Transpiration Of An Chinese Pine And Black Locust Mixed Forest In Hilly-gully Region Of The Loess Plateau

Understanding quantitatively the plant water relations is the key to successfully implement protective afforestation programs in the water limited region of the Loess Plateau, northwest China. It is critically important to study the transpiration of the main tree species and forest stand in manipulating plant water relations by means of tree species selection, stand density arrangement, and spatially protective forests layout. Our study measured sap flow of 17 trees covering 4 diameter classes in a 16 years old Chinese pine and Black locust mixed forest stand from July to October in 2008 using Thermal dissipation probe (TDP) in Caijiachuan watershed in Jixian, county, Shanxi Province. The stand transpiration was derived by scaling up methodology by sapwood area for this 245 stems/hm2 density forest. At same time, we also measured environmental factors such as solar radiation, air relative humidity, air temperature, wind speed, and soil moisture content.The main conclusions of this study are as follows:The diurnal fluctuation of sap flow velocity of Chinese pine and Black locust was basically the same in the sunny day but experiencing a significant difference in the starting time, time to peak, dramatic decreasing time, and lasting time duration of saplow velocity. In different weather conditions, tree sap flow rate and transpiration law is:sunny> rainy. The sap flow rate and transpiration of Chinese pine were significantly larger than Black locust.Mean diurnal transpiration rate for different diameter classes of each species was 12cm>10cm>6cm>8cm for Chinese pine and 12cm>8cm>6cm for Black locust indicating that the diurnal transpiration rate increased against the diameter of breast height except for 6 and 8 cm pine trees. We found that solar radiation(R2>0.568, P=0.00, n>10848) and vapor pressure deficit (R2>0.393, P=0.00, n> 10848) were the major factors to control the canopy transpiration of Chinese pine and Black locust. In addition, monthly stand transpiration scaled up from sapwood area were 30.8mm,24.1mm, 26.3mm, and 18.4mm for July, August, September, and October,2008, respectively. The total transpiration during this four month period was less than the total rainfall during this time suggesting that the water stress was not likely in general.