Land-use changes around remnant savannas alter tree size demographics

Abstract

Anthropogenic land-use change is currently one of the greatest threats to biodiversity across the globe and tropical savannas are likely to be disproportionately impacted by these changes in coming decades. In particular, the loss of larger savanna trees is concerning because of the critical ecological functions they provide. Here, we studied the potential for positive (e.g., increased resources, propagation) and negative (e.g., harvesting, cattle grazing) influence of two increasingly dominant land uses throughout southern African savannas, communal lands and intensive agricultural, on the size class distributions of dominant large tree species (i.e., knobthorn [Senegalia nigrecens] and marula [Scelerocarya birrea]) in savannas surrounded by these transformed lands. We recorded a total of 646 knobthorn and 303 marula individuals. The two species showed individualistic responses to surrounding land use types. Average abundances (mean, ± 95 % CI) of marula were similar between savanna patches surrounded by savanna only, and savannas surrounded by agricultural and communal areas, while the abundance of most size classes of knobthorn were drastically reduced by surrounding land uses, likely from harvesting for fuel wood. In this study we showed that the influence of land use changes on large tree populations spills over onto the savannas they surround. Furthermore, tree population responses depend on both the surrounding land use and species identity, which determine the nature of exploitative activities. The different exploitative activities appear to be neutral with respect to the populations of marula while reducing populations sizes of knobthorn (i.e. through wood fuel harvesting). We conclude with management recommendations that may increase knobthorn populations in communal areas. Specifically, we suggest encouraging people to propagate knobthorn and other native and wood fuel species for sale at nurseries, teaching harvesting approaches that foster sustainable populations, and encouraging alternative sources fuel and diversified livelihoods.