Long-Term Glades in Acacia Bushland and Their Edge Effects in Laikipia, Kenya

Throughout the Laikipia ecosystem in Kenya, isolated glades occur within acacia bushland and woodland communities. These glades are at least several decades old. They are reported to be old settlement sites of traditional pastoralists no longer present, and their size, location, and orientation are consistent with the settlement sites of related pastoralists studied elsewhere. The purpose of this study was to document the effects of these glades at the local and landscape levels. Working in central Laikipia, we documented differences in vegetation, animal use, and soils at four of these glades, and at increasing distances from glade edges. Four ''glade specialists'' dominated the plant communities within glades, and were very rare outside of glades. Pennisetum stramineum, one of six glade edge species, formed a ring of dense tall grass around most glade edges. The transition to acacia bushland at the glade edges was not always abrupt (depending on the trait considered), resulting in edge effects that differed in depth and sharpness. Edge depth, defined as the distance required to attain two-thirds of background levels for a trait, ranged from 0 to 200 m. Understory plant species richness and diversity were lowest inside glades, and gradually increased with distance from glades. However, because glades supported species not found elsewhere, the presence of glades increased overall species diversity. The density of wild and domestic large mammal dung was up to 10 times greater inside the glades, and declined with distance from glade edges. Similarly, soil nitrogen, potassium, carbon, calcium, and sodium were greatest inside glades. Soil phosphorus, magnesium, and manganese were not elevated inside glades or within 100 m of glades, but instead were much more abundant in background samples. These glades may be maintained by high densities of large mammals, either through herbivory or through changes in soil chemistry. Glades may be attractive to mammals because of the high quality of food there, or as part of an anti-predator strategy. The result is a relatively permanent community mosaic that increases ecosystem heterogeneity and resource use by domestic and wild animals. The spatial nature of this heterogeneity differs among species, depending on their distributions relative to the glades.