TREE-CROP INTERACTIONS IN AGROFORESTRY SYSTEMS INVOLVING DIFFERENT TREE SPECIE

Abstract

The present experiment was conducted to quantify the variation of soil water content and growth ofthe annual crop when grown in agroforestry systems involving different tree species as contourhedgerows.The final objective is to select a tree species which has a minimum of negativeinteractions (i.e. competition) and a maximum of positive interactions with the annual crop.

The experiment was conducted at Pallekelle in the mid-country intermediate zone (1M] ) during theyala season in 1997. TIle experimental site contained five tree species established as hedgerowsalong contours at an inter-hedgerow distance of 4 m. The five tree species were Calliandracalothyrus, Desmodium ranson ii, Flemingia congesta, Gliricidia sepium and Tithonia diversifolia.Bush bean (var, Top crop) was grown as the annual crop between hedgerows at a spacing of 30 em x8 cm. Soil water content was measured gravimetrically in two soil depths (0-10 em and 20-40 em) atthree distances (0-30 ern, 150-180 em and 300-360 em ) from the hedgerow. Measurements weremade at IO-day intervals. Annual crop growth was measured by taking the total dry weight of bushbean at flowering. Dry weight measurements were done at six distances (30, 90, 150, 210, 270 and330 em) from the hedgerows. Measurements were also made in a control treatment which was a solecrop of bush bean without.a hedgerow. Each treatment had three replicates.

The soil water content at both depths varied significantly with varying distance from the hedgerowon all dates of measurement. Moreover, there were significant differences between soil watercontents under different hedgerow species at both depths of sampling. The effect of interactionbetween species and distance from the hedge was not significant. Plots with hedgerows had asignificantly greater soil water content in the surface soil layer (0-10 em depth) as compared to thesole crop control on all dates of measurement except at 30 days after sowing (30 DAS). This couldprobably be due to the shading effect of the hedgerows which would reduce evaporation from thesurface layers of the soil. Calliandra and Desmodium had consistently higher soil water contents inthe surface layers. On the other hand, Tithonia had consistently lower surface soil water contents.The soil water content of the 0-10 em depth layer decreased significantly as the distance from thehedgerow increased. TIle greater soil water availability in the surface soil layers closer to thehedgerows could probably be due to the greater shading effect near the hedges.

Plots with hedgerows had significantly greater soil water contents at 20-40 em depth than the solecrop control on all dates of measurement. Consistently greater levels of soil water content wereobserved at this depth under Calliandra, Desmodium and Flemingia. Conversely, plots withGliricidia had consishtentiy lower soil water contents at 20-40 ern depth. In contrast to the 0-10 cmlayer, at 20-40 cm depth, the soil water content closer to the hedge (0-30 em) was significantly lowerthan at 150- 180 em and 300-330 cm distances.

Total dry weight of bush bean plants showed decreases of varying magnitudes in crop rows closer(30 ern) to the hedgerows. This growth decline was observed under all hedgerow species whereas nosuch decline was observed in the sole cropped control. At a distance closer to the hedgerow (30 em),bean plants had a lower total dry weight than the control under all tree species except Desmodium.Conversely, at a distance of 150 em from the hedgerows, bean plants under all tree species exceptFlemingia had greater total dry weights than the control. The distance from the hedgerow at whichbean plant dry weight was maximum varied For different tree species. This indicated that differenttree species exerted varying levels of competition on the annual crop. The least competition wasexerted by Desmodium followed respectively by Gliricidia, Calliandra, Tithonia and Flemingia.

There are significant differences between tree species in the degree of environmental interaction andcompetition exerted by them on the annual crop in agroforestry systems involving tree species ascontour hedgerows.