Woody cover in African savannas: the role of resources, fire and herbivory

Abstract

AimTo determine the functional relationships between, and the relative importance of, different driver variables (mean annual precipitation, soil properties, fire and herbivory) in regulating woody plant cover across broad environmental gradients in African savannas.Location Savanna grasslands of East, West and Southern Africa. MethodsThe dependence of woody cover on mean annual precipitation (MAP), soil properties (texture, nitrogen mineralization potential and total phosphorus), fire regimes, and herbivory (grazer, browser + mixed feeder, and elephant biomass) was determined for 161 savanna sites across Africa using stochastic gradient boosting, a refinement of the regression tree analysis technique. ResultsAll variables were significant predictors of woody cover, collectively explaining 71% of the variance in our data set. However, their relative importance as regulators of woody cover varied. MAP was the most important predictor, followed by fire return periods, soil characteristics and herbivory regimes. Woody cover showed a strong positive dependence on MAP between 200 and 700 mm, but no dependence on MAP above this threshold when the effects of other predictors were accounted for. Fires served to reduce woody cover below rainfall-determined levels. Woody cover showed a complex, non-linear relationship with total soil phosphorus, and was negatively correlated with clay content. There was a strong negative dependence of woody cover on soil nitrogen (N) availability, suggesting that increased N-deposition may cause shifts in savannas towards more grassy states. Elephants, mixed feeders and browsers had negative effects on woody cover. Grazers, on the other hand, depressed woody cover at low biomass, but favoured woody vegetation when their biomass exceeded a certain threshold. Main conclusionsOur results indicate complex and contrasting relationships between woody cover, rainfall, soil properties and disturbance regimes in savannas, and suggest that future environmental changes such as altered precipitation regimes, N-enrichment and elevated levels of CO 2 are likely to have opposing, and potentially interacting, influences on the tree-grass balance in savannas.

Woody encroachment and its consequences on hydrological processes in the savannah

“…Among the possible factors behind woody encroachment are changes in rainfall regime [2,4], elevation of atmospheric CO 2 concentration [5,7], invasion by exotic trees [8], changes in disturbance regimes such as suppression of fire [3,7,9] and decline in browsers or overgrazing [3,7,9]. Increases in woody cover or density can result in altered habitat for specialized fauna, changes in plant species composition [10,11] and also biogeochemical cycles and ecosystem services, such as carbon storage and water provisioning [1,8,[12][13][14][15].…”

The future distribution of the savannah biome: model-based and biogeographic contingency

“…B 371: 20150311 plant growth. Nitrogen availability alters tree/grass ratios in savannahs and reduces the CO 2 fertilization effect on tree growth [45,46]. Indeed, Higgins et al [47] have shown in a field experiment that tree-dominated experimental plots have lower rates of N-mineralization than grass-dominated plots, suggesting that progressive nitrogen limitation may act as a negative feedback on CO 2 fertilization-fuelled expansion of trees in savannahs.…”

Potential aboveground biomass in drought‐prone forest used for rangeland pastoralism

“…Studies from other ecosystems, including Eucalyptus forest from eastern Australia (Keith et al 2010), savanna from northern Australia (Williams et al 1996), and African savanna (Sankaran et al 2008) have also found rainfall an important predictor of structural attributes. Variation in statistical procedures makes a quantitative comparison difficult, but rainfall would appear to have a greater control over biomass in the mulga dry forest than the vegetation in other studies.…”

“…Fire provided an important contribution to continental-scale models describing tree cover in Africa (Sankaran et al 2005(Sankaran et al , 2008, and is also heavily implicated as a determinant of structural change based on experimental findings (Sharam et al 2006, Staver et al 2009). Boundaries between mulga forest and spinifex (Triodia spp.…”

“…A large data set representing tree cover in African savannas revealed that 71% of the variance could be explained by mean annual rainfall, fire return interval, soil attributes, and indices of grazing intensity, in that order of importance (Sankaran et al 2008). The 4 E-mail: rod.fensham@derm.qld.gov.au influence of rainfall was particularly important in the semi-arid spectrum (annual rainfall 200-700 mm), and disturbance variables became important where annual rainfall exceeded 700 mm.…”