BIOPHYSICAL INTERACTIONS IN TROPICAL AGROFORESTRY SYSTEMS

The rate and extent to which biophysical resources are captured and ut ilized by the components of an agroforestry system are determined by t he nature and intensity of interactions between the components. The ne t effect of these interactions is often determined by the influence of the tree component on the other component(s) and/or on the overall sy stem, and is expressed in terms of such quantifiable responses as soil fertility changes, microclimate modification, resource (water, nutrie nts, and light) availability and utilization, pest and disease inciden ce, and allelopathy. The paper reviews such manifestations of biophysi cal interactions in major simultaneous (e.g., hedgerow intercropping a nd trees on croplands) and sequential (e.g., planted tree fallows) agr oforestry systems. In hedgerow intercropping (HI), the hedge/crop inte ractions are dominated by soil fertility improvement and competition f or growth resources. Higher crop yields in HI than in sole cropping ar e noted mostly in inherently fertile soils in humid and subhumid tropi cs, and are caused by large fertility improvement relative to the effe cts of competition. But, yield increases are rare in semiarid tropics and infertile acid soils because fertility improvement does not offset the large competitive effect of hedgerows with crops for water and/or nutrients. Whereas improved soil fertility and microclimate positivel y influence crop yields underneath the canopies of scattered trees in semiarid climates, intense shading caused by large, evergreen trees ne gatively affects the yields. Trees in boundary plantings compete with crops for above-and belowground resources, with belowground competitio n of trees often extending beyond their crown areas. The major biophys ical interactions in improved planted fallows are improvement of soil nitrogen status and reduction of weeds in the fallow phase, and increa sed crop yields in the subsequent cropping phase. In such systems, the negative effects of competition and microclimate modification are avo ided in the absence of direct tree-crop interactions. Future research on biophysical interactions should concentrate on (1) exploiting the d iversity that exists within and between species of trees, (2) determin ing interactions between systems at different spatial (farm and landsc ape) and temporal scales, (3) improving understanding of belowground i nteractions, (4) assessing the environmental implications of agrofores try, particularly in the humid tropics, and (5) devising management sc hedules for agroforestry components in order to maximize benefits.