Altitudinal variation in drought resilience of Tianshan spruce (Picea schrenkiana) in the Middle Tianshan Mountains, China

Understanding how forest resilience varies with altitude is critical for predicting drought responses in mountain ecosystems. As a major water source for arid Central Asia, the Tianshan Mountains have recently experienced severe drought stress, yet the altitudinal variation in forest resilience remains poorly resolved. Here, we analyzed 232 Picea schrenkiana trees from 12 plots spanning the full altitudinal range (1,500–2,700 m) in the Middle Tianshan Mountains of China. We quantified drought responses using multiple resilience indicators, including Lloret indices, recovery period (RE), and the “line of full resilience”. These indicators were applied to compare spruce responses across altitudes under mild (−1.5 < standardized precipitation-evapotranspiration index (SPEI) ≤ −1.0) and severe drought (SPEI ≤ −1.5). The results showed that low-altitude spruce exhibited a typical “low resistance-high recovery” strategy, while high-altitude spruce showed “high resistance-low recovery”, with mid-altitude trees forming a transition. For instance, under severe drought, low-altitude trees showed the greatest growth reduction but recovered within about three years, while high-altitude trees showed the smallest reductions yet failed to fully recover within four years. A three-year window analysis confirmed the robustness of drought resilience patterns. We identified the primary drivers of drought resilience at different altitudes using multiple regression models, with pre-drought growth (RWpre) being the most important for low-altitude resilience; drought intensity and RWpre were the main drivers at mid- and high-altitudes. Boosted regression trees (BRT) analysis revealed that spruce recovery depended less on post-drought moisture with increasing altitude. These results demonstrate that long-term environmental conditions have shaped different adaptive strategies along the altitudinal gradient. Low-altitude forests should prioritize water stress mitigation rather than growth stimulation. Moderate thinning may enhance resistance at mid-altitudes, and high-altitude spruce requires long-term monitoring due to its limited recovery capacity. Given the increased frequency of future drought events, this study provides insights into mountain forest vulnerability and emphasizes the necessity for altitude-tailored management approaches.