Root trait variation along water gradients in the Cape Floristic Region

Questions: Roots are responsible for essential plant functions including water uptake. However, the extent to which root traits (size and structure) determine plant presence in water-limited environments is still poorly understood. Here we investigated how root traits vary across water availability gradients within a dry South African biome. Location: South Africa. Methods: We measured six below-ground (root) and eight above-ground (leaf + stem) traits of 124 individuals of nine dominant woody shrub species from wetter and drier sites (600–700 vs 250–300 mm annual precipitation) in the Fynbos biome of the Cape Floristic Region. Within sites, we sampled from recently burnt and unburnt/more mature vegetation and at three locations along topographical gradients. Results: Drier regions showed greater maximum rooting depth, length, root dry matter content and root to shoot ratio. These trait patterns were consistent at an intraspecific level, along locally drier topographical locations and in post-fire environments. Roots accounted for significant whole-plant trait variation. Additionally, in drier conditions, we found increased root allocation deviating from expected global allometric relationships. Our study also demonstrates that the combination of fire and drought in the driest locations results in poor above-ground vegetation recovery in terms of plant size, cover and individual counts with only resprouters persisting. Conclusions: Our research suggests that root investment in Fynbos shrubs will likely be key for coping with a drier and warmer future and should be a focus of more research for dryland biomes.