The influence of hydrometeorological factors on tree growth in mountainous watersheds of the Qilian mountains in China.

To examine the influence of hydrometeorological factors on forest ecosystems, this study focused on the growth response of the Qinghai spruce (Picea crassifolia Kom.) to hydrometeorological factors, such as soil moisture, relative humidity, vapor pressure deficit, temperature, precipitation and wind speed, in a mountainous watershed. The Dayekou watershed, which is situated in the Qilian Mountains, was used to study the increase in stem diameter based on the daily-monthly fluctuations, cumulative growth, and stem diameter expansion in response to hydrometeorological parameters. The stem diameters of six dominant trees (categorized in three classes) were recorded using the Dendrometer DRL26 tree stem diameter growth monitor and combined with hydrometeorological monitoring data. The influence of hydrometeorological factors on stem diameter growth was statistically analyzed. The results indicated that the daily fluctuation of stem diameter growth of Qinghai spruce exhibited a parabolic pattern, which could be divided into three stages: contraction (from 10:00 to 21:00), expanding (from 21:00 to 5:00 the following day), and growth (from 5:00 to 11:00 the following day). The monthly stem diameter growth also exhibited a trend, which could be divided into three stages: initial growth (May), rapid expansion (June-July) and slow growth (August-October). At a 40 cm depth, soil water content, air humidity, and atmospheric pressure all showed positive correlations with stem diameter growth (P < 0.01), while saturated water pressure differential, wind speed, and photosynthetically active radiation showed negative correlations (P < 0.01). Our results demonstrated that relative air humidity, soil moisture, air temperature, and atmospheric pressure at a 40 cm depth had the highest impact on the Qinghai spruce's growth in stem diameter. Changes in these hydrometeorological factors due to potential climate change will affect forest growth in the future.