Drought resilience and legacy effects in two forest tree species on Loess Plateau of China: Growth and water-use efficiency under different drought conditions

As droughts become more frequent and severer, understanding tree resilience and its role in mediating drought legacy effects (LEs) is critical for predicting forest ecosystem responses to future droughts and informing forest management. Both Pinus tabuliformis and Populus davidiana are widely distributed in the Loess Plateau region of western China and play important roles in provision of ecosystem services. In this study we quantified the LEs and resilience, including resistance (Rt) and recovery (Rc), of radial growth (BAI, basal area increment) and intrinsic water use efficiency (iWUE) of the two species, determined the external and internal factors influencing Rt and Rc, and disentangled the respective contribution of Rt and Rc to LEs in the these two tree species. We found either negative or positive legacy effects in BAI (LE_BAI) and iWUE (LE_iWUE) in both species, mostly lasting for 1–3 years. Species differences were only detected in LE_iWUE during the severer drought event. But species variation in resilience did not differ. P. tabuliformis exhibited lower Rt but higher Rc than P. davidiana. Tree diameter and drought intensity were negatively correlated with Rt and Rc; whereas tree age and growth variability positively influenced both resilience components. In P. tabulaeformis, the influence of Rt was stronger on LE than on Rc during the milder droughts, whereas during the severer droughts LE was affected by Rc. The reversed patterns of the effects were exhibited by P. davidiana. Our findings help advance current understanding on the factors driving resilience and how trees use different resilience strategies under different drought conditions to alleviate negative LEs.