Search

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.

Linking the response of tree growth to global warming is a key to fully appreciating the impact of climate change on forests. To examine the impacts of temperature and precipitation on tree growth, we studied the radial growth of Pinus tabulaeformis along an altitude gradient from 2032 m a.s.l. to 2361 m a.s.l. on the Helan Mountains, which is almost the northwestern limit of P. tabulaeformis distribution in China. The results showed that, radial growth of P. tabulaeformis decreased significantly (P

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.

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.

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.

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.