Woody plant biomass and carbon exchange depend on elephant‐fire interactions across a productivity gradient in African savanna

Abstract

Summary1. Elephants and fire are individually well-known disturbance agents within savanna ecosystems, but their interactive role in governing tree-cover dynamics and savanna-forest biome boundaries remains unresolved. Of central importance are the mechanisms by which elephants vs. fire affect tree biomass and cover, and how -over long time periods -both factors interact with rainfall and soils to govern tree biomass and carbon dynamics. 2. Here, we evaluated the response of woody vegetation to 56 years of fire manipulation in South Africa's Kruger National Park, with three fire regimes (annual, triennial and unburned) replicated across a productivity gradient and subject to two periods of contrasting elephant abundances (generated by the cessation of culling in 1994). 3. Higher fire frequencies had a negative effect on woody biomass in the low-elephant period, but this effect was weak to negligible in the high-elephant period as the difference among fire treatments diminished. Moreover, elephants removed increasing amounts of woody biomass as productivity increased across study sites, but fire did not. We infer that elephant-induced tree mortality could overcome increases in woody-plant productivity, while fire-induced mortality alone could not. 4. Elephants caused woody-plant carbon to shift from a sink to a source; this effect was independent of fire treatment, with highest rates of net carbon removal in the wettest and most productive site. 5. Synthesis. Our results reveal a context-dependent interaction between fire and elephants as disturbance agents in savanna: the influence of fire on woody plants was sensitive to the abundance of elephants and diminished with increased plant productivity. In contrast, elephants were capable of shifting landscapes from relatively dense woodland to open savanna, even in unburned sites, and exerted strong impacts irrespective of site conditions and plant productivity.

Sensitivity of woody carbon stocks to bark investment strategy in Neotropical savannas and forests

“…The results from this study show a strong dependence of tree growth on precipitation and fire frequency, supporting observations (Pellegrini et al, 2017b;Higgins et al, 2007) and other modeling studies (Baudena et al, 2015). However, we found that including bark investment as a fire survival mechanism broadened the range of climate and fire conditions under which savannas occur by reducing the range of conditions leading to either complete tree loss or complete grass loss.…”

Elephants limit aboveground carbon gains in African savannas

“…The measured increases in ACD between 2008 and 2014 likely represent current day patterns driven by woody encroachment. Indeed, Pellegrini et al () also measured substantially lower biomass declines between 2002 and 2012 in experimental field plots, in contrast to earlier time periods.…”

“…A moratorium on culling in Kruger National Park (KNP), South Africa, has led to prodigious elephant population growth from about 8,000 individuals in 1994 to more than 17,000 in 2015 (Ferreira, Greaver, & Simms, ). This population growth was accompanied by large declines in aboveground woody biomass between 1998 and 2004, as measured in experimental plots in hillcrest locations across the park (Pellegrini, Pringle, Govender, & Hedin, ). While this is suggestive of an elephant‐driven effect, evidence from one of the experimental sites for which additional data were collected in 2012 revealed little biomass change between 2002 and 2012.…”

Inserting Tropical Dry Forests Into the Discussion on Biome Transitions in the Tropics

“…It may be that in Africa, a higher abundance of large herbivores, including elephants, favors grasses over trees, leading to a more open savanna vegetation with more frequent fires, even in areas of higher soil fertility (Charles-Dominique et al, 2016;Pellegrini et al, 2017). If dry forests are not found on fertile soils in more mesic areas of Africa, there may not be moist forest-dry forest transitions on this continent, because the areas mapped as belonging to the "arid flora" or "succulent biome" are completely separated from moist forest regions by large areas of savanna (Schrire et al, 2005;Linder, 2014).…”