Global variability in hydraulic traits and water use strategies of mountain shrubs and dwarf shrubs.

Shrubs are perennial, multi-stemmed woody plants whose adaptation to stress factors allows them to colonise extreme habitats, including high elevations. Accordingly, shrubs are one of the most important growth forms in mountain regions, but their hydraulic properties are poorly understood. We conducted a literature search on the water use strategies of mountain shrubs, focusing on their main hydraulic traits related to water uptake, transport and release, as well as hydraulic limitations in summer and winter. In addition, the leaf cuticular conductance was measured in selected Alpine species. A total of 104 publications were found, mainly from North America, Europe and Asia, and a few from Africa and South America, with snow and steppe habitats strongly underrepresented. The dataset revealed a wide range of specific hydraulic conductivity (k; 0.8-25.8 × 10 ms MPa), with highest values in tundra shrubs, and of the water potential at 50% conductivity loss (Ψ; -11.8 to -0.29 MPa), with lowest values in steppe and temperate dry summer species. Deep-rooted shrubs from arid environments had access to more reliable water sources, while others relied on shallow but nutrient-rich soil water. No clear trend was observed along elevation or precipitation gradients, suggesting a wide range of hydraulic strategies to achieve a balanced water status. Shrub species from arid regions have to withstand low water potentials during the dry season, whereas temperate shrubs experience frost drought and freeze-thaw-induced embolism in winter. The literature review revealed major gaps in the geographic distribution of available studies, and in our knowledge of root characteristics, recovery from embolism, and water storage capacity.