Effects of site aridity and species on stand transpiration in high-elevation dryland ecosystems

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.