| The loess hilly area of west Shanxi is characterized by its fragile ecosystems. Severe soil erosion is one of the major ecological problems in this region. Sustainable restoration of vegetation is the key approach to the control of soil erosion. It is critically important to study the transpiration of the two typical forests because it will be significant in regulating plant water relations, selecting proper afforestation species. This paper selected Pinus tabulaeformis Carr. and Robinia pseudoacacia L. as the research object, we also studied the transpiration characteristics of them and finally total transpiration of forest stand was estimated. The main conclusions are as follows:(l)The diurnal variation of sap flow velocity for Pinus tabulaeformis Carr. and Robinia pseudoacacia L. was analogous and apparent. Commonly the starting time is between5:00and7:00, and it will reach its peak during the noon. The lowest sap flow velocity appeared in the morning. The start time of sap flow velocity for Robinia pseudoacacia L. was earlier than Pinus tabulaeformis Carr. and the peak of Robinia pseudoacacia L. was also higher.(2)The order of sap flow velocity under different weather conditions is:sunny>cloudy>overcast. The sap flow velocity of Pinus tabulaeformis Carr. and Robinia pseudoacacia L. decreased rapidly when showery rain appeared. When the rain ended, sap flow velocity rose again.(3)During June and October, the monthly variation of sap flow velocity was basically the same. Sap flow velocity increased at the beginning and reached its peak in August, which proved that transpiration was the most severely in August. Sap flow velocity would decrease after August.(4)We found that the sap flow velocity was highly significantly correlated with solar radiation, vapor pressure deficit and relative air humidity. Also sap flow velocity was significantly related with wind speed. These environmental factors all affected the transpiration.(5)Mean diurnal transpiration order for the two different diameter classes were:Pinus tabulaeformis Carr.,16cm>12cm>8cm.Robinia pseudoacacia L.,17cm>13cm>9cm>5cm.We concluded that forests with bigger diameter at breast height had higher transpiration.(6)The monthly forest stand transpiration of Pinus tabulaeformis Carr. were11.24mm,12.45mm,19.32mm,14.09mm,12.10mm for each month, the total was69.20mm.The forest stand transpiration for Robinia pseudoacacia L. were13.01mm,13.47mm,15.92mm,9.55mm,8.93mm for each month, with total60.87mm. So the transpiration was less than the rainfall and the forest stand transpiration of Robinia pseudoacacia L. was less than the other one. |
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