Intra-annual radial growth and its climate response for Masson pine and Chinese fir in subtropical China

KEY MESSAGE: Chinese fir has strong transpiration and assimilation activity. Daily stem radial increments were closely related to moisture conditions. Tree growth responds to a climate in similar ways but with different magnitudes. High temporal resolution dendrometric measurements offer a great opportunity to acquire valuable information about stem size dynamics at half-hour intervals and improve our understanding of climate influence on tree growth and physiological processes. In the present study, we continuously monitored intra-annual stem radial size changes with the help of automated dendrometers in pure Masson pine and Chinese fir forests widely distributed in subtropical China across two consecutive years of 2017 and 2018. The main goal of this study was to analyze intra-annual stem growth dynamics as well as their climatic forcing. Our results indicate a similar growth onset between Masson pine and Chinese fir but prolonged growth duration for Chinese fir in 2018. Large stem-size fluctuations with particularly strong stem shrinkage especially in summer were obtained for Chinese fir. The analysis of climatic forcing on daily stem radial increments indicated similar correlation trends with different magnitudes for the two species, with changes in the daily stem growth positively correlated with relative humidity (RH), precipitation, and soil water content (SWC), and negatively correlated with photosynthetically active radiation (PAR), vapor pressure deficit (VPD) and temperature factors. In general, daily stem radial growth is closely associated with moisture conditions. Lagging the climatic factors showed decreasing impacts on daily stem growth. We conclude that tree growth is primarily responsive to moisture-related climatic factors, and Chinese fir shows more robust transpiration and assimilation activity.