Drought reduces grassland biodiversity and productivity globally. Therefore, it is important to understand the mechanisms of drought resistance in grassland species. However, the traits underlying species differential drought resistance are still poorly resolved. Here we tested the roles of various root traits in determining grassland species drought resistance. We hypothesized that root traits that enhance plants water uptake capacity increase species survivals under drought.
We transplanted 41 common temperate grassland species into field plots exposed to a dry and a wet treatment, and assessed plant survival, rooting depth, root:shoot ratio (RSR) and midday leaf water potential under drought (ΨMD). Additionally, root diameter, specific root length (SRL) and root density (RD) were measured in plants growing in pots in a greenhouse.
We found that rooting depth in dry and wet plots was positively associated with ΨMD, and with plant survival under drought. In forbs, higher plasticity of rooting depth response to drought enabled drought resistant species to extend their root to deeper soil layers than drought sensitive ones, while in grasses root plasticity was unrelated to survival. Unexpectedly, RSR was negatively associated with survival. SRL and RD did not relate to survival, and root diameter showed opposite associations with survivals for grasses (positive) and forbs (negative).
The significant association of rooting depth and its plasticity with drought resistance of grassland species support the idea that desiccation avoidance strategy is important for grassland species to tolerate drought. The general lack of significant association of root morphology traits and the negative association of RSR with plants survival suggest that access to deep soil layer is more important to maintain high midday leaf water potentials and survivals under drought than biomass allocation or root morphology. Our study shows the importance of rooting depth in promoting plants drought resistance in grassland species.