Abstract
Aim Biomes are defined according to the growth forms of their dominant species but also contrast in floristic composition. Biome boundaries thus represent areas of taxonomic turnover. The degree of turnover is dependent on the difference in environmental conditions between biomes and the ability of lineages to evolve adaptive traits. Open‐ and closed‐canopy biomes differ in structure, disturbance and resource availability. We aimed to determine whether these boundaries impose greater adaptive challenges to the movement of floristic lineages than do structurally similar ones. Location South Africa. Methods We determined genus‐ and family‐level dissimilarity between neighbouring biomes, and tested their correlations with differences in climate and soil properties, fire frequency and leaf area index ( LAI : a measure of canopy cover used as a proxy for understorey light availability). Results Genus‐ and family‐level dissimilarities between biomes were positively correlated with differences in LAI . High taxonomic turnover was also associated with differences in fire frequency and soil organic carbon. LAI was strongly predicted by differences in precipitation in the driest quarter. Although environmental correlates with floristic turnover varied, the taxonomically dissimilar Fynbos boundaries were linked to differences in cation exchange capacity. Main conclusions Biome boundaries with edaphic contrasts or emergent differences in vegetation structure caused by plant–water–fire interactions present possibly the hardest boundaries for floristic transitions. The infrequency of transitions across these boundaries is probably a consequence of environments contrasting in light and moisture availability, fire frequency and soil properties, requiring co‐adaptation across multiple traits. Variation in floristic turnover between boundaries, however, implies that adaptive challenges and the ease with which they overcome vary with biome.