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 (R-2 = 0.23), while at the other sites, J(s) was mostly controlled by atmospheric vapor pressure deficit (VPD) (R-2 > 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.