LAUSR

Abstract Droughts of increasing frequency and severity are expected to have a great impact on vegetation restoration in semiarid ecosystems by reducing water availability to plants. Understanding the functional plasticity of root systems in plant water uptake and the response to extreme droughts is of critical importance for predicting revegetation dynamics under future extreme climates. Sampling was undertaken over a two-year period to investigate the seasonality of water use strategy and growing status of one mature exotic shrub species Caragana korshinskii and one native subshrub species Artemisia gmelinii in both its young and mature growing stages. We investigated stable isotopes, made in situ soil water observations, and recorded functional root distribution. The spring and summer drought in 2015, especially the record low precipitation in July, allowed us to examine the impact of extreme drought on plant water sources and stem mortality. The results revealed that both exotic and native species exhibited significant seasonal patterns in water source use, following shifts in soil water content; they were overall positively correlated with soil water availability. The exotic species C. korshinskii used more soil water from subsurface layers compared to the native species A. gmelinii. The prolonged drought reduced the plasticity of water use strategy and resulted in clear stem mortality of both the young and mature native species but not the exotic species. The results indicate that the native shrub A. gmelinii is less resistant to extreme natural drought than exotic shrub C. korshinskii and hence could be more severely affected by future extreme climatic episodes. This implies that vegetation restoration may be greatly disturbed by extreme droughts in semiarid ecosystems.