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The soil moisture (SM) influences tree growth with climate change. However, the spatial and temporal dynamics of tree water use strategies in climate-sensitive areas remain uncertain. Therefore, we collected the tree-ring oxygen isotope (δ18OTR) chronologies and divided the wet–dry gradients according to the precipitation on the Tibetan Plateau (TP). Further, the relationship between the δ18OTR and environmental factors was analyzed across different gradients. We found the following: (1) The SM during the growing season was the most important factor for δ18OTR. (2) The response of the δ18OTR to the SM had a lag in arid areas than in humid areas. (3) Trees absorbed the SM on the surface in humid areas (r = −0.49 to −0.41, p < 0.01), while trees absorbed the SM from deep in the soil in arid areas (r = −0.48 to −0.29, p < 0.01). The results demonstrated that trees were better able to cope with drought stress in arid regions because they used more stable deep soil water than in humid regions. Therefore, the findings will provide a scientific basis for water use of trees using the δ18OTR in complex environmental contexts. Trees with single water use strategies should be given more attention to keep ecosystems healthy. [ABSTRACT FROM AUTHOR]

Understanding the resource allocation strategies of dominant conifer species in mountain forest ecosystem can provide decision-making basis for mountain ecosystem conservation and sustainable use. However, the resource allocation trade-offs of different tree species under the heterogeneous environments in arid and semi-arid mountains were not fully understood. Therefore, we analyzed the stoichiometric characteristics of two dominant conifer species in the Qilian Mountains at the eastern margin of the Tibetan Plateau with three altitudes and four typical regions to explore the resource allocation strategies of ecological adaptations of dominant tree species under different environmental gradients. The results showed that there were differences in stoichiometric characteristics for different tree species, altitudes and regions. 1) Compared with Qinghai spruce, Qilian juniper stored more nutrients in the underground part when it was resistant to drought. 2) Plants tended to invest more N and P to compensate for reduced physiological efficiency as altitude increases and temperatures decrease. 3) Different dominant conifer species stored more nutrients in the roots in the western section under drought stress. However, in the eastern section with favorable moisture and temperature they invested nutrients in the leaves and stems. 4) Mean annual temperature (MAT), mean annual precipitation (MAP) and soil water content (SWC) were the common controlling factors affecting stoichiometric characteristics of the different tree species, altitudes and regions. The findings of this study provided a theoretical basis for understanding of plant adaptation mechanisms at different spatial and temporal scales and species effects, as well as technical support for forest ecological control and restoration in ecologically sensitive and fragile areas. [Display omitted] • There were differences in stoichiometric characteristics for different tree species, altitudes and regions. • Qilian Juniper stored more nutrients in the underground part when it was resistant to drought. • The results of this study support the Temperature-Plant Physiology Hypothesis. • In the eastern section with favorable moisture and temperature they invested nutrients in the leaves and stems. • MAT, MAP and SWC were the common controlling factors. [ABSTRACT FROM AUTHOR]

• Coupling coordination degree can better evaluate the water-carbon function. • Water conservation trended downward, while carbon reserve trended upward. • The level of water-carbon coupling coordination on the Tibetan Plateau is low. • Climate is an important cause of the water-carbon coupling coordination level. • Ecological management should be adapted to water-carbon mechanism and dynamic tend. The Qilian Mountains on the eastern edge of the Tibetan Plateau are an important ecological security barrier in northwestern China and an extremely critical water-carbon conservation area. The water and carbon cycles are closely linked, and the water-carbon coupling process has become further complicated under global climate change and through the intensification of human activities. A better understanding of the impacts and potential mechanisms of water-carbon interactions is the key to improving the efficiency of water-carbon resource utilization. To rationalize the relationship between water and carbon, we used data from statistical almanacs, land use, DEM and various climatic elements to estimate the water-carbon nutrition in the Qilian Mountains from 2000 to 2020 based on the InVEST model and the Coupled Coordination Degree Model (CCDM) and to explore the coupling mechanism of water-carbon, spatial-temporal dynamics, and the influencing factors. The results showed that: 1). the average annual water and carbon storage in the Qilian Mountains are 63.21 × 108 m3 and 77.82 × 107 t, respectively, showing a spatial distribution pattern of high in the northeast and low in the southwest. The inter-annual changes in water (y =-0.451x+968.920) and carbon (y = 0.082x-87.808) showed different trends. 2). The annual average of the water-carbon coupling coordination degree of 0.280 in the Qilian Mountains was low in general, and the trends declined over time from 0.273 in 2000 to 0.264 in 2020. However, the spatial distribution was relatively clustered, and the spatial correlation improved from 0.69 in 2000 to 0.74 in 2020. 3). Precipitation, potential evapotranspiration and temperature were important elements driving water-carbon coupling. Among them, precipitation was significantly positively correlated with water-carbon coupling (r = 0.88), and potential evapotranspiration (r =-0.56) and temperature (r =-0.39) showed significant negative correlations with water-carbon coupling. This study can provide information for management and decision-making in carbon sequestration, water resource utilization, ecological protection and climate change, and promote the study of the spatial and temporal coupling functions of water and carbon with in the Qilian Mountains. Meanwhile, this study can provide a reference for the environmental management and protection of the Qilian Mountain ecosystem. [ABSTRACT FROM AUTHOR]