Elephant damage, not fire or rainfall, explains mortality of overstorey trees in Serengeti

Abstract

Generalizations about the drivers of tree demography in tropical savannas continue to prove difficult because of the complex and dynamic interactions involved, and because multi-year data sets spanning meaningful gradients in potential drivers are lacking. Overstorey trees play disproportionate roles in the long-term dynamics and functioning of savanna ecosystems. Understanding demographic patterns in these trees is complicated by their resprouting ability after being top-killed and few studies have attempted to separate top-kill from true mortality events. We examined the interactive effects of fire frequency, mean annual precipitation and elephant herbivory on overstorey (>2 m) tree mortality between 2009 and 2014 across 32 permanent vegetation plots in the Serengeti Ecosystem, Tanzania. Mean tree mortality over the study period was 0.28 ± 0.02 (0.07 year-1). Among trees that were top-killed (19.1% of all individuals), 31.2% resprouted. Mortality was driven largely by elephant herbivory, though mortality rates varied considerably across space and tree species. Fire frequency and mean annual rainfall were weak predictors of tree mortality. Over the 5 year period, chronic elephant herbivory (i.e. repeated through time) was a stronger predictor of tree mortality than maximum elephant herbivory, suggesting that repeated, low-intensity damage from elephants was more important to mortality than acute, but infrequent, damage. Elephants disproportionately damaged certain tree species relative to their availability on the landscape, and the tolerance to damage differed by species as well. Acacia senegal was strongly selected and appeared to have low tolerance to damage when it occurred, resulting in a mortality rate of 0.736 ± 0.052 (0.234 year-1) over the study period. Top-killing and resprouting rates varied across species, with Acacias generally having low resprout rates. Synthesis. Our study highlights the considerable role that chronic herbivory plays in structuring savanna tree populations, irrespective of prevailing fire and rainfall conditions, and provides important vegetative context to the dramatic recent declines and (in the case of Serengeti) increases in savanna elephant densities in sub-Saharan Africa. This study examines drivers of overstorey tree mortality, top-kill and resprouting rates in African savannas. The work highlights the considerable role that chronic elephant herbivory plays in structuring savanna tree populations, irrespective of prevailing fire and rainfall conditions, and provides important vegetative context to the dramatic recent changes in savanna elephant densities in sub-Saharan Africa.