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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.

The frequency of stem-growth cessation in Picea crassifolia has increased significantly as the frequency of extreme climate events has increased. Extreme climatic events are increasingly recognized as important drivers of tree growth, forest dynamics, and range contractions. Understanding tree growth responses to extreme events is important for forest conservation and management, especially under climate change. Here, we studied the patterns of growth cessation of Picea crassifolia Kom., an endemic species to Central Asia, across its distributional range in the Helan Mountains of Northwestern China, to test the hypotheses that (1) tree growth and growth cessations are limited by moisture availability, and (2) that this relationship is constant over the diverse set of conditions covered in our elevational gradient. While tree growth across our gradient was significantly limited by low precipitation in June, we found growth cessation events at the lower distributional margin to increase in severity and frequency in recent decades. We found that the combination of low precipitation early in the monsoon season (June) and high mid-summer (July) vapor pressure deficit likely caused the increased frequency of growth cessation events. Because these populations are already experiencing a high frequency of growth cessation events, the mortality rate of the lower distributional margin trees could further increase if the current trends of decreasing moisture and increasing heat stress continue into the future. Our results strongly suggest that growth cessation events are more than the dendrochronological curiosity they are usually considered to be, and can be important indicators of increased tree growth stress and be potentially useful in identifying tipping points prior to forest change.

The increment duration and rate were equally important in controlling annual stem radial increments of Picea meyeri at its upper limit of distribution. Stem radial increment rate and duration are the most important parameters in determining the width of annual tree rings. To identify the contributions of rate and duration to annual radial increments and their relationships with environmental factors, we analyzed intra-annual stem increments of five Picea meyeri trees in a sub-alpine habitat of North-Central China over 7 years (2008–2015, except for the year 2012) with point dendrometers in this study. The results showed the following. (1) We estimated that approximately 53% of the variability in the annual radial increments is attributable to the rate of radial increment and approximately 47% to its duration. (2) The contribution of cessation time to the annual increments was more than three times that of the initiation time. (3) The initiation of radial increment was primarily controlled by soil temperature and warmer soil temperature could advance the initiation time. The cessation and rate of radial increment were mainly influenced by thermal and light-related environmental factors. During growing seasons, low temperatures and insufficient light caused by many rainy and cloudy days at the high altitudes of the Luya Mountains may result in earlier cessations and lower rates of radial increment. Overall, our results may have further applications in modeling the responses of tree stem growth to climate change in a sub-alpine habitat of North-Central China.

Understanding interspecific differences in tree water use will aid in the assessment of both tree-level ecophysiological adaptation to climate change and forecasts of forest dynamics. We investigated the seasonal variation of water sources between two co-occurring trees species with contrasting leaf phenology and rooting traits: the deciduous Larix principis-rupprechtii Mayr. and the evergreen Picea meyeri Rehd. et Wils. At weekly/biweekly intervals from September 2013 to October 2014 in the Luya Mountains in North-Central China, we collected and analyzed a total of approximately 2400 samples of δD and δ18O in tree xylem water, potential water sources for all study trees, and the contribution of water at different soil depths. Concurrently, we monitored leaf phenology by direct observation and wood phenology with the microcore method. Microcoring allowed us to trace intra-annual dynamics of word formation (i.e., onset, end, and maximum growth rate). These results, including a seasonal origin index, indicated that winter snowmelt water is sourced for growth initiation for both larch and spruce, although larch relies on it more than spruce. Larch and spruce mainly absorbed water from the same soil layer of 10–20 cm during the growing season (circa 38.9% and 37.5% of total water uptake, respectively). However, this potential inter-specific water competition did not increase until growth rates reached the maximum for the year; larch used more water from deeper soil layers while spruce used water generally equally from each soil layer. Unlike deeper-rooting larch, the more shallow-rooted spruce showed a greater ability to shift water uptake among various soil layers. This plasticity in water uptake was accompanied by tighter stomatal regulation, suggesting spruce growth is generally more tightly coupled to water availability. Such diverging species-specific water use strategies improve our knowledge on tree-level ecophysiological mechanisms, with implications for understanding ecosystem-level forest dynamics and potential resilience to environmental stress.

Changing localized environmental conditions along altitude gradients could modify the responses of tree xylem structures to climate variations. To explore how trees adapt to local environment and respond to climate variants, we analysed the variation of Picea meyeri xylem tracheid features and the relationships with climate variables along an altitude gradient (1970, 2240, 2490 and 2650 m above sea level) of the Luyashan Mountains in the North China. The results mainly indicated the following: (1) tracheid number and diameter of P. meyeri showed significant differences among the four sites along the altitude gradient; (2) trees at site 2 (2240 m) and site 3 (2490 m) are similar in age, but the xylem tracheid number and diameter of these trees were significantly different, which may indicate different functional adaptation; (3) the relationships between xylem features' residual chronologies and the monthly climate data were inconsistent along altitude gradients, which indicated that the limiting factor of P. meyeri growth along the altitude gradients, shifted from drought stress at lower altitudes to low-temperature stress at higher altitudes.

Fine-resolution studies of stem radial variation over short timescales throughout the year can provide insight into intra-annual stem dynamics and improve our understanding of climate impacts on tree physiology and growth processes. Using data from high-resolution point dendrometers collected from Platycladus orientalis (Linn.) trees between September 2013 and December 2014, this study investigated the daily and seasonal patterns of stem radial variation in addition to the relationships between daily stem radial variation and environmental factors over the growing season. Two contrasting daily cycle patterns were observed for warm and cold seasons. A daily mean air temperature of 0 &deg, C was a critical threshold that was related to seasonal shifts in stem diurnal cycle patterns, indicating that air temperature critically influences diurnal stem cycles. The annual variation in P. orientalis stem radius variation can be divided into four distinct periods including (1) spring rehydration, (2) the summer growing season, (3) autumn stagnation, and (4) winter contraction. These periods reflect seasonal changes in tree water status that are especially pronounced in spring and winter. During the growing season, the maximum daily shrinkage (MDS) of P. orientalis was positively correlated with air temperature (Ta) and negatively correlated with soil water content (SWC) and precipitation (P). The vapor pressure deficit (VPD) also exhibited a threshold-based control on MDS at values below or above 0.8 kPa. Daily radial changes (DRC) were negatively correlated with Ta and VPD but positively correlated with relative air humidity (RH) and P. These results suggest that the above environmental factors are associated with tree water status via their influence on moisture availability to trees, which in turn affects the metrics of daily stem variation including MDS and DRC.

Responses of tree growth to climate are usually spatially heterogeneous. Besides regionally varying external environments, species specificity is a crucial factor in determining said spatial heterogeneity. A better understanding of this species specificity would improve our estimations of the warming effects on forests. In this study, we selected two widely-distributed boreal conifers, Dahurian larch (Larix gmelinii) and Mongolian pine (Pinus sylvestris var. mongolica), to compare their growth-climate responses, including long-term growth-climate correlations and short-term growth resilience to drought. We sampled 160 trees and 481 tree-ring cores from the two species in two pure and two mixed forests, located in the Greater Khingan Range, northeast China. We found that Dahurian larch was generally positively correlated with spring temperature and negatively correlated with summer temperature. In contrast, Mongolian pine was more sensitive to summer moisture. Our results suggest that the main climatic limitations were low spring temperatures for Dahurian larch and summer moisture deficits for Mongolian pine. Dahurian larch represented higher growth resistance to drought, while Mongolia pine represented higher recovery. Based on this, we inferred that Dahurian larch was more vulnerable to extreme droughts, while Mongolian pine was more vulnerable to frequent droughts. We also demonstrated the effects of forest type on growth-climate responses. The negative effects of summer temperatures on Mongolian pine seemed to be more significant in mixed forests. As warming continued, Mongolian pine in this area would suffer severer moisture deficits, especially when coexisting with Dahurian larch. Our results suggest that Dahurian larch gained an advantage in the competition with Mongolian pine during high moisture stress. Driven by the warming trends, the species specificity in growth response would ultimately promote the separation of the two species in distribution. This study will help improve our estimations of the warming effects on forests and develop more species-targeted forest management practices.

Key message The daily stem radial growth of P. ori- entalis during the main growing period in semi-arid areas of North China is mainly limited by moisture availability. Abstract Finer resolution studies of tree stem radial growth on short temporal scales can be useful for exploring the main environmental variables that trigger radial growth. The objective of this study was to determine the most important environmental factors that impact the intra- annual radial growth of Platycladus orientalis (Linn.) Franco in a semi-arid area of North China. The stem radial growth of P. orientalis was studied daily during the growing season (May to October 2013) using high-reso- lution automatic point dendrometers. Our results showed that the daily stem radial increment (SRI) of P. orientalis during the main growing period was primarily impacted by relative air humidity, precipitation, vapor pressure deficit and daily maximum air temperature. Those environmental factors mainly influenced the moisture availability to the trees, thereby affecting their daily stem radial growth. The rapid release of stem radial growth was observed when heavy precipitation events occurred after a drought period. The maximum stem growth rate of P. orientalis was observed in early August and was regulated by soil water content in the summer; this was regarded as an adaptation to the local climate conditions. Our results have further implications for sensitivity analyses of tree growth to cli- mate changes in semi-arid areas.

Daily mean air temperatures from 81 meteorological stations in Northeast China were analyzed for the spatiotemporal change of the climatic growing season during the period 1960–2009. Our results showed that latitude strongly influenced the spatial patterns of the mean start (GSS), end (GSE), and length (GSL) of the growing season. For the area studied, a significant increasing trend in GSL during 1960–2009 was detected at a significance level of 0.01, especially after the early 1980s. The area-average GSL has extended 13.3 days during the last 50 years, mainly due to the advanced GSS evident in the spring (7.9 days). The variations of GSS and GSE were closely correlated with the monthly mean temperature (T mean) of April and October, respectively, while GSL was closely related to the monthly minimum temperatures (T min) of spring (March to April) and autumn (September to October). The distributions of the trends in growing season parameters (GSS, GSE, and GSL) showed great spatial variability over Northeast China. Significant relationships between altitude and the trend rates of the GSS and GSL were detected, while geographic parameters had little direct effect on the change in GSE. This extended growing season may provide favorable conditions for agriculture and forest, and improve their potential production.

North-Central China is a region in which the air temperature has clearly increased for several decades. Picea meyeri and Larix principis-rupprechtii are the most dominant co-occurring tree species within the cold coniferous forest belt ranging vertically from 1800 m to 2800 m a.s.l. in this region. Based on a tree-ring analysis of 292 increment cores sampled from 146 trees at different elevations, this study aimed to examine if the radial growth of the two species in response to climate is similar, whether the responses are consistent along altitudinal gradients and which species might be favored in the future driven by the changing climate. The results indicated the following: (1) The two species grew in different rhythms at low and high elevation respectively; (2) Both species displayed inconsistent relationships between radial growth and climate data along altitudinal gradients. The correlation between radial growth and the monthly mean temperature in the spring or summer changed from negative at low elevation into positive at high elevation, whereas those between the radial growth and the total monthly precipitation displayed a change from positive into negative along the elevation gradient. These indicate the different influences of the horizontal climate and vertical mountainous climate on the radial growth of the two species; (3) The species-dependent different response to climate in radial growth appeared mainly in autumn of the previous year. The radial growth of L. principis-rupprechtii displayed negative responses both to temperature and to precipitation in the previous September, October or November, which was not observed in the radial growth of P. meyeri. (4) The radial growth of both species will tend to be increased at high elevation and limited at low elevation, and L. principis-rupprechtii might be more favored in the future, if the temperature keeps rising.

The Stem Radial Growth (SRG) was measured by dendrometer from 6 sample trees of Picea meyeri at the tree-line of Luya Mountain, North China, combined with synchronized instrumental observation and measurement of habitat temperature, moisture, etc., from July the 15 th to August the 7 th (summer phase) and from September the 5 th to October the 9 th (autumn phase) in 2007. The preliminary analytic results were as follows: 1) the diameter of stems varied on sunny days with a daily fluctuation but steadily widened, while on rainy days it continuously increased during the rain but shrank rapidly after the rain. 2) The SRG pattern in both phases was fitted with a quadratic curve, showing a positive value in summer phase; whereas declining to a negative figure in the first stage of autumn phase --early September, and then kept as a constant zero from middle September on. 3) The distance analysis showed a relative warmer and moister environment was suitable for the growth of Picea meyeri in autumn phase, whereas an environment with a high relative humidity (RH) in the air and moisture in the soil was suitable in summer phase. Thus the water regime was the controlling factor for Picea meyeri growth on treeline of Luya Mountain.