Study On Water Use Mechanism And Its Sable Isotope Of Typical Tree Species In Beijing Mountainous Areas

Beijing mountainous area is not only the key ecological shelter for Beijing but also an important part for the whole North China Plain. The soil of this area is shallow and poor, and the water resources is limited. The rainfall distribution of this area is uneven, which caused the forest must endure the seasonal variations of soil water availability, the forest is extremely easy to dry and be in a state of severe water shortage. To reveal the way forests survive and thrive in such conditions, the water use sources and the water use efficiency in different seasons need to be investigated to enhance the understanding of water use mechanism. Our study deepened the exploration of how plants adapt seasonal drought, and has significant importance for the mountain vegetation restoration and reconstruction.In this study, P. orientalis, Q. variabilis, R. pseudoacacia and P. tabulaeformis were selected to formulate the plant water use mechanism. By field investigation, different samples based on different temporal scales were collected for isotopic analysis. The source of plant water use was analyzed by the comparison of soil water isotope compositions and plant water isotope compositions. Isosource method was used to qualify the contribution rate of different water sources. By measuring isotopic compositions of different plant’s leaves, water use efficiency was analyzed in different seasons. Finally we conclude the water use mechanism through plant’s water use sources and water use efficiency.The major results were as follows:(1) During the dry season, the main water sources of plants are the middle and deep soil water. At the beginning of the dry season, P. orientalis, R. pseudoacacia and P. tabulaeformis manly used 20-40cm soil water; Q. variabilis’s water use sources is 40-60 cm. In the late dry season, after experienced a long period of drought stress, P. Orientalis and P. tabulaeformis tend to obtain water from deeper area of soil, their water use source depths is 40-80cm and 40-60cm; R. pseudoacacia and P. tabulaeformis’s water use sources maintained in 20-40cm. (2) During the rainy season, the main water use of plants is the shallow soil water. At the beginning of the rainy season, in addition to P. tabulaeformis which mainly use deeper soil water, other three typical species use shallow soil water:P. orientalis’s water use sources is 0-10cm, R. pseudoacacia’s water use sources is 0-20cm, Q. variabilis water use sources’s is 0-10cm and 20-40cm. (3) In response to rainfall, Plants shows flexible water use mechanism which use the surface soil water in the early stage after the rain, and use the former layer soil water as time continues. (4) The stable carbon isotope value of plant’s leaves indicate water use efficiency. Q. variabilis has the highest water use efficiency and water of P. tabulaeformis has the lowest efficiency. Overall, the water use efficiency of 2014-2015 is lower than the water use efficiency in 2013-2014. (5) The stable carbon isotope value of the dry season is higher than the rainy season, which shows that the plants increase water use efficiency to adapt to the arid environment.