The climate in arid Central Asia (ACA) has changed rapidly in recent decades, but the ecological consequences of this are far from clear. To predict the impacts of climate change on ecosystem functioning, greater attention should be given to the relationships between leaf functional traits and environmental heterogeneity. As a dominant constructive shrub widely distributed in ACA, Reaumuria soongarica provided us with an ideal model to understand how leaf functional traits of desert ecosystems responded to the heterogeneous environments of ACA. Here, to determine the influences of genetic and ecological factors, we characterized species-wide variations in leaf traits among 30 wild populations of R. soongarica and 16 populations grown in a common garden. We found that the leaf length, width, and leaf length to width ratio (L/W) of the northern lineage were significantly larger than those of other genetic lineages, and principal component analysis based on the in situ environmental factors distinguished the northern lineage from the other lineages studied. With increasing latitude, leaf length, width, and L/W in the wild populations increased significantly. Leaf length and L/W were negatively correlated with altitude, and first increased and then decreased with increasing mean annual temperature (MAT) and mean annual precipitation (MAP). Stepwise regression analyses further indicated that leaf length variation was mainly affected by latitude. However, leaf width was uncorrelated with altitude, MAT or MAP. The common garden trial showed that leaf width variation among the eastern populations was caused by both local adaptation and phenotypic plasticity. Our findings suggest that R. soongarica preferentially changes leaf length to adjust leaf size to cope with environmental change. We also reveal phenotypic evidence for ecological speciation of R. soongarica. These results will help us better understand and predict the consequences of climate change for desert ecosystem functioning.

Leaves were collected from in situ and common garden populations of Reaumuria soongarica in northwest China. A total of 287 R. soongarica individuals were sampled in situ from 30 wild populations. To reduce the sampling bias, all individuals sampled in each population were spaced more than 30 m apart. Fresh leaves of ten individuals of R. trigyna and R. kaschgarica were also sampled in the wild. Additionally, a total of 17 populations spaced at least 100 km apart were included in the common garden trial of R. soongarica. Finally, several undamaged leaves were randomly selected, 10 for each wild individual and 20 per cultivated shrub. All the selected leaves were then placed on a black velvet background and digitally photographed. Measurements of leaf length and leaf width were acquired using the ImageJ image processing software.