Grass mortality in semi-arid savanna: The role of fire, competition and self-shading

Perennial grasses are a dominant component of many vegetation formations and provide important ecosystem services including supporting herbivores and preventing soil erosion. Despite their importance, our understanding of the processes that influence their mortality rates is surprisingly limited. This study explores the effects of local and landscape-scale processes on mortality of a perennial grass (Stipagrostis uniplumis) in semi-arid savanna. We focussed on three local-scale factors: self-shading by the standing dead biomass of a tuft, plant size, and neighbour abundance as a measure of intra-specific competition. Three indices of neighbour abundance were calculated: number of neighbours, sum of the neighbours’ basal area, and sum of the neighbours’ living basal area. At the landscape scale, we explored the influence of fire on tuft mortality. The amount of standing dead biomass increased the mortality rates of tufts. Neighbour abundance, indexed as the sum of the living basal area of neighbours, was also associated with higher mortality rates, whereas the other indices of neighbour abundance had no influence on mortality rates. On a landscape level, fire significantly increased tuft mortality rates, from up to 31% for unburned tufts, to 73% for burned tufts. Fire, on the other hand, indirectly reduces the risk of future mortality by reducing self-shading and competitive pressure. Our results imply that the timing and frequency of fires is crucial for their positive indirect effects on plant fitness. As the onset of local effects on plant mortality is highly dependent on grazing pressure and stochastic rainfall, fire management should flexibly take into account the accumulation of dead plant material on a site.