Ecotones – where biomes meet

Abstract

news and update ISSN 1948-6596 commentary Ecotones – where biomes meet The classification of biomes (or vegetation types) represents one of the most basic summaries of ecological units and provides part of the basis of biogeography. The first systematic continen- tal biome map was a map of Africa produced by Schantz in 1923 (Kuchler 1960). Since then, bi- ome mapping has continually developed to in- form biodiversity sciences. Initially such devel- opments were mainly in the form of expert mapping, where vegetation scientists with good knowledge of the area being mapped, and often drawing on actual plant community data, drew the boundaries of biomes (Kent 2012). More recently, ever more reliance has been put on remote-sensing data, where algo- rithms are used to interpret satellite imagery to classify biome distributions (and other land use types; Xie et al. 2008). With the concomitant development of other large-scale spatial datasets, such as of climate and soil, there has been an increased interest in the drivers of biome distribution, or elements of vegetation such as woody cover, at macroecological scales (e.g., Stephenson 1990, Greve et al. 2011, Campo-Bescos et al. 2013). Understanding what drives biome distribution is especially important in the face of global change, in response to which the distribution of biomes may be expected to shift. It has been suggested that if such shifts are to take place, the transition zones between vegetation types, i.e. the ecotones, would be most suitable for monitoring such changes (e.g., Berner et al. 2013; though see Kupfer and Cairns 1996). Eco- tones are often also areas of high richness as they may harbour species from different bi- omes (e.g., van Rensburg et al. 2009), and may be important areas of gene flow and hybridiza- tion (Kark and van Rensburg 2006). Against this background of the im- portance of ecotones, and a lack of attention historically afforded them (Kark and van Rens- burg 2006), it is exciting that the South African Journal of Botany will shortly be releasing a special issue dealing with “biome boundaries” in the South African context (Potts et al. 2015a). South Africa harbours a particularly high diversity of biomes (Mucina and Ruther- ford 2006), with some regions of high biome turnover across relatively small geographic dis- tances (Fig. 1). This, together with the rich his- tory for biome research in the country (Potts et al. 2015a), makes the region a natural labora- tory to understand ecotones and their drivers. The special issue presents work conduct- ed at a range of spatial scales (Potts et al. 2015a), ranging from transplant experiments across biome boundaries (Esler et al. 2015) to works conducted across all biomes in the coun- try. One of the country-scale studies examines the use and limitations of dynamic vegetation models in understanding the shifts of biome boundaries in response to climate change (Moncrieff et al. 2015), while another examines the extent to which ecotones can be also used to differentiate between alien plant composi- tion (Rouget et al. 2015). Other studies look at the role of soils in determining biome distribution (reviewed in Potts et al. 2015a). One of these used soil car- bon isotope analyses to ask whether soil type might drive biome boundaries in a grassland- forest mosaic (Gray and Bond 2015). By exam- ining the depth profile of carbon isotopes they concluded the opposite: that biome affects soil carbon. In the fire-prone grasslands, fires de- plete nutrients, while nutrient build-up hap- pens in forests, which are protected from fires. In this system, disturbances, rather than the abiotic environment, drive the position of eco- tones. A number of papers in the special issue look at changes in vegetation structure and boundaries through time using repeat aerial and ground-based photography (reviewed in Potts et al. 2015a). These studies, which could illustrate how shifts in biome boundaries occur (e.g., Masubelele et al. 2015), illustrate the im- frontiers of biogeography 7.3, 2015 — © 2015 the authors; journal compilation © 2015 The International Biogeography Society