Many researchers are currently investigating the processes by which agroforestry systems can be sustained, adopting modelling approaches such as those outlined by Lawson et al. (1995) to determine the factors limiting the success of such systems in the field. Successful agroforestry systems may succeed where the tree and crop components are complementary to each other in the way they exploit the available resources. Anderson and Sinclair (1993) considered agroforestry systems in these terms, suggesting that success depended largely on niche differentiationÐ either temporal, for example, where the trees accessed resources like rainfall occurring outside the conventional cropping seasonsÐor spatial, for example, where deep-rooted tree species could utilise resources
outside the reach of crop roots. In the case of intensive
systems, such as the Grevillea/maize system reported
in this paper, it may be the case that such complementarity
has to be imposed on the system, through
site management practices such as canopy pruning, or
root pruning by trenching or deep root ploughing.
However, such below-ground practices are labourintensive
or require powerful machinery