Water uptake depth is coordinated with leaf water potential, water‐use efficiency and drought vulnerability in karst vegetation

Summary Root access to bedrock water storage or groundwater is an important trait allowing plant survival in seasonally dry environments. However, the degree of coordination between water uptake depth, leaf‐level water‐use efficiency (WUEi) and water potential in drought‐prone plant communities is not well understood. We conducted a 135‐d rainfall exclusion experiment in a subtropical karst ecosystem with thin skeletal soils to evaluate the responses of 11 co‐occurring woody species of contrasting life forms and leaf habits to a severe drought during the wet growing season. Marked differences in xylem water isotopic composition during drought revealed distinct ecohydrological niche separation among species. The contrasting behaviour of leaf water potential in coexisting species during drought was largely explained by differences in root access to deeper, temporally stable water sources. Smaller‐diameter species with shallower water uptake, more negative water potentials and lower WUEi showed extensive drought‐induced canopy defoliation and/or mortality. By contrast, larger‐diameter species with deeper water uptake, higher leaf‐level WUEi and more isohydric behaviour survived drought with only moderate canopy defoliation. Severe water limitation imposes strong environmental filtering and/or selective pressures resulting in tight coordination between tree diameter, water uptake depth, iso/anisohydric behaviour, WUEi and drought vulnerability in karst plant communities.