Influence of climate on seasonal and diurnal stem radius variations in Picea meyeri during cold seasons.

• 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]