Effects of shade on the growth and mineral nutrition of tropical grasses in silvopastoral systems

Interactions between tree and grass strata in silvopastoral systems have be en studied for a targe number of species and environmental conditions. In t ropical environment the increase of growth and/or mineral nutrition of the grass stratum due to tree shade has been very often reported. This effect h as been observed even for non-leguminous tree component and under artificia l shading, and occurs most frequently when water and nitrogen are limiting factors for grass growth. This paper reviews the literature on morphogeneti c and trophic responses of tropical C-4 grasses to shading, as well as on t he ecological, physiological and physical mechanisms involved in these resp onses. It is concluded that shade enhances the availability of nutrients in the soil, which concerns the chemical and/or physical fertility. The princ ipal processes involved are: (i) recycling of nitrogen fixed by legume tree s via leaf-root-nodules decomposition and/or animal excreta, (ii) changes i n microclimatic conditions by shade, favoring the biological activity of th e soil, and (iii) transfer of nutrients absorbed by trees from the deeper l ayers of the soil to the surface via leaf drop and/or animal excreta. The n eeds and perspectives for future research are discussed.