Annual and spatial variation in herbaceous biomass production in a Zambian dry miombo woodland

Herbaceous biomass production was studied for six years (1991-1996) at four dry miombo woodland sites (two on hillsides and two on interfluves) in central Zambia by methods of harvest and nutrient analysis. Frequent man-made fires at the study sites destroyed most of the necromass by the end of the dry season; the residual necromass disappeared by the middle of the rainy season through decomposition. Production was confined to the wet season (November-April). Above-ground total biomass peaked in March or April but this varied from year to year over the six-year period. The hillside sites produced a lower mean biomass (<100 g m(-2) yr(-1)) than interfluve sites (100-200 g m(-2) yr(-1)). When mass loss due to decomposition of necromass and nutrient leaching from live biomass was taken into account, the peak above-ground total biomass estimates represented 64-92% of actual production. The effect of woodland clearing on above-ground herbaceous biomass production was most significant at the hillside sites and production was negatively correlated with soil extractable phosphorus. Tree basal area, stem density and height, leaf biomass in cut plots, soil cation exchange capacity and mineral nitrogen had no significant effect on herbaceous biomass production. Annual variations in herbaceous biomass production appeared to be linked to variations in soil moisture availability. The nutrient content of herbaceous biomass changed throughout the growing season. Nitrogen concentration in biomass was highest at the beginning of the rainy season while phosphorus concentration was highest during the latter half of the season. The concentration of potassium increased from the beginning of the rainy season and peaked in mid-season before declining to low dry season levels.