Qinghai spruce (Picea crassifolia) forest transpiration and canopy conductance in the upper Heihe River Basin of arid northwestern China

Tree transpiration plays a determining role in the water balance of forest stands and in water yields from forested catchments, especially in arid regions. This study describes direct measurements of tree sap flow by heat pulse velocity method, and calculations of stand transpiration and canopy conductance, during the 2011 and 2012 growing seasons, in an 80- to 120-year-old Picea crassifolia stand in a natural forest. The experiment was carried out on Qilianshan Mountain, located in the upper Heihe River Basin, in the arid region of Northwest China. It was found that stand transpiration (Ec) ranged from 1.0 mm d−1 to 2.4 mm d−1, and from 1.2 mm d−1 to 2.4 mm d−1, during the 2011 and 2012 growing seasons, respectively. Total Ec was 195.2 mm, 219.6 mm during the 2011 and 2012 growing seasons, respectively. And total Ec during the 2012 growing season was 12.5% higher than during the 2011 growing season. Based on the half-hourly temporal scale, canopy conductance was between 0.3 mm s−1 and 51.3 mm s−1, and between 1.2 and 57.0 mm s−1, during the 2011 and 2012 growing seasons, respectively. Canopy conductance was sensitive to variations in climatic variables such as vapor pressure deficit (D), air temperature (T), wind speed (W) and global short-wave radiation (R). It is also evident that an exponential decay function, including D and T, accounted for 81% and 19% of the variations in canopy conductance, respectively. These results was not only providing the basis for more detailed analyses of water physiology and growth of Qinghai spruce trees for the later application of a climate-driven process model, but also might have implications for forest management in Qilianshan Mountains.