Contrasting Drought Thresholds Between Natural and Planted Forests on the Tibetan Plateau: Implications for Adaptive Plantation Transformation

Climate-induced drought is a major threat to forest growth and ecosystem stability. However, the long-term effects of drought on tree growth, especially across forests with contrasting stand structures, remain poorly understood. Based on 136,988 live tree records monitored from 1979 to 2017 in the southeastern Tibetan Plateau, we assessed how drought intensity, species richness, tree density, and stand age influence long-term forest growth responses in natural and plantation forests. Plantation forests experienced more intense drought and showed lower drought tolerance thresholds (Palmer Drought Severity Index (PDSI) = -1.65) than natural forests (PDSI = -2.50). Species richness improved drought resistance under prolonged drought, yet suppressed growth in highly diverse natural forests. In plantations, moderate diversity levels (richness = 3-5) best balance growth and resistance. Younger stands (< 35 years), particularly plantations under 20 years old, were most sensitive to drought. High tree density (> 1250 stems/ha) amplified negative density dependence, reducing growth, and drought resistance. This study integrates natural and plantation forests to propose a novel structural threshold framework for enhancing plantation drought resistance. It suggests that targeted thinning and enrichment planting to moderate diversity levels in young, dense plantations can enhance drought resistance without compromising productivity. Promoting near-natural restoration in plantations may further strengthen key ecosystem services-such as carbon sequestration and water retention-under a changing climate.