Why tree-crop interactions in agroforestry appear at odds with tree-grass interactions in tropical savannahs |
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| Authors: | C K Ong R R B Leakey |
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| Institution: | (1) International Centre for Research in Agroforestry, P.O. Box 30677, Nairobi, Kenya;(2) Institute of Terrestrial Ecology, Bush Estate, Penicuik, Midlothian, Scotland, EH26 0QB (Author for correspondence: E-mail |
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| Abstract: | This paper describes recent research findings on resource sharing between trees and crops in the semiarid tropics and attempts
to reconcile this information with current knowledge of the interactions between savannah trees and understorey vegetation
by examining agroforestry systems from the perspective of succession. In general, productivity of natural vegetation under
savannah trees increases as rainfall decreases, while the opposite occurs in agroforestry. One explanation is that in the
savannah, the beneficial effects of microclimatic improvements (e.g. lower temperatures and evaporation losses) are greater
in more xeric environments. Mature savannah trees have a high proportion of woody above-ground structure compared to foliage,
so that the amount of water 'saved' (largely by reduction in soil evaporation) is greater than water 'lost' through transpiration
by trees. By contrast, in agroforestry practices such as alley cropping where tree density is high, any beneficial effects
of the trees on microclimate are negated by reductions in soil moisture due to increasing interception losses and tree transpiration.
While investment in woody structure can improve the water economy beneath agroforestry trees, it inevitably reduces the growth
rate of the trees and thus increases the time required for improved understorey productivity. Therefore, agroforesters prefer
trees with more direct and immediate benefits to farmers. The greatest opportunity for simultaneous agroforestry practices
is therefore to fill niches within the landscape where resources are currently under-utilised by crops. In this way, agroforestry
can mimic the large scale patch dynamics and successional progression of a natural ecosystem.
This revised version was published online in June 2006 with corrections to the Cover Date. |
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| Keywords: | alley cropping competition complementarity light interception resource capture water use |
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