Leaf carbon isotope tracks the facilitation pattern of legume shrubs shaped by water availability and species replacement along a large elevation gradient in Trans-Himalayas

Understanding patterns and mechanisms of nurse plant facilitation is important to predict the resilience of arid/semiarid ecosystems to climate change. We demonstrate whether water availability and nurse species turnover interactively shape the facilitation pattern of widespread legume shrubs along a large elevation gradient. We further investigate whether leaf δ13C of nurse plants can track the facilitation pattern.We measured the relative interaction index (RII) of species number within and outside the canopy of two widespread legume shrub species (Caragana gerardiana and C. versicolor) alternatively distributed along a large elevation gradient in Trans-Himalayas. We further assessed the proportional increase of species richness (ISR) at community level using the paired plot data. To determine site-specific water availability, we measured leaf δ13C of nurse shrubs and calculated Thornthwaite moisture index (MI) for each elevation site.Elevational variations in RII, ISR and δ13C were mainly explained by MI when the effects of soil nitrogen and plant traits (leaf nitrogen and shrub size) were controlled. Variations in RII and ISR across the two nurse species were better explained by δ13C than by smoothly changing climatic factors along elevation. At the transition zone between upper limit of C. gerardiana (4100 m) and lower limit of C. versicolor (4200 m), RII and ISR were much higher in C. versicolor than in C. gerardiana under a similar MI. Such an abrupt increase in facilitation induced by nurse species replacement was well tracked by the variation of δ13C.Water availability and nurse species replacement are critical to shaping facilitation patterns by legume shrubs along a large elevation gradient in dry mountainous regions like Trans-Himalayas. Turnover in nurse species under global change may significantly alter the pattern of nurse plant facilitation associated with water availability, which can be well tracked by leaf δ13C.