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.