Although stand age has been shown to strongly affect forest ecosystem processes, little is known about the role that forest stand age plays in tree radial growth processes. The knowledge on geographical patterns of the stand age effect on radial growth along climatic gradients is also scarce. Based on dendrological methods and tree-ring cores from 2276 trees at 14 sampling plots, we confirmed that differences in stand age could result in radial growth dissimilarities of Qinghai spruce (Picea crassifolia Kom.), a moisture-sensitive forest tree species endemic to the Tibetan Plateau. However, the effect that stand age has on the radial growth of Qinghai spruce is not consistent. There is evident dissimilarity in the radial growth dynamics of Qinghai spruce from different-aged forest stands in semi-humid regions, but this dissimilarity is minimal in regions under higher drought limitation. Additionally, we observed significant negative correlations in temporal changes of growth concordance of Qinghai spruce from different-aged stands and regional moisture conditions at each study site. It can therefore be concluded that Qinghai spruce will exhibit greater stand age related growth dissimilarities under lower drought limitation. Findings from this study can improve our understanding of biogeographical patterns of moisture-sensitive tree growth that will be necessary to improve future projections of forest dynamics and to guide forest management under a changing climate.

Understanding the spatial-temporal tree growth variability and its associative climatic response is fundamental in the assessment of forest vulnerability and the appraisal of forest risk under climate change. Based on 4219 tree-ring cores from 23 sample plots, this study demonstrates divergent radial growth dynamics of Qinghai spruce (Picea crassifolia Kom.) across a moisture gradient. On the one hand, contrary to the warming-induced growth enhancement of trees in cold regions reported in previous studies, our study find that Qinghai spruce growth has persistently declined under recent warming trends in multiple cold and moist sites that are representative of the Tibetan Plateau, China. On the other hand, Qinghai spruce growth exhibited W-shaped dynamic trajectories that were similar to changes in regional precipitation at all warmer and drier sites investigated in this study. Along with differences in regional evaporative change, differences in the growth-climate response can explain divergences in Qinghai spruce growth observed throughout various hydrological niches. Drought is believed to be the primary limiting factor of Qinghai spruce growth in all regions, resulting in a lower radial growth rate in warmer and drier regions. However, Qinghai spruce growth exhibited a higher dependency on the negatively-affected temperature in cold and moist regions, while the growth dependency respective to the positively-affected precipitation was higher than the temperature in warmer and drier regions. Results from this study are intended to add to the growing knowledgebase of forest response under persistent climate change.