Fire season affects size and architecture of Colophospermum mopane in southern African savannas

摘要

Wildfires may be started naturally by lightning or artificially by humans. In the savanna regions of southern Africa, lightning fires tend to occur at the start of the wet season, during October and November, while anthropogenic fires are usually started during the dry season, between July and August. A long-term field manipulation experiment initiated in the Kruger National Park in 1952 was used to explore whether this seasonal divergence affects tree abundance, spatial pattern, size and architecture. After 44 years of prescribed burning treatments that simulated the seasonal incidence of lightning and anthropogenic fires, mean densities of the locally-dominant shrub, Colophospermum mopane, were 638 and 500 trees ha(-1) respectively. Trees in burnt plots had aggregated distributions while trees in unburnt plots had random distributions. Significant differences (p < 0.001) were recorded in a range of morphological parameters including tree height, canopy diameter, mean stem circumference and number of stems. The incidence of resprouting also differed significantly between treatments, with burnt trees containing a high proportion of coppiced stems. The differences in tree size and architecture between the mid-dry season and early-wet season burning plots suggest that anthropogenic fires applied during July and August cannot substitute for a natural lightning fire regime. Anthropogenic fire yields a landscape that is shorter, more scrubby and populated by numerous coppiced shrubs than the landscape generated by natural lightning fire conditions.