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To determine why herbaceous productivity in tropical and subtropical savannas is often significantly higher under crowns of isolated trees than in adjacent grass—lands, experimental plots were established in three concentric zones, crown, tree—root, and grassland, surrounding isolated trees of Acacia tortilis in low—rainfall and high—rainfall savannas in Tsavo National Park, Kenya. Plots were fertilized (to determine the importance of nutrient enrichment by trees), shaded (to determine the importance of crown shade), fertilized and shaded (to identify fertilizer x shade interactions), or trenched (tree roots entering plots were severed to determine the importance of belowground competition between overstory trees and understory herbaceous plants). In addition, vertical root distributions of trees and herbaceous species were determined, and root systems of A. tortilis saplings were excavated. At both sites fertilization significantly increased herbaceous productivity in tree—root and grassland zones, but not in canopy zones; artificial shade had no effect on productivity at the low—rainfall site but increased productivity in the tree—root zone at the high—rainfall site; and severing tree roots had no effect on herbaceous productivity at the low—rainfall site, but increased productivity in the crown and tree—root zones at the high—rainfall site. Roots of herbaceous and woody species co—occurred within the same soil horizons, but tree roots extended farther into grasslands at the low—rainfall site than at the high—rainfall site. These studies suggest that savanna trees completed more intensely with understory plants at wetters sites, where their roots terminated in or near crown zones, than at drier sites, where their roots extended farther into open grassland. Nutrients added by trees to crown zones in the form of tree litter and animal droppings increased understory productivity by fertilizing nutrient—limited soils. Shade contributed more to regrowth after severe defoliation than to growth under more normal conditions.

Abstract. The effects of grazing, physical disturbance, interspecific competition, and fire on populations of individual species and on the structure and diversity of grassland communities were investigated in short‐, mid‐, and tall‐grasslands in the Serengeti National Park, Tanzania. The treatments included deep and shallow artificial disturbances, early dry‐season burning, and removal of dominant grass species from 2m2 plots located inside and outside of large‐mammal‐proof exclosures. Species‐cover data were collected for five years and analyzed by analysis of variance and diversity indices. In all three communities, protection from grazing and physical disturbance had greater impacts on species cover and diversity than removal of dominant species or fire. More species were significantly affected (positively and negatively) by disturbance and protection from grazing (50 ‐ 100%) than by species removals and fire (< 41%). In most cases, the cover of tall, perennial species increased following protection from grazing while the cover of annual and short, perennial species decreased. Over the five years of the study, vegetatively reproducing species benefited more than sexually reproducing species from protection from grazing, and tall rhizomatous species benefited more than tall stoloniferous species. Disturbance caused annual and short perennial species to increase and tall perennials to decrease in cover. Following species removals, tall species were either unaffected or they increased in cover while some short species increased and others decreased in cover. Species diversity in two of the three communities decreased when the communities were protected from grazing but increased when disturbed, when dominant species were removed, and when burned.