| Artificial forest area has been increasing with the people's the attention of environmental problem,which causes the contradictions between forest and water has also become one of the emphasis of forest hydrology abroad,while understanding the relationship between forest structure and water consumption will be the key that coordinates the contradictions between forest and water.In this study, stand structure of Larix principis-rupprechtii plantation forest and Pinus armandii Natural Forest was contrastively researched through the method of Thermal Dissipation technique, Lysimeter,Traditional hydrology in Xiangshui river small watershed on the south of Liupan Moutain from May 2009 to October 2009. the tree sap flow velocity and tree stand evaptranspiration and its component in two tree stands were contrastively analyzed, the conclusions are as follows:1. dynamic variations of sap flow velocity.In typical weather, the daily sap flow rate are with the order of B.platyphylla >L.principis-rupprechtii >P.armandii in the sunny day. when it is cloudy or rainy,the sap flow rate of L.principis-rupprechtii is the highest, while the P.armandii's is the lowest. The sap flow rate in different forest types are also obvious different during different stage of growing season,the sap flow rate are with the order of B.albo-sinensis > B.platyphylla > L.principis-rupprechtii > P.armandii during the early growing seasons(may-June), B.albo-sinensis > P.armandii > B.platyphylla > L.principis-rupprechtii in the mid-growing seasons(July-Aug),P.armandii> L.principis-rupprechtii > B.albo-sinensi > B.platyphylla in the late growing season (Sep-Oct).2. Correlation analysis between the sap flow rate and meteorological factor.The correlation coefficient is in the range of 0.89-0.93 between the sap flow rate of four forest types and saturated VPD, is in the range of 0.76-0.87 between the sap flow rate of four forest types and solar radiation, However, there is no correlation between the sap flow rate of four forest types and wind speed.The method of stepwise regression analysis is adopted between instantaneous sap flow rate,average daily sap flow rate and meteorological factor.The result shows that the saturated VPD is the main impact-factor to instantaneous sap flow rate,however, the influence of solar radiation is a lagging phenomenon. In day time scales showed that only solar radiation involves in the equaltion, which futher proves that solar radiation is the main factor to the trees water consumption.while solar radiation is entered in the regression equation,which indicates that solar radiation is the main factor to water consumption of the tree species in daily time scales3.The study on evapotranspiration components.In the growing season, canopy interception of L.principis-rupprechtii plantation forest ranges from 6.2 to 33.7mm which is 1.14 times as P.armandii Natural Forest. The canopy interception differ greatly because of the influence of LAI, LAI value ranges from 2.97 to 4.8 in L.principis-rupprechtii,while LAI value ranges from 2.26 to 4.06 in P.armandii. The former monthly LAI value is larger than the latter.The average transpiration of L.principis-rupprechtii ranges from 0.25 to 1.33mm/d, P.armandii's is between 0.07 and 0.63mm/d, the cumulative sapwood area ia a determining factor in forest transpiration.The mean daily forest transpiration of L.principis-rupprechtii plantation forest shrub is only 0.14mm/d, while the P.armandii Natural Forest is 0.61mm/d. the latter is 3.3 -4.8 times larger than the former's during the whole growing seasons.the L.principis-rupprechtii plantation underwood average transpiration is 1.21mm/d which is 1.5-2 times larger than P.armandii Natural forest,forest transpiration is influenced by forest physiology characteristic,besides,it is also influenced by canopy density and cover degree of different layers.Forest transpiration increase with the canopy density and cover degree increasing.4.The comparison of total evapotranspirationThe sum precipitation of the growing season is 495.4mm in 2009, average evaptranspiration of L.principis-rupprechtii plantation forest is 2.87mm/d during the whole growing season, the evaptranspiration of tree stand(transpiration and interception),shrub, underwood is 53.2%,5.0%, 41.8% respectly. The total transpiration takes up 106.7% of the total rainfall during the growing season. The total amount of average daily evaptranspiration is 2.35mm/d in P.armandii Natural Forest, the evaptranspiration of tree stand(transpiration and interception),shrub,underwood is 44.8%,26.1%, 29.1% respectly. The total evaptranspiration takes up 87.4% of the total rainfall during the growing season. From this we can see that although the component of total evaptranspiration in two forest stands is the same, the evaptranspiration of tree stand is the highest in total transpiration of two forest stands. The evaptranspiration of underwood takes second place, the transpiration of shrub is smallest.The components made up different percentage obviously. It is because of the different stand structure in two forest.The total evaptranspiratio of L.principis-rupprechtii plantation is larger than the local homochronous rainfall, so it may lead to the lack of the water in the soil, therefore, it is necessary to adjust density of the tree stand properly,it is useful for decreasing the over consumption of the water.
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