Postdoctoral Associate University of Wyoming, United States
Abstract: Evolutionary radiations of woody taxa within arid landscapes were enabled by multiple trait innovations that confer drought avoidance or resistance. Drought avoiders maintain photosynthesis and transpiration rates in dry conditions by extending roots deep into the soil to avoid water limitation. By contrast, drought resisters construct xylem that allows sufficient sap flow under substantial negative xylem pressure. Little is known about how these traits have coevolved across the phylogeny of woody plants, nor about how these traits jointly impact species’ distributions and global change responses. We synthesized global trait datasets to ask how rooting depth and xylem vulnerability across 208 species interact with aridity and water table depth to influence species occurrence probabilities across all major biomes.
Results of a phylogenetic generalized least squares regression demonstrate that xylem vulnerability and maximum rooting depth are independent traits. Results of generalized linear mixed effects models demonstrate that xylem vulnerability and maximum rooting depth are selected by aridity and water table depth. Species with resistant xylem are more likely to occur in arid climates with deep water tables regardless of rooting depth. Species with vulnerable xylem are more likely to occur in humid climates at any water table depth if their roots are shallow, but if their roots are deep then vulnerable species can occur in arid climates. Our results expand the scope of the hydrological niche segregation hypothesis and show that traits related to water uptake and transport explain biogeographic-scale species occurrences across regional and local gradients in water availability. Given that water table depth is relatively independent of regional climate, if evaporative demand changes faster than water table depth under climate change, then vegetation responses to climate change may in the near-term be buffered by stable water table depths.
