Tree dieback and regeneration in secondary Acacia zanzibarica woodlands on an abandoned cattle ranch in coastal Tanzania

Abstract

Questions: Bush encroachment is a major problem when African savanna ecosystems are used for cattle ranching. How do secondary woodlands develop after ranching is abandoned? What are the patterns and rates of tree mortality and regeneration? Location: Mkwaja Ranch (now part of Saadani National Park) in coastal Tanzania. Methods: Ninety-seven circular plots (4-m radius) were set in secondary Acacia zanzibarica woodland along a gradient of tree density. Variables relating to tree and grass layers and soil characteristics of plots were recorded. Seedlings were counted twice in the wet seasons, and resprouts once. Tree flowering and pod production were assessed during the fruiting season, while survival of trees initially present was recorded after 12 and 32 months. Interrelationships among variables were investigated using multiple linear regression, binary logistic regression and mixed effects models. Results: After 32 months, over one-third of trees in plots had died. Most died after fire, especially on heavy soils, and mortality was significantly related to the tree live biomass ratio and soil conditions. Seed production was very low, especially in denser stands. Numbers of seedlings correlated with soil and grass variables but not with seed rain. Half of trees killed above-ground produced coppice shoots from the base; in contrast, root suckering was independent of topkill. By the end of the study, no seedlings survived and no resprouts emerged above the grass layer. Conclusions: A. zanzibarica woodlands at Mkwaja Ranch were able to develop because of ranching, and can only persist under intensive grazing. The woodlands do not represent a successional stage towards forest and will probably revert to predominantly grassland vegetation within 10–20 years unless grazing pressure from wild ungulates increases considerably and/or fire regimes change.