It is important to develop a better understanding of the climatic and soil factors controlling the stem diameter growth of Qinghai spruce (Picea crassifolia Kom.) forest. The results will provide basic information for the scientific prediction of trends in the future development of forests. To explain the seasonal pattern of stem diameter growth of Qinghai spruce and its response to environmental factors in the Qilian Mountains, northwest China, the stem diameter changes of 10 sample trees with different sizes and soil and meteorological conditions were observed from May to October of 2015 and 2016. Our results showed that the growth initiation of the stem diameter of Qinghai spruce was on approximately 25 May 2015 and 20 June 2016, and stem diameter growth commenced when the average air and soil temperatures were more than 10 °C and 3 °C, respectively. The cessation of growth occurred on approximately 21 August 2015 and 14 September 2016, and it was probably controlled by soil moisture. Stem diameter growth began earlier, ended later, and exhibited a larger growth rate as tree size increased. For the period May–October, the cumulative stem diameter growth of individual trees was 400 and 380 μm in 2015 and 2016, respectively. The cumulative stem diameter growth had a clear seasonal pattern, which could be divided into three growth stages, i.e., the beginning (from day of year (DOY) 120 to the timing of growth initiation with the daily growth rate of less than 2 μm·day⁻¹), rapid growth (from the timing of growth initiation to the timing of growth cessation with the daily growth rate of more than 2 μm·day⁻¹), and ending stages (from the timing of growth cessation to DOY 300 with the daily growth rate of less than 2 μm·day⁻¹). The correlation of daily stem growth and environmental factors varied with growth stages; however, temperature, vapor pressure deficit (VPD), and soil moisture were the key factors controlling daily stem diameter growth. Overall, these results indicated that the seasonal variation in stem growth was regulated by soil and climatic triggers. Consequently, changes in climate seasonality may have considerable effects on the seasonal patterns of both stem growth and tree growth.

Dryland montane forests are important agents for soil and water resource conservation. The growth of these forests under climate warming is strongly affected by local environmental factors. However, how environmental factors impact intra-annual stem growth dynamics across environmental gradients in these regions remains unclear. This work focused on assessing seasonal patterns of stem growth across different elevations and how environmental factors impact stem growth in the Qilian Mountains, northwestern China. The stem growth of 50 Qinghai spruce trees was monitored for two years across an elevation gradient from 2500 m to 3300 m above sea level (a.s.l.). We found that growth initiation occurred later as the elevation increased, and growth commenced when elevation-specific temperature thresholds were reached. However, growth cessation presented large elevational differences: cessation occurred much earlier at low elevations (2500 m and 2700 m a.s.l.). Exceptionally early growth cessation occurred predominantly at 2700 m a.s.l., which was correlated with seasonal drought/insufficient rainfall and low soil moisture occurring since mid-July 2015. Temperature and soil moisture were the key factors governing the daily rate of stem growth in the beginning, rapid growth, and end stages. Overall, due to effects of seasonal drought and low temperature on growth cessation and growth rate, the annual growth of Qinghai spruce was rather low at both low (2500–2700 m a.s.l.) and high (3100–3300 m a.s.l.) elevations; middle elevations (approximately 2900 m a.s.l.) might be the most favorable Qinghai spruce growth. Our results implied that tree growth will likely decline at low elevations and that the optimal elevation for Qinghai spruce growth in northwestern China is expected to shift upward under future climate warming.

It is important for integrated forest-water management to develop a better understanding of the variation of tree transpiration among different canopy layers in the forests and its response to soil moisture and weather conditions. The results will provide insights into water consumption by trees occupying different social positions of the forests. In the present study, an experiment was conducted in the Qilian Mountains, northwest China, and 13 trees, i.e., 4–5 trees from each one of dominant (the relative tree height (H_R) > 1.65), subdominant (1.25 < H_R ≤ 1.65) and intermediate-suppressed (H_R ≤ 1.25) layers) were chosen as sample trees in a pure Qinghai spruce (Picea crassifolia Kom.) forest stand. The sap flux density of sample trees, soil moisture of main root zone (0 to 60 cm) and meteorological conditions in open field were observed simultaneously from July to October of 2015 and 2016. The results showed that (1) The mean daily stand transpiration for the study period in 2015 and 2016 (July–October), was 0.408 and 0.313 mm·day⁻¹, and the cumulative stand transpiration was 54.84 and 40.97 mm, accounting for 24.14% (227.2 mm) and 16.39% (249.9 mm) of the total precipitation over the same periods, respectively. (2) The transpiration varied greatly among canopy layers, and the transpiration of the dominant and codominant layers was the main contributors to the stand transpiration, contributing 86.05% and 81.28% of the stand transpiration, respectively, in 2015 and 2016. (3) The stand transpiration was strongly affected by potential evapotranspiration (PET) and volumetric soil moisture (VSM). However, the transpiration of trees from the dominant and codominant layers was more sensitive to PET changes and that from the intermediate-suppressed layer was more susceptible to soil drought. This implied that in dry period, such as in drought events, the dominant and codominant trees would transpire more water, while the intermediate-suppressed trees almost stopped transpiration. These remind us that the canopy structure was the essential factor affecting single-tree and forest transpiration in the dryland areas.

Forests in water source areas are important factors for water supply security, soil, and water conservation, and their water consumption from transpiration is strongly affected by site conditions, including the slope aspect. However, the lack of research on how the slope aspect interferes with the response of stand transpiration to drought has hindered researchers from developing climate-resilient forest–water coordinated, sustainable development plans for different stand conditions. This study was conducted on Quercus wutaishansea forests in the southern part of Liupan Mountain in northwest China, and two sample plots were built on sunny and shady slopes. The responses of stand transpiration to various soil moisture and meteorological conditions on different slope orientations were analyzed. The results showed better-growing stands on shady slopes transpired more and consumed more soil moisture than those on sunny slopes. The soil moisture on shady slopes decreased rapidly below the threshold level during the drought, leading to a limitation of stand transpiration; however, its transpiration recovered rapidly after the drought. In contrast, stand transpiration on sunny slopes was not affected by this drought and maintained its pre-drought rate. Our results suggested that stands with higher water demand on shady slopes were more susceptible to drought when it occurred. This indicated that in the case of frequent droughts, the vegetation should be managed according to the vegetation-carrying capacities resulting from different site conditions.

Climate change has dramatically altered the frequency and intensity of droughts, thereby altering tree growth. Understanding whether tree growth in semiarid areas in response to long-term drought and the post-drought recovery rate of tree growth vary along moisture gradients is crucial for predicting future forest change. Here, we assessed the spatial variation in both the growth resistance of Qinghai spruce (Picea crassifolia Kom.) to long-term drought and its post-drought recovery using a stand-total sampling strategy along a moisture gradient that covered three sites (with an annual precipitation of 330.4, 394.2, and 515.9 mm for the western, middle, and eastern sites, respectively) with six plots. Resistance and recovery were evaluated by analyzing the tree growth trends during a long-term drought period (1980–2001) and the subsequent post-drought period (2001–2013), respectively. Our results indicate that the trees with the highest temporal stability were those at the wetter eastern site; specifically, during the long-term drought period, the trees at the wetter eastern site showed the highest resistance (−0.015) and the lowest recovery (0.002). The trees in moderately arid conditions were much more sensitive to climate change than those at the relatively arid western site, showing the lowest resistance (−0.050) and highest recovery (0.020). Climate change had the strongest impact on tree growth at the moderately arid site, contributing 60.6% to the tree growth decreasing trend during the long-term drought period and 65.4% to the tree recovery during the post-drought period, respectively. Climate change had a lower impact on tree growth at the wet and dry sites, contributing less than 50% to the tree growth trends at these sites. The results indicate that a trade-off relationship exists between resistance and recovery at the different sites; that is, the highly resistant trees at the wetter eastern site tend to have lower recovery, whereas the weakly resistant trees at the moderately arid site tend to have higher recovery. These results have implications for predicting tree growth in response to future climate change.