Abstract
Plant species range shifts are predicted to occur in response to climate change. The predictions are often based on the assumption that climate is the primary factor limiting the distribution of species. However the distribution of grassy biomes in Africa cannot be predicted by climate alone, instead interactions between vegetation, climate and disturbance structure the ecosystems. To test if climatic variables, as predicted by an environmental niche model, determine the distribution limits of two common savanna tree species we established a transplant experiment at a range of latitudes and altitudes much broader than the distribution limits of our study species. We planted seedlings of two common savanna trees, Senegalia nigrescens and Colophospermum mopane, at eight paired high and low elevation sites across an 850 km latitudinal gradient in South African savannas. At each site seedlings were planted in both grassy and cleared plots. After two years of growth, rainfall, temperature and location inside or outside their distribution range did not explain species success. Grass competition was the only variable that significantly affected plant growth rates across all sites, but grass competition alone could not explain the distribution limit. Species distributions were best predicted when maximum tree growth rates were considered in relation to local fire return intervals. The probability of sapling escape from the fire trap was the most likely determinant of distribution limits of these two species. As trees grew and survived 100s of kilometers south of their current range limits we conclude that climate alone does not explain the current distribution of these trees, and that climate change adaptation strategies for savanna environments based only on climatic envelope modelling will be inappropriate.
Highlights
Global change is transforming the planet (Ellis, 2011; Scheffers et al, 2016)
The realized niche describes where a species occurs after accounting for the effects of inter-specific competition. This concept has been applied almost globally as a framework to describe the geographic distribution of species (Gaston, 2003; Thomas, 2010). This framework is generally simplified to assume that climate shapes a plant’s distribution range, and is the basis of attempts to predict the future ranges of plants using the correlative climate envelope approach (Thomas et al, 2004; Hijmans et al, 2005) where the current distribution of a species is mapped in climate space and any future shifts in climate space are expected to produce an accompanying shift in the future species distribution range (Elith and Leathwick, 2009)
VN, the low latitude higher elevation site was the exception, as it is had the highest mean annual temperature (MAT), the mean monthly maximums were higher at the low elevation site
Summary
Global change is transforming the planet (Ellis, 2011; Scheffers et al, 2016). One of the most pervasive impacts, which has severe economic and ecological impacts, is that of shifting plant and animal ranges (Parmesan and Yohe, 2003; Thomas et al, 2004; Chen et al, 2011). The shifts are most frequently attributed to the warming of the earth, with the expectation that plant and animal distributions will shift toward the cooler poles or higher elevations (Lenoir et al, 2008; Chen et al, 2011) These expectations (Hutchinson, 1957; Chase and Leibold, 2003) are often derived from the. The realized niche describes where a species occurs after accounting for the effects of inter-specific competition This concept has been applied almost globally as a framework to describe the geographic distribution of species (Gaston, 2003; Thomas, 2010). There is debate as to the broad applicability of climate models, (Davis et al, 1998; Araújo and Luoto, 2007; Svenning and Skov, 2007; Araújo and Peterson, 2012), the underlying general assumption that climate alone is the primary determinant of species distribution limits (Holdridge, 1967; Gaston, 2003) is seldom questioned
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