Climatic, edaphic and fire regime determinants of biome boundaries in the eastern Cape Floristic Region

• The most important variables in differentiating amongst biomes were rainfall, fire exposure, soil fertility and soil drainage.
• The patterns observed can be used to generate hypotheses for experimental investigation.
• This region has much to offer researchers interested in biome boundaries.

Landscapes of the eastern Cape Floristic Region are extremely rich in biomes. Analysis of the determinants of boundaries between biomes can generate insights and predictions that are useful for the applied ecological sciences. Here we used a historical data set and multivariate methods to explore the determinants of the boundaries of Forest, Fynbos, Grassland, Renosterveld and Thicket biomes in a 1200 km2 area of the Kouga region. The data set comprised 203 (10 m × 10 m) sites with corresponding landscape-scale environmental variables, and 100 sites with corresponding site-bound edaphic (physical and chemical) variables. Of the landscape variables, soil drainage and fire exposure had the strongest biome-specific associations: most Grassland occurred on seasonally waterlogged soils and Renosterveld on moderately drained soils. While Forest was exclusively associated with landscapes protected from fire, much Fynbos and Thicket were found in areas exposed to fires of intermediate frequency; however, in these situations Fynbos is restricted to sandstone-derived soils and thicket to shale-derived soils. Other strong patterns were the restriction of Forest to sandstone-derived soils and of Thicket to soils of deep to intermediate depth. Soils of the majority of Fynbos and Grassland were shallow, rocky and infertile, while those of Forest and Thicket were deep, rock-free and fertile, especially in terms of oxidizable carbon and total nitrogen. The relatively high fertility of Forest and Thicket soils is attributed canopy-induced enrichment in the prolonged absence of fire. Renosterveld occupied soils of intermediate fertility. These patterns can provide useful insights for developing experimental approached to simultaneously assess the determinants of multiple boundaries in the biome-rich landscape of the eastern Cape Floristic Region.