Study On Ecological Water Circle Mechanism Of Populus Euphratica Forest In Ejina Oasis

Ejina Oasis is an important ecological shelter for the desert zone in Northwest China, Populus euphratica is the dominant tree species of supporting Ejina Oasis, while "water" is the major restraining factor for survival and development of Populus euphratica in Ejina Oasis. In this paper, water physiological characteristics and water circulation mechanism of SPAC system of Populus euphratica were analyzed. We hope to find effective ways and provide scientific basis for the protection and restoration of ecological environment of Ejina Oasis through revealing this mechanism. The main results are as follows:(1) The total growth process of DBH, height and volumn of natural Populus euphratica in Ejina can be divided into three stages,namely slow - fast - slow,fast-growing period is from the 6th to15th years,fast-growing piont is in the sixth year. The optimal DBH growth model of Populus euphratica is Masao Yoshida equation; the optimal height growth model is Rолясрequation; the optimal volumn growth model is mixed equation, the second best model is Weibull equation. According to the survey results of standard plot, the natural Populus euphratica forest in Ejina could be divided into four typical forest types: namely the flood plain mature and over-mature stand of Populus euphratica, young and strong stand of Tamarix ramosissima and Populus euphratica, young stand of Elaeag nusangastifolia,Tamarix ramosissima and Populus euphratica, desertificated open forest of Populus euphratica.(2) The findings for the root system of Populus euphratica are: 88.88% of the root system of Populus euphratica distributes in the soil layers of 40～100cm, the more the soil layers close to the trunk, the more roots distribute. Root distribution amount reduces along with the increase of the distance from the trunk; the overall distribution characteristic of the roots with different level of diameter is that assimilating roots(d<2mm)approximately take up 92.21% of the total root length density, and transporting roots(d>2mm)only take up 8.79%, which indicates that Populus euphratica absorb necessary water for survival using the huge assimilating root system(d<2mm)from the soil, it is a kind of resisting and adapting performance of Populus euphratica to extreme drought.(3) The unit leaf area index of natural Populus euphratica is quite high, which can be 2.90m2/m2, therefore, the energy utilization efficiency is relatively high. The total biomass of 20 years old Populus euphratica is 74.99t/hm2, its annual mean productivity could be 3.75t/hm2·a. (4) The soil sand grain contents of different forest types of Populus euphratica are generally on the high side, especially the content of coarse sand with 0.5 ~ 1.0 mm grain diameter is large. The deep soil in young forest of Populus euphratica contains clay particles. The general characteristics of soil moisture content is: the surface soil of 0~30 cm has dry sand bed, the soil moisture content is almost zero; in the range of 30~150 cm, the soil moisture content gradually increases, while varied gently along with the increase of soil depth, however, the phenomenon of obviously rise in the depth of 60~100cm could be observed. The soil moisture contents of various forest types, in the order from high to low, are: flood plain mature and over-mature forest of Populus euphratica > young stand of Populus euphratica > young and strong forest of Populus euphratica; the order of the water holding capacities of the soils are: flood plain mature and over-mature forest of Populus euphratica > Gobi> young and strong forest of Populus euphratica; the scope of effective soil moisture content for judging the growth of Populus euphratic in Ejina Oasis is between 1.18%～20.05%; the measured numerical value of hydrolic pressure conductivity Kfs of water saturated soil shows that the infiltration capacity of the local soil is quite strong.(5) The diurnal variation curves of photosynthetic rate and evaporation rate for Populus euphratic can be divided into two types, namely the single-peak type and the double-peak type, and midday depression phenomenon emerged in the double-peak type; The seasonal variations of photosynthetic rate and water utilization efficiency for Populus euphratic with different ages are both in the orders of July> May> September; The diurnal variation law of the leaf water potential for Populus euphratic is that the highest water potential value emerged at the approximate time of the dawn, it began to decrease along with the rising of light intensity and air temperature and the declining of relative air humidity, the order of the leaf water potential at the daybreak time was young trees>middle age trees>old trees, which indicates that the water absorbing capacity of the old trees is the strongest, that of the young trees is the weakest; The diurnal variations of sap flow velocity for the tree trunks of Populus euphratic appears to be a single-peak curve in the sunny days of the growing season, seasonal variation laws of sap flow velocity for Populus euphratic with different diameter levels are as follows: that of Populus euphratic with large (60cm) and middle (28cm) diameter levels are both in the order of May>August, that of Populus euphratic with small diameter level is in the order of August > May; A multivariate linear regression analysis was carried out for the major influencing factors of net photosynthetic rate (Pn), evaporation rate (E) and sap flow of the tree trunks, including air temperature (Ta), leaf temperature (Tl), relative air humidity (RH), photosynthetic active radiation (PAR), stomatal conductance (Gs), intercellular CO2 concentration (Ci), vapor pressure deficit of leaf (VPDL) and wind speed (WS), the result shows that the correlation coefficients between Pn, in the order from large to small, are PAR, Tl, RH, Ci, Ta, Gs; the correlation coefficients between E, in the order from large to small, are Gs, PAR, Tl, RH, Ta, VPDL; the correlation coefficients between the sap flow of the tree trunks, in the order from large to small, are E, PAR, WS, Tl, Pn, RH, Ci, Ta, Gs.(6) Through selecting a suitable calculating method, a conclusion was made that the total water consumption of Ejina Oasis was 314.56 million m3, which includes water consumption for production, survival and ecological purposes, the water availability of the Oasis was 441.55 million m3, therefore, the water for ecological, production and survival purposes of the Oasis could basically be satisfied at present. The annual water inflow of Heihe River has to be guaranteed over than 320 million m3, in that case, the ecological environment would not get worse and adverse succession could be prevented.(7) The water potential gradients of Soil-Populus euphratic-Atmosphere (SPAC system) in Populus euphratic with different ages are, respectively, in the system of soil-old tree-atmosphere, 1:94:2757; in that of soil-middle aged tree-atmosphere, 1:171:2757; in that of soil-young tree–atmosphere, 1:140:2757. The water potential gradient of the middle aged Populus euphratic is the maximum, the second is that of the young trees, the smallest is that of the old trees, which indicates that the middle aged Populus euphratic is the most active for absorbing water, and its absorbing capacity is the strongest; the young aged Populus euphratic, though relatively tender, still needs a lot of water supply; the old aged Populus euphratic, with slow metabolism, is tending towards recession, therefore, the power of absorbing water is inadequate.