The growth resilience of forests to extreme drought event has become an urgent topic in global change ecology because of exacerbated water constraints upon trees' growth over the last few decades. Yet, surprisingly little is empirically known about the contribution of stand age, a key factor influencing forest structure and ecological processes, to variation in growth resilience among stands. This study revealed discrepancies in the drought resilience of forests of different stand ages by analyzing an extensive tree-ring dataset from Qinghai spruce (Picea crassifolia Kom.), a typical moisture-sensitive tree species in northwestern China. We found that older growth Qinghai spruce forest stands have higher resistance to droughts than do younger growth ones. Conversely, however, the post-drought recoveries of these older growth forest stands are lower than those of the younger growth stands. Patterns in the variation of resilience indices were consistent between two contrasting hydrological niche regions, whereas the stand age-related discrepancies in drought resilience became significantly smaller going from the wetter region to the drier region. These findings imply that, instead of a one-size-fits-all strategy, more meticulous and more targeted strategies are needed to enhance forest management and strengthen forest conservation given the experienced and projected climate trends, which feature increasing precipitation but higher extreme-drought frequency across this spruce tree's habitat and distribution in northwestern China.

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.