Comparative Studies on Transpiration Characteristics of Vitex negundo at Different Geomorphological Positions at Guilin Karst Experimental Site

Transpiration characteristics of Vitex negundo at different geomorphological positions(depression,slope,saddle) at Guilin Karst Experimental Site were studied.The results showed that there were differences in daily variation of transpiration rate of V.negundo growing in different geomorphological positions.The transpiration rate at saddle displayed one-peak pattern,which reached peak value at about 12∶30,and did not appear obvious noon depression phenomena.Although the transpiration rates at depression and slope displayed two-peak pattern,it was truly attributed to noon depression phenomena at slope,while at depression,it was related to the instantaneous decreasing of light intensity.The average daily transpiration rates were in sequence as saddle>depression>slope.The differences in transpiration rate of V.negundo among different geomorphological positions related to the differences in hydrothermal conditions,the thickness of soil layer,and the time length of accepting sunlight.Stepwise regression analysis and best subsets regression analysis between transpiration rate of V.negundo and its influence factors were made,and the results indicated that the dominant factors to affect transpiration rate of V.negundo were stomatal conductance,air relative humidity and leaf temperature.The influence of stomatal conductance was the greatest,and the influencing extent varied with the geomorphological positions.