Porcupines, Fires and the Dynamics of the Tree Layer of the Burkea Africana Savanna

Abstract

(1) The tree layer of the Eragrostis pallens-Burkea africana savanna is composed primarily of Terminalia sericea and B. africana with other tree species occurring infrequently. Burkea africana is dominant on well drained soils at the upper portions of a soil catena, co-dominant with T. sericea at midslope and replaced on seasonally waterlogged soils at the base of the slope by T. sericea. (2) There is a successional process occurring within the tree layer. Terminalia sericea establishes only on open sites and never under itself or other members of the tree layer. Burkea africana establishes on open sites, under T. sericea and under itself. Dombeya rotundifolia, another common tree, establishes under B. africana but not under T. sericea or itself and never in the open. The other tree species establish either under the canopies of large individuals of B. africana or on termite mounds. (3) The general successional sequence is T. sericea followed by B. africana and later D. rotundifolia or other shade-tolerant tree species. The actual successional sequence varies over the slope. In the upper portions, where T. sericea is uncommon, B. africana initiates the sequence and is followed by more shade-tolerant species. At midslope, T. sericea generally establishes first, followed by B. africana and later by shade-tolerant species. At the base of the slope, T. sericea is dominant since it is able to withstand the seasonally waterlogged soils there. During long periods of drought B. africana may establish secondarily. Other tree species rarely establish in this zone and then usually upon termite mounds. (4) A mature, closed-canopy woodland does not develop due to the disturbance by porcupines and fire. Porcupines ringbark trees and expose the heartwood to fire. They preferentially attack D. rotundifolia and B. africana in that order and rarely scar T. sericea. The strong preference of porcupines for D. rotundifolia results in many coppiced shrubs and a reduction in the number of individuals reaching tree status. The secondary preference for B. africana, which is abundant over much of the slope, results in large individuals being removed by fire more frequently than T. sericea. Of the large, live trees that fell after a fire in September 1985, 57% were B. africana. (5) The Eragrostispallens-Burkea africana savanna is maintained by porcupines and fire as a mosaic of grassland and small woodland patches undergoing cyclical successions developing after the fall of large individuals of B. africana.