Microhabitat variation in an African savanna: effects of woody cover and herbivores in Kenya

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Abstract
Effects of extreme livestock pressure on the abundance and distribution of a drought-deciduous shrubSericocomopsis pallida, and effects of the shrub canopy on microclimate, soil fertility and grass production, were studied in savanna grasslands of southern Kenya. Canopy volume declined with increasing herbivore pressure, but shrub density was not systematically affected, suggesting strong resilience against destruction by herbivores. However, shrubs became more aggregated with increasing herbivory, suggesting that clumps of individuals are more resilient to destruction than are isolated individuals. Grass production was three times greater under the canopy ofS. pallidathan in the open. Comparisons of physical-chemical properties among soils derived from four microsites revealed far higher nutrient levels in sub-canopy soil than in soil derived from open ground between canopies, and radically different properties compared to soil heavily enriched with livestock dung and urine. Higher nutrient levels beneath the canopy most likely resulted from Utter decomposition.A pot experiment, designed to simulate shading and soil conditions in the field, showed that grass growth promotion was largely due to a substantial increase in soil fertility beneath the canopy ofS. pallida, comparable to grass productivity in soil enriched with livestock excreta. However, this increase in sub-canopy grass production did not significantly increase regional grass layer production.