. Sixteen access tubes were installed in eachagroforestry system, in four lines of four tubes, withtwo lines on either side of the tree row. Each linehad an access tube at 75, 150, 300 and 525 cmfrom a tree row. Four tubes were also installed in thesole maize crop. Measurements were made weekly,between 18 March (7 DAP) and 5 July (117 DAP) at20, 30, 40, 50, 75, 105, 135, 165, 195 and 225 cmdepths. Measurements were taken during the morningand required about 3 hours to complete. On each mea-surement occasion, a mean water count was made in atank of water to reduce errors due to instrument drift.Neutron probe measurements were calibrated byconcurrently measuring the gravimetric soil watercontent on three occasions in March twice in thegrevillea-maize system and once in the senna-maizesystem. Mean neutron probe counts were establishedat each depth and distance from the tree row and undis-turbed soil samples collected from close to each tubeusing steel rings (5 5 cm, 98.2 cm3). A trench wasdug close to a line of four access tubes and samplestaken from 12 depths (15, 30, 40, 50, 75, 105, 135,165, 195, 225, 255 and 300 cm) at each of the fourdistances from a tree row. The rings were weighed,oven dried at 105 C for 48 h and then re-weighed todetermine the volumetric water content.Linear regressions between the volumetric watercontents and the ratio of neutron probe count to waterwere produced for each depth. The significance of thedifference between the slopes and intercepts of theselinear regressions was tested using an ANOVA (Meadet al., 1993) and depths with similar regressions (20,30, 40, 50, 75, 105, 135 and 225 cm) were combined;the 165 and 195 cm depths were different from theremainder but similar and were therefore combinedseparately. Values of r2 varied from 0.78 to 0.17 forindividual depths but the combined regressions (Fig-ure 1) had r2 values of 0.52 (n = 88) and 0.22 (165and 195 cm, n = 25). Livesley (1999) provides furtherdetails of the calibration process.Profile soil water storage was determined as thesum of the product of volumetric water content andthickness of each layer. The amount of stored water(mm) was calculated to a depth of 240 cm at eachdistance from a tree row, and in the sole maize system,using the measurement at 20 cm depth to represent the025 cm layer, and each subsequent measurement torepresent the appropriate layer (i.e., 30 cm representedthe 2535 cm layer and 195 cm the 170210 cm layeretc.).Water balanceWater use by the trees and crops of an agroforestry sys-tem can be estimated using the water balance equation(Wallace, 1996):P = R + D + I + E + Tc + Tt + S, (1)Where P is rainfall, R is runoff, D is drainage, I isinterception by the canopy, E is evaporation from thesoil surface, Tc and Tt are transpiration by the crop andtree respectively, and S is the change in the amountof water stored in the soil. R and I were not measuredin this study but no run-off was observed and canopyof the senna hedgerow was small following pruningwhen most rainfall occurred. R and I were assumed tobe zero.An estimate of the water available for drainage oruse by the trees was made by assuming that evapo-transpiration from the sole maize crop was equal topotential evapotranspitation (Eo) and, in turn equiva-lent to the water used by the maize in the agroforestrysystems (i.e., D + Tt = P E S).Data analysis and presentationProfiles of soil water content at 75, 150, 300 and525 cm from the grevillea and senna tree rows wereselected at six times during the maize cropping seasonto prepare contour-fill figures of the distribution of soilwater content with time. Each contour-fill figure con-tains data from 40 points (ten depths four distances)on a vertical plane 20225 cm from the soil surfaceand 75525 cm from the tree row.The difference between amounts of stored water inthe two agroforestry systems and that in the sole maizewas tested for statistical significance using a homosce-dastic two-tailed t-test. A mean value of stored waterin each agroforestry system was determined by appro-priately weighting each of the four values of watercontent obtained at different distances from the treerows. The distance between 0 and 6 m from a tree132Figure 1. Neutron probe calibration coefficients and 95% confidence limits. Soil count to water count ratios plotted against measured volumetricsoil water contents at depths 20, 30, 40, 50, 75, 105, 135 and 225 cm (A) and at depths 165 and 195 cm (B).row was divided into sixteen sections each of 37.5 cm.Each distance at which soil water content was mea-sured was then weighted according to the number of37.5 cm sections it represented. So, 75 and 150 cmmeasurements were each weighted by 3, and 300 and525 cm measurements were each weighted by 5.The neutron probe calibration regressions were notstrong, therefore it was possible that the error in thecalibration coefficient was greater than observed dif-ferences between treatments and locations. The t-testanalyses were repeated, but stored water in the gre-villea system was calculated using the upper 95%confidence limit as the calibration coefficient (Meadet al., 1996) and stored water in the senna and solemaize systems was calculated using the lower 95%confidence limits. Likewise, t-tests analyses were re-peated between the senna system (upper 95% limits)and sole maize (lower 95% limits).ResultsRainfall and potential evapotranspirationThe distribution of rainfall through the season isshown in Figure 2. The monthly rainfall was 85 mmin February, 245 mm in March, 197 mm in April,275 mm in May, 51 mm in June, 135 mm in July and105 mm in August. A 25 mm rainfall event on 3 Marchprompted maize planting eight days later. During thefirst 60 days of maize growth there were regular andintense rainfall events, but after 10 May (60 DAP) the133Figure 2. The daily rainfall (mm) and potential evaporation (mm) from a sole maize crop during the first growing season in 1996 in westernKenya.Figure 3. The soil water profile (with mean standard errors) beneatha sole maize crop on 5 occasions after maize planting in westernKenya.intensity of rain events decreased, and from 90 DAPonwards rainfall was less frequent.Potential evapotranspiration from the sole maizecrop (Eo) was generally between 4 and 7 mm d1throughout the cropping season. There were only threedays in the second half of the growing season when Eowas < 4 mm d1.Soil water contentProfiles of soil water content for the sole maize cropare shown in Figure 3. Typically, values were leastat 150 cm depth where the texture of the profile wasdifferent (less dense with a micro-aggregate structure)and greatest between 5090 cm and 190240 cm.Maximum values were recorded at most depths at67 DAP and water contents were greater at maturity(116 DAP) than shortly after planting (7 DAP).In the agroforestry systems, soil water content atthe start of the cropping season generally increasedwith depth between 20 and 50 cm, decreased withdepth between 50 and 135165 cm, and then increasedslightly to a depth of 225 cm (Figure 4). At 7 DAP,water contents at 75 and 150 cm from the grevillea treerow were typically about 23% less than those in thesenna-maize system at every depth. At 300 and 525 cmfrom the grevillea tree row, the water content profileswere similar to those in the senna-maize system exceptin the upper 50 cm where they were also 23% less.Values of soil water content under the sole maize cropwere similar to those in the senna-maize system.Between 7 and 18 DAP, there was a large increasein water content of the upper 100150 cm of every pro-file (Figure 4). This wetting was most pronounced at150 and 300 cm from the grevillea tree row. At 525 cmfrom the senna hedgerow, profile wetting reached adepth of 200 cm. Between 18 and 39 DAP, profile wet-ting extended to 225 cm depth at each location. Exceptat 300 cm from the senna hedgerow and within 300 cmof the grevillea tree row where profile wetting was lesspronounced or incomplete. The values of soil watercontent measured at 39 DAP were generally the largestmeasured during the cropping season, and thereafterthe profiles generally dried.Between 39 and 60 DAP, water contents decreasedslightly or remained constant. The exceptions were at300 cm from the senna hedgerow where they increasedbelow 150 cm, and at 75 cm from the grevillea treerow where they increased between 100 and 200 cm134Figure 4. Mean volumetric water content with distance (75 to 525 cm) and depth (7.5 to 225 cm) from the grevillea tree row, and sennahedgerow intercropped with maize on six occasions after maize planting in western Kenya.depths (Figure 4). Between 60 and 116 DAP, soilwater content decreased throughout the profile. Inthe grevillea-maize system, the values measured at116 DAP were similar to those measured at the startof the maize cropping season (7 DAP). In contrast, thevalues measured in the senna-maize and sole maizesystems did not return to values measured 7 DAP, butremained 12% greater.In summary, the measured soil water contents inthe sole maize system were greater than those in theagroforestry systems, especially when compared withthe grevillea-maize system. In the grevillea-maizesystem, soil water contents generally increased withincreasing distance from the tree row, whereas soilwater content in the senna-maize system varied littlewith distance from the hedgerow.Stored water dynamicsAt the start of the maize cropping season, there wasbetween 881 and 917 mm of stored water in thesoil profile (0 to 240 cm) of the grevillea system,between 921 and 948 mm in the senna system andabout 964 mm under the sole maize crop. Through-out the cropping season, stored water was least at75 cm from the grevillea tree row and increased withincreasing distance from the tree row (Figure 5). Incontrast, stored water was generally highest at 75 cmfrom the senna hedgerow, and then decreased between75 and 300 cm. The pattern at 525 cm from thesenna hedgerow was more complex, stored water be-ing initially comparable at 525 and 75 cm from thesenna hedgerow, but after early season rainfall (after39 DAP) stored water was slightly greater at 75 cmfrom the senna hedgerow.135Figure 5. Changes in stored water to 240 cm depth under sole maize, grevillea-maize and senna-maize systems in western Kenya. The 75, 150,300 and 525 cm refer to perpendicular distances from a tree row. S.E. refers to standard error of the mean.Stored water in the grevillea-maize system in-creased between 7 and 39 DAP, to reach maxima of919 mm at 75 cm from the tree row and 986 mmat 525 cm, an increase in soil water storage of 38and 69 mm, respectively (Figure 5). After 39 DAP,stored water in the grevillea-maize system generallydecreased so that at maize harvest stored water inthe profile was comparable with that measured at thestart of the cropping season (5 mm less at 75 cm,5, 4 and 12 mm more at 150, 300 and 525 cm, re-spectively; Table 1). Stored water was depleted mostrapidly closest to the grevillea tree row. The increaseof 38 mm of stored water at 75 cm from the grevilleatree row measured at 39 DAP had disappeared 35 dayslater.Stored water in the senna-maize system increasedmost rapidly between 7 and 39 DAP, after which itremained relatively constant until 60 DAP (Figure 5).Stored water in the senna-maize system reached amaximum of 1008 mm at 75 and 525 cm from thehedgerow, an increase in soil water storage of 60 and62 mm, respectively. After 81 DAP, stored water inthe senna-maize system generally decreased, but incontrast to the grevillea-maize system, the amount ofwater stored at the end of the season was substan-tially greater at all distances than that measured 7 DAP(25 mm at 75 cm, 22 mm at 150 and 300 cm, and 9 mmat 525 cm from the hedgerow; Table 1).Stored water in the sole maize system generallyincreased in the first half of the growing season reach-ing a maximum (1030 mm) at 67 DAP, and decreasedin the second half. By the end of the growing sea-son, the profile contained 24 mm more water thanthat measured at the start (Table 1.). The amount ofstored water in the sole maize system was signifi-cantly greater (P 0.05) than that at 75 cm fromthe grevillea tree row, and occasionally so at 150 cmfrom the tree row. Although stored water in the solemaize system was consistently greater than that in thesenna-maize system at all measured distances from thehedgerow, the uncertainty in the neutron probe calibra-tion coefficient meant that the differences could not beinterpreted as significant. Stored water in the grevillea-maize system was consistently smaller at all distances136Table1.Changesinsoilwaterstorage(mm)beneathgrevillea-maize,senna-maizeandasolemaizecropsinwesternKenya.Thelocations(75,150,300and525cm)aredistancesfromatreerowDAP71111181825253232393946465353606067677474818188889696102102109109116TotalLocation(cm)Grevillea7551969271311417207477745Grevillea150333144209812275613131175Grevillea300132763261005126393201214Grevillea525131982261567619699912112Senna75815145181112422111713914725Senna150126188211195169114412201022Senna30051324122435222111831316622Senna52593360141301127291314069Solemaize109302111766275121415161425Table2.Wateravailablefordrainageorplantuptake(mm)beneathgrevillea-maize,senna-maizeandasolemaizecropsinwesternKenya.Thelocations(75,150,300and525cm)aredistancesfromatreerowDAP71111181825253232393946465353606067677474818188889696102102109109116TotalRainfall(mm)4459123327118100191104421621812114757E o(mm)21344741404440454036343230303545592Location(cm)Grevillea75296701413594088282026522635170Grevillea1502086251117522573351936016238160Grevillea300101716516602171341621109230161Grevillea5251066811511541976271921320130153Senna75151163141315482369291213120037140Senna15011195817915512187181416817741143Senna3001812523723552473302422916337143Senna52514770816136025713515210141336156Solemaize131646103037673265361818214245140137from the tree row than stored water in the senna-maizesystem, but again the differences were only significant(P 0.05) at 75 cm from the tree row, for 14 of themeasured 16 weeks.Soil water balanceChanges in soil water storage varied with distancefrom the tree row although the general pattern wasof increasing storage early in the season, alternateperiods of accretion and depletion in mid-season (3988 DAP), followed by depletion of stored water in lateseason when Eo exceeded rainfall (Table 1). Rainfallwas 757 mm and the potential evaporation (Eo) was592 mm during the cropping season (Table 2). If it isassumed that evapotranspiration from the sole maizecrop occurred at the potential rate (Eo) then there was141 mm of drainage during the season. In the agro-forestry systems, the estimated drainage or uptake bythe trees was between 140 and 170 mm, depending ondistance from the tree row and the tree species.DiscussionSoil water distribution and dynamicsStored water content increased with increasing dis-tance from the grevillea tree row. Similar observa-tions have been made with tree rows in semi-aridareas, such as with Eucalyptus tereticornis (Malikand Sharma, 1990; Onyewoyu and Stigter, 1995) andBauhania rufescens (Mayus et al., 1994). Similarly,Okorio (2000) working with five tree species (Alnusacuminata, Casuarina equestifolia, Grevillia robusta,Maesopsis eminii and Markhamia lutea) on a clayloam soil in sub-humid (1250 mm rain a1) CentralUganda, generally found smaller water contents closer(1 m) to tree rows than further away (13 m) althoughthere were no significant differences in soil watercontent and patterns of soil water depletion betweenspecies.The smaller amount of stored water under thegrevillea tree canopy was probably a result of theinterception loss of rainfall by that canopy and thepreferential uptake of water from regions close to thetree stem. There was an increase of only 38 mm instored soil water at 75 cm from the grevillea tree rowas compared with > 65 mm at every other distancefrom the tree row. An almost closed canopy of 4-year-old grevillea trees reduced the annual input of rainfall(782 mm) to the soil surface by 1428% in Machakos,Kenya (Wallace et al., 1995). Trees can substantiallyreduce the amount of rain reaching the soil surfaceduring light rainfall events, but the proportion of rainlost decreases as the intensity of a storm increases(Young, 1997).The amount of stored soil water at 75 cm fromthe senna hedgerow was generally greater than thatat 300 cm (Figure 5). Similar patterns of storagewere found in hedgerow intercropping with Leucaenaleucocephala (Lal, 1989; Govindarajan et al., 1996;Chirwa et al., 1994), Flemingia macrophylla (Chirwaet al., 1994), Gliricidia sepium (Lal, 1989; Mazzarinoet al., 1993), Erythrina poeppiginia (Mazzarino et al.,1993) and Senna siamea (Kiepe, 1995). These studiesspan several different rainfall regimes and soil types,but the general explanation of this pattern was thathigher water contents near a hedgerow were caused bya combination of microclimatic modification, reducedevaporative loss (canopy shade and mulch barrier) andimproved infiltration.In this study, all senna prunings, leaf fall and stoverwere removed; consequently the enhanced soil waternear the senna hedgerow could not have been a mulcheffect. The larger water content next to the sennahedgerow was probably related to reduced evapora-tion due to canopy shade and reduced crop transpir-ation because of reduced wind speed and enhancedhumidity conditions. Senna may have taken up wa-ter from soil profiles at all four distances from thehedgerow, but the beneficial effects of the hedgerowwith regards to evaporation and crop transpiration onlycompensated for interception and uptake losses nearthe senna canopy (Huxley et al., 1994; Wallace et al.,1995). Lal (1989) suggested that in his study of Leu-caena and Gliricidia hedgerows, topsoil water con-tents were greater under and near hedgerows becausethe trees preferentially abstracted water from depth.However, this explanation is challenged by the morecommon observation that when soil water and nutri-ents are distributed uniformly trees will preferentiallyabstract water from the topsoil (ICRAF, 1994).The profile (0240 cm) at 75 cm from the grevilleatree row wetted up more slowly than at greater dis-tances from the tree row and wetting only reached adepth of 200 cm. This was attributed to preferentialuptake of water from this region and the reduced in-put of rainfall through canopy interception. However,at the start of the rainy season the profile water con-tent near to the grevillea tree row was smaller thanthat beyond 300 cm, so a greater amount of rainfallwould have been required to wet the profile to the138same water content. Huxley et al. (1994) observedthat below hedgerows of Grevillea robusta grown withmaize, profiles wet up more slowly than further awayand, conversely, that drying was always observed firstbelow the hedgerow. Similarly, Wallace et al. (1995),working in semi-arid Kenya, observed that profile wet-ting after a 60 mm rainfall event was less pronouncedbelow grevillea grown with maize than below sole gre-villea or sole maize and that drying was also evidentearlier.The soil water balance and the uptake of water bytreesIt is hypothesized that in agroforestry systems treeswill take up a portion of the water that would otherwisedrain or be taken up by the crop, and that this portionwill probably increase with increasing proximity to atree row. However, juxtaposed to this is the possibilitythat drainage might be increased close to tree rowsbecause of preferential drainage along perennial rootchannels (Van Noordwijk et al., 1991).At 525 cm from a tree row the uptake of water bythe trees was probably small, and the drainage losseswere probably similar to those under sole maize. The-oretically, it is possible that grevillea required andabstracted twice as much water as senna, because itproduced twice as much dry weight biomass ha1 assenna (Livesley et al., 2002), assuming grevillea andsenna had a similar water use ratio. As the water useratio of tropical tree species has rarely been measured,it is difficult to gauge species variation (Monteith,1986; Ong et al., 1992).Despite its fast growth rate, grevillea has not beenregarded as a resource competitive tree when grownwith crops. This assumption has been based on ob-servations of excavated root systems, which revealedthat grevillea had few lateral roots and a tendency to-wards deep rooting (Mwihomeke, 1992; Lott, 1997).The sap flux through the proximal roots of a grevilleatree was measured in a dry season at Machakos, Kenyaand vertically orientated roots took up almost all thewater from soil below 100 cm. However, within 24 hof the first rainfall event, lateral roots in the topsoilprovided about 80% of the water taken up (ICRAF,1996; Howard et al., 1997). Similarly, in this study, themore rapid topsoil drying in the grevillea agroforestrysystem suggests preferential abstraction of water fromsuperficial layers during the rainy season, whilst thesmaller soil water content below 100 cm close to thegrevillea tree row at the start of the cropping sea-son suggests uptake of water from depth during thedry season. Grevillea may then perform an impor-tant nutrient conservation function by reducing deepdrainage.In contrast with grevillea, senna has been regardedas a fast growing and competitive tree species (ICRAF,1996). The senna hedgerow in this study did not ex-ploit as much available stored water as grevillea duringthe previous 3 years growth, but did exploit more thanthe sole maize crop during the season studied. A soilwater balance study at Machakos, Kenya revealed thatsoil water contents under a Senna spectablilis tree rowgrown with maize were similar to those below a solemaize crop during the rainy season, but in the dry sea-son soil water contents near senna were significantlysmaller (ICRAF, 1996). The pruning of hedgerows isclearly likely to have an effect both on the amount andseasonal pattern of water use by trees and on the com-petitive interaction with the crop; these effects couldnot be studied in the this study.In this study, the grevillea-maize system preven-ted the soil profile from wetting up to the same extentas soil beneath a senna-maize system or a sole maizecrop, and depleted stored water to a greater extent. Thesoil water content beneath the grevillea-maize systemincreased with increasing distance from the tree row,probably because of preferential water uptake beneaththe canopy and reduced rainfall input through can-opy interception. Because interception losses were notmeasured in this study, the proportional importance ofthese two mechanisms upon soil water distribution anddynamics in the grevillea system cannot be identified.The senna-maize system exploited more stored waterthan a sole maize crop, but the beneficial effects ofthe hedgerow (reduced evaporation and crop transpir-ation) appeared to greatly compensate for interceptionand uptake losses near the senna canopy.AcknowledgementsThis work was financed by DFID Forestry ResearchProgramme, as a sub-contract of the AgroforestryModelling Project (R5651). The field research andlaboratory analyses were supported by a grant fromthe Swedish International Development CooperationAgency (Sida) to ICRAF. 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