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Internal water storage in the stem provides additional water for transpiration and photosynthesis and thus ensures tree hydraulic safety and growth under stressed conditions. Comprehensive understandings in elevation- and species-specific patterns of stem water relations and the underlying mechanism could shed light on the physiological foundation of forest conservation and management in the montane area. Here, stem radius variations of co-occurring Larix principis-rupprechtii and Picea meyeri were monitored at three sites of different elevations (2200 m, 2400 m, and 2600 m a.s.L.) in 2018 and 2019 using point dendrometers. The seasonal cycle was divided into four periods: spring rehydration, summer growth, autumn contraction, and winter dormancy. Tree water deficit-induced stem shrinkage (TWD) during the summer growth and daily stem cycles in each period were extracted and compared between species and among elevations, and their responses to environmental factors were determined. The results showed an elevational trend for averaged TWD in L. principis-rupprechtii instead of P. meyeri. TWD in both tree species had a closer relationship with vapor pressure deficit (VPD), while the TWD showed piecewise changes with species-specific differences as the VPD increased. The daily amplitude of L. principis-rupprechtii was smaller than that of P. meyeri in periods of winter dormancy and spring rehydration, however, the comparison was opposite during summer growth and autumn contraction. Furthermore, the daily amplitude of L. principis-rupprechtii and P. meyeri had different responses to the air and soil hydrothermal conditions in different periods over the year. Our study highlights interspecific water-use behaviors between larch and spruce, indicating their contrasting responses to a global increase in drought stress, especially in low elevations.

In the context of rapid warming, a better understanding of the spatially varying growth-climate responses of widely-distributed forests is essential for predicting the warming effects on trees, so as to guide the forest management. Here, we characterized the growth-climate relationships of the southern part of Asian boreal forests in northeast China at multiple levels to deduce how warming impacts tree growth. Specially, we aimed to distinguish climatic limitations to growth and clarify how local climate determines the response pattern. We established a tree-ring network, which includes 516 trees from 20 populations across the study area. We then correlated regional, population, and tree chronologies with climate data. In the north of the study area, radial growth responded positively to temperature before the growing season and precipitation during the growing season. In contrast, growth in the south responded positively to precipitation and negatively to temperature during the growing season. Thus, diverse growth patterns exist in response to climate. The extent to which growth was limited by climate varied along the latitudinal climate gradient. The north parts were limited by both frost and moisture, whereas the central-south parts were limited by moisture deficit induced by high temperature. The growth-climate response was quantified as the percentage of significant tree-level correlations. Through associating this quantitative index of each population with local climate, we confirmed the decisive role of local temperature in forming the spatial heterogeneity of response. We also confirmed that warming negatively affected tree growth across the study area. Our models predict that as warming continues, the trees threatened by high temperature in summer will increase by 45.5%–63.6% by the end of this century. All these findings warn of growth declines in Asian boreal forests, and suggest the need for urgent efficient regionally-specific measures to sustain the forest productivity in this region.