Abstract
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 °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.
Highlights
A complete assessment of the response of tree growth to climates over different time scales is critical for understanding the physiological mechanisms of tree growth and their response to environments in the context of global climate change [1]
Ta and vapor pressure deficit (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 maximum daily shrinkage (MDS) and Daily radial changes (DRC)
The drought period occurred in mid–late May and July–August, with soil water content (SWC) generally below 10% and Ta above 20 ◦ C
Summary
A complete assessment of the response of tree growth to climates over different time scales is critical for understanding the physiological mechanisms of tree growth and their response to environments in the context of global climate change [1]. Long-term relationships between tree radial growth and climatic variables have been evaluated in recent decades by mostly using dendrochronological approaches that are based on the statistical relationships between tree-ring widths and meteorological factors at monthly, seasonal, and annual scales [2,3]. Dendrometers continuously and automatically monitor intra-annual stem radial variation at a high temporal resolution of minutes to hours without invasive sampling [10,11,12] Such technology has provided opportunities to study stem radial variation and its response to environmental factors on daily and seasonal scales [3,7]. Two opposing patterns of diurnal cycles of stem radial variations are observed in warm and cold seasons that are related to the physiological processes of transpiration and water uptake, and freeze-thaw cycles, respectively [17,18,19]
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