Qilian Mountain situated at the northwest arid region of China and surrounded by desert and Gobi, receive higher precipitation and play a role in supplying water to lowlands. Many inland rivers, such as Shiyanghe River, Heihe River and Shulehe River, originate from Qilian Mountain. 

Forest vegetation of Qilian Mountain is not only valuable forest resource, but also have the ecological function of water reservation which is very important for retain oasis ecosystem of middle and lower reaches of inland river system. 

Qinghai spruce (Picea crassifolia) is the dominant tree species in Qilian Mountains. In the upper Heihe Basin, Qinghai spruce forests occupies about 25% of the total forest area and 78% of arbor stand. To this type, it is important to understand physiological processes and components of the water balance in stands, and understanding water use in Qinghai spruce tree is the first and crucial step. 

In order to accurately estimate whole-tree water use and individual tree transpiration, it is important to have reliable information on radial patterns of sap velocity and responses of sap flow to local environmental conditions.  

Variation in sap flow and environmental conditions was investigated in a mature Qinghai spruce (Picea crassifolia) stands during the growing season of 2011 at the Pailougou watershed, in Qilianshan Mountain, located in the upper Heihe River Basin, in the arid region of Northwest China. Daily sap flow was measured by the heat-pulse technique on nine trees during the growing season.  

It was found that the highest daily sap flow velocity in sap flow radial distribution was at 20 mm sapwood depth, and that from 10 mm to 20 mm sapwood depth, the daily sap flow velocity gradually increased, whereas from 20 mm to 40 mm sapwood depth, sap flow velocity gradually diminished.  

A simple Gaussian regression model for the radial distribution of sap flux velocity was formulated, which explained 92% of the radial profile variation of sap flow velocity. Sap flow velocity was heightened by increasing the global short-wave radiation (R, W m−2), vapour pressure deficit (D, kPa), and air temperature (T, °C) when R < 800 W m−2, D < 1.4 kPa, and T < 18.0 °C.  

However, inherently difficult to establish firm relationships between sap flow velocity and R, D, and T because of the complex crown environment. The correlation of daily sap flow velocity to soil moisture content on a clear day was fitted by a logistic regression.  

Scientists conclude that the measurement of radial flow pattern provides a reliable method of integrating sap flow from individual measuring points to the whole tree. And D, R, T and soil moisture had varying influences on sap flow velocity in the Qinghai Spruce. 

This research achievement has been published on the Agricultural and Forest Meteorology.