Aboveground dry-matter allocation in ungrazed and grazed stands of Indigofera spinosa in the arid zone of Turkana, Kenya

The dwarf shrub Indigofera spinosa, indigenous to arid and semi-arid rangel ands of northeastern Africa, is an important food source for livestock. Pro per management of the shrub requires improved understanding of the effects of grazing and climatic variability on aboveground dry-matter allocation. B etween 1986 and 1990, we compared the temporal variability of aboveground d ry-matter allocation to different plant biomass compartments. We also compa red dry-matter transfers between components; total live biomass to litter, standing dead to litter and live biomass to standing dead between continuou sly grazed and an ungrazed treatments. Partitioning of combined total dry-m atter production among different structural organs (called allocation ratio ) is influenced by phenological changes, episodic rainfall and herbivory. D ry-matter production in the grazed treatment responded more markedly to epi sodic rainfall events more than in the ungrazed treatment. Exclusion of gra zers failed to improve the relative growth rate (RGR) of shrub biomass, whi le grazing improved it. RGR declined in the ungrazed treatment following th e accumulation of standing dead dry-matter, while in the grazed treatment i t declined following the shedding of leaves. The shrub allocated more to to tal live biomass than to standing dead. Greater reduction of total live all ocation ratio in the grazed than in the ungrazed treatment occurred during a dry year. The ungrazed treatment had higher standing dead allocation rati o than did the grazed treatment. Plants transferred more dry-matter from to tal live biomass compartment to litter, than from standing dead to litter o r from total live biomass to standing dead independent of treatment. The ra tes of transfer were higher in the ungrazed than in the grazed treatment. T he results suggest that I. spinosa has evolved to respond to climatic varia bility and grazing by allocating dry-matter differently between various com partments.