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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.

Under the background of global warming, the trends and variabilities of different grades of precipitation have significant effects on the management of regional ecosystems and water resources. Based on a daily precipitation dataset collected from 148 meteorological stations in the Yangtze River Basin from 1960 to 2017, precipitation events were divided into four grades (small, moderate, large, and heavy precipitation events) according to the precipitation intensity to analyze the temporal and spatial change trends of different grades of precipitation amounts and frequencies, and the influence of different grades of precipitation on total precipitation was also discussed in this study. The results revealed that small precipitation amounts over the Yangtze River Basin decreased significantly, with a rate of −1.22%/10a, while heavy precipitation amounts showed a significant increasing trend (4.27%/10a) during the study period. The precipitation frequency of small and total events decreased significantly, with rates of −3.86%/10a and −2.97%/10a, respectively. Regionally, from the upper reaches to the lower reaches of the Yangtze River Basin, the contribution rate of small precipitation amounts and frequencies to the total precipitation gradually decreased, while heavy precipitation amounts and frequencies increased. The different grades of precipitation in region II showed a decreasing trend due to its unique geographical features. Furthermore, a Pearson correlation analysis was used to analyze the response of precipitation to long-term air temperature, demonstrating that small and moderate precipitation amounts and frequencies were mainly negatively correlated with long-term air temperature and that heavy precipitation amounts showed a stronger positive correlation with long-term air temperature (13.35%/K). Based on this, the rate of change in heavy precipitation in the Yangtze River Basin may be higher under the background of climate warming, which will lead to greater risks of extreme floods in the future. Evaluating and predicting the trends of different grades can provide a theoretical reference for agricultural production, flood control, and drought mitigation.

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.

Based on daily mean temperature measurements at 54 meteorological stations throughout the Tibetan Plateau (TP) from 1960 to 2009, the trends in the thermal growing season were investigated. The thermal growing season was defined as the period beginning on the last day of the first 6-day period with a daily mean temperature greater than 5 °C after the last spring frost (daily mean temperature

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.