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• Soil N availability increased at alpine treeline ecotones. • Plant N and P stoichiometry indicates nutrient patterns at alpine treeline ecotones. • Soil N pools drive plant growth and might have a strong effect on treeline dynamics. • Alpine treeline might respond quickly to available nutrients change with global warming. Alpine treeline ecotone is one of the most vulnerable ecosystems and considered an ideal "warning line" for the monitoring of global climate change. However, shifts in the nutrient traits of plant biomass and soils of alpine treeline ecotones along the elevation gradient are poorly investigated. This study aimed to quantify nitrogen (N) and phosphorus (P) traits of Qinghai spruce (Picea crassifolia) and their driving factors for seven typical treelines (timberline - treeline - tree species line) covering 800 km southeast-to-northwest transect within the northeastern Tibetan Plateau. The biomass of plant tissue (leaves, twigs, and fine roots) and the N and P contents in them and soil nutrients (total and available N and P) were determined. Two mechanisms, which can drive the nutrient content in plants, namely, temperature-plant physiology and the sink (growth) limitation hypotheses were checked here. The N and P contents in plant tissue increased with the elevation, and temperature negatively impacted tissue N and N:P in all of the seven typical treelines, which supported both of the hypotheses. Soil N availability increased along the altitudinal gradient in 1.23 times. Soil TN explained 13.4% (p < 0.01), NO 3 –-N 10.1% (p < 0.05), and NH 4 +-N 8.6% (p < 0.05) of the variations in plant tissue N and P stoichiometry and biomass. Thus, soil N pools stimulated Qinghai spruce growth and have a strong effect on treeline dynamics. [ABSTRACT FROM AUTHOR]

• The macroecology of plant tissues and soil nutrients in an arid area were studied. • Climate, soil depth, latitude, and longitude significantly affected soil nutrients. • Plant tissue N and N:P were positively correlated with precipitation. • Plant and soil were closely linked and interacted with each other. • Precipitation is critical to mediating the element circulation in arid areas. Knowledge of nutrient traits, especially carbon (C), nitrogen (N), and phosphorus (P), is essential for understanding the biogeochemical cycle and ecosystem functioning. However, spatial patterns in soil and plant tissues stoichiometry in the arid and semiarid forest ecosystem are not yet fully understood. Here, a regional-scale study was conducted on the Qilian Mountains to illustrate the macroecological patterns of soil C:N:P stoichiometry and the plant tissue N and P contents in Qinghai spruce (Picea crassifolia) forest and their driving forces. Soil C, N, and P contents and C:N:P ratios at all four soil depth layers (0–5, 5–10, 10–20, and 20–40 cm) showed depths, latitudinal, and longitudinal trends (p < 0.01), except for the soil P content and C:N ratios. The plant N and P contents and N:P ratios were found to be correlated with the latitude and longitude (p < 0.05). The N content and N:P ratios of plant tissue showed a negative correlation with the temperature (p < 0.05), which coincides with the temperature-plant physiological hypothesis. Additionally, the N and N:P of both the soil and plant tissue were positively correlated with precipitation (p < 0.05), which led to the plant tissue N and N:P and the soil TN and N:P being tightly related. Variation partitioning analysis revealed that the soil properties (39.31%) contributed more to the plant tissue N:P stoichiometry variations than climate (6.47%). The results demonstrated that the macroecological pattern of plant tissue N:P stoichiometry is mainly regulated by soil traits. These findings contribute to our understanding of the C:N:P macroecological pattern of forest ecosystems in arid regions, providing basic data for future forest cultivation and management in those areas. [ABSTRACT FROM AUTHOR]

• We monitored sap flow for 16 spruces and 14 junipers at 4 arid to semi-arid sites. • Sap flow is almost completely controlled by VPD except for spruce in the arid site. • Increasing site aridity drives a shift towards more anisohydric stomatal behavior. • Spruce exhibits stronger stomatal regulation of transpiration than juniper. • Transpiration may increase first and then decrease from semi-arid to arid forests. Climate change is altering regional aridity and species composition in dryland ecosystems. Understanding the effects of long-term increasing aridity and species-specific stomatal behaviors on transpiration is therefore important for water-resource forecasting. To assess the effects of site aridity levels and species on stand transpiration rate (E s), we monitored sap flux density (J s) and relevant environmental parameters for 16 isohydric Picea crassifolia (spruce) and 14 anisohydric Juniperus przewalskii (juniper) trees over three growing seasons at four arid to semi-arid high-elevation sites on the Tibetan Plateau. Our results show that E s was about 5–9 times higher in semi-arid sites than in arid sites, and about 6–9 times higher in spruce than in juniper. Spruce exhibited stronger stomatal regulation of transpiration than juniper. Soil water supply strongly promoted J s only for spruce in the arid environment (R2 = 0.23), while at the other sites, J s was mostly controlled by atmospheric vapor pressure deficit (VPD) (R2 > 0.82). However, increasing site aridity greatly reduced the sensitivities of both J s and canopy conductance to VPD , especially for juniper. We expect that in the transition from semi-arid to arid alpine forests, E s will initially rise due to increasing VPD. However, in the long term, there may be a stronger decline in E s since both the sensitivity of J s to VPD and the stand sapwood area will decrease. These findings could be used to reduce the impacts of climate change on water resources in high-elevation drylands. [ABSTRACT FROM AUTHOR]