INFLUENCE OF TREES ON ABOVEGROUND PRODUCTION DYNAMICS OF GRASSES IN AHUMID SAVANNA

Above-ground grass biomass, necromass and tree litter were measured mo nthly over a vegetation cycle under tree clumps and in the open, in a humid savanna in Cote d'Ivoire. Grass production was calculated using several meth ods to better discriminate the contribution of the differ ent grass compartments. Above-ground grass biomass is higher in the op en than under canopies during the second part of the growing season, b ut there is no difference in grass necromass dynamics. Physical protec tion of grass necromass by tree litter against decaying under tree can opies was assumed to explain this discrepancy. Grass production, calcu lated as the sum of positive increments of biomass and necromass, equa ls 1073 g m(-2) yr(-1) in the open, against 74 % underneath trees. How ever, basal ground cover is only 50 % of that in the open. Comparison with other savanna studies as a whole does not show any significant ef fect of rainfall on the relationship between under-canopy and outside- canopy grass production. However, in arid conditions, grass production tends to increase under light-canopied trees (mostly Acacia legumes) which hardly affect grass photosynthesis, but add high quality litter to the soil surface.