Response of sap flow in Qinghai spruce (Picea crassifolia) to environmental variables in the Qilian Mountains of China

Key message Vapor pressure deficit, soil water content, and daily maximum air temperature were the most important variables to affect sap flow. Tree transpiration has a vital role in maintaining water balance in forest ecosystems, but how environmental variables affect transpiration is not clear at different altitudes in the regions of the arid mountains of China. With significant changes in water regimes in arid areas due to climate change, a greater understanding of transpiration responses to environmental variables will determine ecosystem health and management. Therefore, using the heat ratio method, sap flow in Qinghai spruce (Picea crassifolia) at different microenvironment conditions was monitored over 2 years (2016 and 2017) in the Qilian Mountains. Environmental variables were measured synchronously with sap flow. The random forests model showed that vapor pressure deficit (VPD), soil water content (SWC), and daily maximum air temperature (T-max) were the most important variables to affect sap flow across four sites and two years. These results differed within sites and years. The important variables were VPD and SWC in 2016 due to lower precipitation, and VPD and T-max in 2017. Meanwhile, the most important variables were VPD in 2016 and T-max in 2017 at the high-elevation site, while opposite at the low-elevation site. Nonlinear models showed that sap flow decreased when VPD > 0.7-1.0 kPa, and saturated when T-max > 15-20 degrees C. Sap flow was more sensitive to SWC at low than at high elevation. We concluded that VPD, SWC, and T-max controlled sap flow in Qinghai spruce at these sites. Precipitation played a key role in controlling sap flow in different years, and altitudes affected the responses of sap flow to environmental variables within years.