Growing season extension does not lead to uniform growth response in Qinghai spruce

Mountain forests experience some of the strongest impacts of global warming among terrestrial ecosystems, particularly due to exacerbated warming-induced moisture deficits. Previous tree-ring width-based studies have sometimes yielded controversial perspectives on historical and projected relationships between tree growth and climate, likely also due to underestimation of the effects of alteration in the thermal growing season. Here, we established a tree-ring network of Qinghai spruce (Picea crassifolia Kom.) across the northeastern Qinghai-Tibet Plateau, and applied a generalized additive model (GAM) framework with temporally flexible and physiologically relevant climate series to (ⅰ) assess species' climate-dependent growth dynamics and (ⅱ) project its future (2021–2060) growth trend. Over the generated site-specific optimal growth thermal range (OGTR), our results revealed an obvious upward trend of growing season length, mean temperature, and total precipitation. Specifically, the extension of growing season length was characterized by both earlier start and later end of the anchoring dates. Meanwhile, the temporal dynamics of growth–climate relationships at most sampling sites indicated that drought limitation on Qinghai spruce has weakened in recent decades. Following episodic extreme drought events, trees can recover their growth to pre-drought levels within no more than four years. Model projections suggest that tree growth will benefit from future climate changes and facilitate forest carbon sequestration at the species-level. However, growth changes will not be uniform across the species' distribution range, particularly the westernmost populations, which are projected to experience a decline in growth. Overall, this study provides new insight into how mountain conifers respond to concurrent warming and moisture changes.