Climate Warming Alters Age-Dependent Growth Sensitivity to Temperature in Eurasian Alpine Treelines

Age-specific response to climate factors and extreme drought events in radial growth of Picea likiangensis

The influence of tree age on the growth response of Picea likiangensis, a predominant timber species in southwestern China, to climatic factors has been under-researched. In this study, we examined the relationships between tree age and the response of P. likiangensis to climatic factors and extreme drought events using tree-ring samples procured from the southeastern edge of the Tibetan Plateau. The results revealed differential responses of the radial growth of P. likiangensis trees of varying ages to climatic factors and extreme drought events. Specifically, deficient water availability during the early growing season emerged as the principal factor constraining radial growth across all age classes. Young and middle-aged trees (<100 years) demonstrated greater responsiveness to water availability than did mature trees (>100 years). Mature trees, in contrast, demonstrated markedly greater resistance to extreme drought events than young and middle-aged trees. Comparative studies of individual trees across different ages revealed negligible differences in the response of young and middle-aged trees to climatic factors and extreme drought events. Given these responses, future forest management practices should prioritize young and middle-aged trees that are more affected by drought to maximize the ecological value of the species. According to the specific research objectives, sample collection processes should classify mature trees and young and middle-aged trees, to minimize the influence of tree age on the final findings of the study.

Global fading of the temperature-growth coupling at alpine and polar treelines

Climate warming is expected to positively alter upward and poleward treelines which are controlled by low temperature and a short growing season. Despite the importance of treelines as a bioassay of climate change, a global field assessment and posterior forecasting of tree growth at annual scales is lacking. Using annually resolved tree-ring data located across Eurasia and the Americas, we quantified and modeled the relationship between temperature and radial growth at treeline during the 20th century. We then tested whether this temperature-growth association will remain stable during the 21st century using a forward model under two climate scenarios (RCP 4.5 and 8.5). During the 20th century, growth enhancements were common in most sites, and temperature and growth showed positive trends. Interestingly, the relationship between temperature and growth trends was contingent on tree age suggesting biogeographic patterns in treeline growth are contingent on local factors besides climate warming. Simulations forecast temperature-growth decoupling during the 21st century. The growing season at treeline is projected to lengthen and growth rates would increase and become less dependent on temperature rise. These forecasts illustrate how growth may decouple from climate warming in cold regions and near the margins of tree existence. Such projected temperature-growth decoupling could impact ecosystem processes in mountain and polar biomes, with feedbacks on climate warming.