Effects of nutrients and shade on tree-grass interactions in an East African savanna

Savanna trees have a multitude of positive and negative effects on understo rey grass production. but little is known about how these effects interact. We report on a fertilization and shading experiment carried out in a Tanza nian tropical city savanna around Acacia tortilis trees. In two years of st udy there was no difference in grass production under tree canopies or in o pen grassland, Fertilization, however, indicate that trees do affect the nu trient limitation of the grass layer with an N-limited system in open grass land to a P-limited system under the trees. The NT ratios of grass gave a r eliable indication of the nature of nutrient limitation, but only when asse ssed at the end of the wet season, Mid-wet season nutrient concentrations o f grasses were higher under than outside the tree canopy, suggesting that f actors other than nutrients limit grass production. A shading experiment in dicated that light may be such a limiting factor during the wet season when water and nutrients are sufficiently available. However, in the dry season when water is scarce, the effect of shade on plant production became posit ive. We conclude that whether trees increase or decrease production of the herbaceous layer depends on how positive effects (increased soil fertility) and negative effects (shade and soil water availability) interact and that these interactions may significantly change between wet and dry seasons.