Variations in stem radii of Larix principis-rupprechtii to environmental factors at two slope locations in the Liupan Mountains, northwest China

Relationships between stem growth and climatic and edaphic factors, notably air temperatures and soil moisture for different slopes, are not completely understood. Stem radial variations were monitored at the bottom and top slope positions in a Larix principis-rupprechtii plantation during the 2017 and 2018 growing seasons. Total precipitation during the growing season in 2017 and 2018 was 566 mm and 728 mm, respectively. Stem contractions typically occurred after mid-morning followed by swelling in the late afternoon in both plots, reflecting the diurnal cycle of water uptake and loss. Trees at the two locations showed the same growth initiation (mid-May) because of the small differences in air and soil temperatures. There were no significant differences in cumulative stem radial growth between the bottom plot (1.57 +/- 0.34 mm) and the top plot (1.55 +/- 0.26 mm) in 2018. However, in 2017, the main growth period of the bottom plot ceased 17 days earlier than in the top plot, while cumulative seasonal growth of the bottom plot (1.08 +/- 0.25 mm) was significantly less than the top plot (1.54 +/- 0.43 mm). Maximum daily stem shrinkage was positively correlated with air and soil temperatures, solar radiation, vapor pressure deficits, and negatively correlated with volumetric soil moisture content. The maximum daily shrinkage reflected transpiration rates as affected by environmental factors. Daily radial stem increment was correlated with precipitation and volumetric soil moisture in both years, but with air temperatures only in 2017. The seasonal growth of L. principis-rupprechtii Mayr thus shows interannual dynamics, while precipitation constitutes a key driving factor.