Abstract:

Sap flow of 13 trees of Larix principisrupprechtii was measured with the thermal dissipation probe between June and October in 2005. Those trees were randomly selected from a 20 m×20 m plot, wihch is located on a slope with a gradient of 45° in the Xixia forestry station along the south side of Liupan Mountains, Ningxia. There was a large variation in daily sap flow among the individual trees ranged from 11.27 kg·d^－1 to 24.46 kg·d , with a variation coefficient of 0.298 from June to July (n=5); and ranged from 5.01 kg·d^－1 to 22.25 kg·d^－1, with a variation coefficient of 0.454 from August to October (n=13). Analysis of variation indicated that diameter at breast height (DBH) and sap flux density (SFD) were two main factors that significantly affected the variation of daily sap flow. DBH and SFD were able to respectively explain 56.9% and 34.7% of the total variation of daily sap flow. The relation between sap flow and DBH may be well interpreted by the close relationship between DBH and sapwood area. Regression analysis showed that SFD was not correlated with DBH, tree height, crown area and sapwood area, however it was significantly and negatively correlated with the canopy overlapping(r=-0.668). This result suggested that SFD was strongly influenced by the spatial position of trees and their shading condition by surrounding trees. At last, a new method used for estimating the stand transpiration was proposed based on the relationship between SFD and the canopy overlapping which is characterized by the spatial position difference of individual trees. The change pattern of the daily stand transpiration estimated by the method was basically similar to that estimated by the traditional method of sapwood area. The new method produced an estimation of stand transpiration of 1.15 mm·d^－1, which was 13.13% lower than the value of 1.32 mm·d^－1estimated by the traditional method. It was concluded that the stand transpiration could be overestimated without considering the difference of spatial characters of individual trees.

Key words: Larix principisrupprechtii,  plantation, sap flow, variation of individual trees, transpiration, scaling up, canopy overlapping