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• Different periods were determined according to the pattern of the daily stem cycle. • Interspecific differences in tree water deficit declined with elevations. • Spruce showed greater daily amplitude than larch in spring rehydration rather than summer growth. • Opposite responses of stem water relations to high vapor pressure deficit between the two conifers. 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. [ABSTRACT FROM AUTHOR]

Transitional climate zones (TCZ) are characterized by instability due to rapid changes in climate and biological variables, and trees growing there are particularly sensitive to climate change. Therefore, knowledge about the shifted relationships of tree growth in response to climate warming will shape regional forest conservation and management strategies. China has experienced rapid warming in recent decades. However, how tree growth in semihumid to semiarid regions, such as the Guandi Mountains, responds to more sophisticated changes in the hydrothermal combination is not yet clear. In this study, we used tree-ring width data from three sites along an elevational gradient in the Guandi Mountains to present the response of Picea wilsonii Mast. radial growth to increasing temperature and elevational differences in the relationship between tree growth and climate. The results indicated that the Guandi Mountains have experienced rapid warming with a clear trend toward aridity. From 1959 to 1995, the radial growth of P. wilsonii was mainly influenced by temperature, while it was controlled by both temperature and precipitation after rapid warming in 1996. From 1959 to 2017, this species showed a generally consistent growth–climate relationship at different elevations in the Guandi Mountains. However, the radial growth of trees at higher elevations had a higher climatic correlation than at lower elevations, and it was more conditioned by higher summer temperatures and precipitation in December of the previous year. These results suggested that P. wilsonii was more susceptible to drought and high temperatures due to a warming climate and that more attention should be devoted to forest management, especially the adverse consequences of summer drought on P. wilsonii. [ABSTRACT FROM AUTHOR]

• The Weibull function was used to fit stem radius variations in cold seasons. • Stem shrinkage in Picea meyeri was greater on the forest edge after stem freeze. • The diurnal temperature range regulated the freeze-induced seasonal and diurnal variations in stem size. • Stem freeze or thaw was active when the daily minimum air temperature was around −2.5 °C. Dendrometer-derived stem radius variations (SRV) have been demonstrated as useful tools to test cold acclimation, which could reflect the tree water (ice) balance in winter and thus the frost stress. However, most previous studies focus on stem increment and water relations during growing seasons, stem radius variations in cold seasons and the underlying tree physiological information are not fully exploited, limiting our understanding of the response of mid- and high-latitudinal forests to warming winter. Hourly SRV in spruces (Picea meyeri) across an elevational transect of 700 m were observed over 6 consecutive years (2012–2017) in the present study. Elevation-specific variations in the onset of stem freeze and thaw, and the magnitude of seasonal and diurnal SRV in cold seasons were explored, and ultimately their dominant environmental factors were determined. Before stem freeze, seasonal and diurnal SRV showed linear trends with elevation, which were mainly affected by air and soil water conditions. Whereas, after stem freeze, the seasonal SRV and thus the stem dehydration was greater at the lower and upper limits of the forest belt, and the daily amplitude of freeze-thaw cycles was greater at high elevations. The diurnal air temperature range dominantly controlled the SRV on different time scales after stem freeze. Stem freeze and thaw in P. meyeri was active when the daily minimum air temperature was -2.5 °C in our study region. The onset of stem freeze and thaw advanced and delayed with elevation corresponding to a rate of 5.2 and 7.5 days/ °C when adjusted for the monitored lapse rate of 0.63 °C per 100 m, respectively. Together these findings highlight the value of high-precision variations in stem size to indicate the potential freezing stress, and trees growing at high elevations might suffer greater stress caused by frost dehydration and freeze-thaw under ongoing climate change. [ABSTRACT FROM AUTHOR]

• P.meyeri tends to increase soluble sugars and starch under low soil moisture conditions. • Needle soluble sugars were more sensitive to air temperature than other organs. • NSCs in the shoots and stems were affected by both soil moisture and atmospheric factors. • Stem soluble sugars had a different response to soil moisture and vapor pressure deficit. • Root NSC reserves may be important for P.meyeri to respond to low soil moisture. Nonstructural carbohydrate (NSC) dynamics have been suggested to be an important trait reflecting carbon balance under environmental changes. However, our understanding of NSC storage dynamics, their controls, and their responses to environmental factors remain unclear. In this study, we selected the evergreen conifer Picea meyeri at five altitudes (2040 m, 2260 m, 2440 m, 2600 m, and 2740 m a.s.l.) on Luya Mountain, North-central China. NSC concentrations in the needles, shoots, stems, and roots were measured, and the environmental variables were monitored during the growing seasons in 2018 and 2019. The results showed that the NSCs first decreased slightly and then increased gradually as altitude increased in the needles, shoots, stems, and roots. The NSC concentrations with the highest levels occurring in the needles and the lowest in the stems, whereas the shoots and roots showed intermediate concentrations. Moreover, soil moisture was the major factor influencing the dynamics of NSC storage in P. meyeri. The concentrations of soluble sugars and starch in all four organs increased with decreasing soil moisture, except for needle starch. In particular, the soluble sugars in the needles were significantly negatively correlated with air temperature, and those in the stems had a different response to soil moisture and vapor pressure deficit (VPD). Furthermore, root NSCs reserves may be important for P. meyeri to respond to low soil moisture. These findings provide new insights for understanding tree NSC allocation and the mechanism of stress resistance. [ABSTRACT FROM AUTHOR]

• The magnitude and duration of tree water deficit were combined to study drought intensity. • Larch showed less limited stem water and a greater increment rate than spruce. • Interspecific differences in annual growth declined in dry years. • Species-specific responses of stem increment to the drought intensity. The heterogeneous responses of mixed forests to climate change are integral to the promotion of the resilience and resistance of forest ecosystems, yet the hydraulic and growth sensitivity of mixed tree species to environmental drivers remain largely unexplored. In this study, stem radial variations in larch (Larix principis-rupprechtii) and spruce (Picea meyeri) were monitored using high-resolution point dendrometers over six years (2014–2019) in a mixed coniferous stand in semi-humid China. The extracted hourly tree water deficit-induced stem shrinkage (TWD) and stem radial increment (SRI) were compared between tree species and among years, and the species-specific responses of the daily maximum TWD and cumulative SRI to environmental variables were fitted by linear mixed models. Averagely, larch had a lower TWD and greater SRI rate than spruce in all six growing seasons. The interspecific differences in the TWD (length, magnitude, and intensity) were significant in dry years, while significant differences in the SRI rates were in wet years. The SRI rate of larch showed a more rapid decrease than that of spruce with increasing intensity of TWD. Furthermore, the TWD of both species was mainly regulated by vapor pressure deficit (VPD) and soil water content (SWC). The TWD of larch had a stronger and earlier response to VPD, while TWD of spruce was more affected by SWC (10 cm depth). The daily SRI of larch was more significantly related to all the measured environmental variables, indicating a more sensitive response to environmental drivers than that of spruce. We identified a shift in competitive advantage from sensitive L. principis-rupprechtii to conservative P. meyeri under extreme drought conditions, providing deep insights into species‐specific water and carbon dynamics in mixed forests. [ABSTRACT FROM AUTHOR]