[Differences and mechanisms on water use of var forests with different origins].

Determining the differences of water use characteristics of a tree species with different origins (natural forests and introduced plantations) is significantly important for forest sustainable management. var. is an important tree species of afforestation in the 'Three North' project in China. In this study, with var. from two origins, we monitored the sap flow velocity of sapwood () of trees by thermal dissipation sap flow probes, and analyzed the relationship between water transportation and the environmental factors during the growing season. The results showed that under the typical sunny day, daily sap flow velocity () of trees from plantations was significantly higher than that from natural forests. The mean value of was 132.98 and 114.86 cm·d for the two origins, respectively. Trees from plantations showed higher water transportation potential than natural forests. Vapor pressure deficit (VPD) mainly showed the driving effect on the water use process of trees from natural forests. In the plantations, there was an obvious threshold effect, and the inflection point of VPD was about 1.91 kPa, with the boundary function of increased to the maximum of 17.88 cm·h. Atmospheric driven transpiration potential (/VPD) of var. trees with two origins decreased with the aggravation of soil drought, but sensitivity to drought was higher in the plantations than in the natural forests, suggesting the strong ability of var. to regulate water use process.