Acacia species turnover in space and time in an African savanna

Aim Patterns of species turnover along environmental gradients are better s tudied than their causes. Competitive interactions, or physiological tolera nce are most often cited as determinants of turnover. Here we investigate d ifferential tree species response to disturbance by fire and mammal browsin g as causes of changing dominance of species within and among sites along a n altitudinal gradient. Methods We documented the distribution of two Acacia species using maps and sample transects. We explored possible causes of species turnover by study ing differences between the species in tolerance to grass competition using pot experiments, to browsers by observing patterns of shoot damage, and to fire by comparing the size structure of populations burnt at different fre quencies and intensities. Results Acacia karroo woodlands were rare and occur at higher elevations th an the much more common A. nilotica woodlands. Woodland composition seems s et to change in future since the pattern of dominance was reversed in juven ile stages. A. karroo juveniles were very widespread and far more abundant than A. nilotica juveniles. A. karroo juveniles were most abundant in tall fire-prone grasslands and were rare on grazing lawns whereas A. nilotica sh owed the reverse pattern. In the pot experiments, growth of both species wa s suppressed by grasses but there were no significant differences in respon se between the two species. Juveniles of A. karroo were more heavily browse d than those of A. nilotica. However juveniles of A. nilotica were less tol erant of frequent intense burns than juvenile A. Karroo. Main conclusions Disturbance gradients, from high fire frequency and low he rbivore density at high altitudes, to lower fire frequency and higher herbi vore density at low altitudes, are responsible for the shift in community s tructure along the spatial gradient. Differential responses to browsing and fire may also explain temporal turnover from A. nilotica in the past to A. karroo in the present. Changes in the area burnt annually, and in faunal c omposition, suggest a landscape-scale shift from grazing-dominated short-gr ass landscapes in the 1960s, favouring A. nilotica, to fire-dominated tall grasslands in the 1990s favouring A. karroo. We suggest that species turnov er due to differential responses along disturbance gradients may be much mo re widespread than the current paucity of studies suggests.