Divergence in plant water relations mediates the clear niche differentiation of two congeneric desert shrub species.

Desert plants, despite overall well adapted to drought, exhibit considerable inter-specific differentiation of micro-habitats and variation in adaptive characteristics in the generally much water-limited yet spatially heterogeneous water regimes. Investigating the interplay between fine-scale spacial variation of environmental water regime and inter-specific differences in water-related physiology can provide an important basis for understanding the functional diversity and co-existence of desert plants. Here, we addressed this question by taking advantage of the remarkable niche differentiation of two dominant shrub species, Haloxylon ammodendron occupying the inter-dune lowlands and H. persicum colonizing the sand dunes in Gurbantunggut Desert of Northwest China. We precisely quantified the locations of each individual of the two congeneric species in transects established across the inter-dune lowland to dune crest landscape, measured soil water contents at different transect positions, and investigated water-related physiological traits of each individual. We found that the clear niche differentiation between the two sister species was paralleled by divergences in a suite of water-related functional traits. Particularly, such divergence indicates a clear trade-off between efficient water acquisition and drought tolerance. Higher hydraulic efficiency in H. persicum contributed to greater competitiveness in the sand dunes with relatively high soil water availability. In contrast, greater hydraulic safety and higher tissue water storage capacity in H. ammodendron contributed to its endurance in the more drought stressed inter-dune lowlands. Our findings indicate that variation of fine-scale soil moisture due to local factors like topography is a critical driver in shaping the species diversity in the heterogeneous environments of deserts.