| Water is one of the important factors effects the ecosystem stability in arid area, regional hydrological processes dominate ecological processes. Excess development and utilization of water resources caused by population growth and social and economic development abounds in arid areas, giving rise to the contradiction between production water and domestic water and ecological water and a series of environmental problems like ecological environment deterioration, and seriously confining sustainable development of regional economy and sustainable improvement of people’s living environment. The primary cause of the deterioration of ecological environment is irrational distribution of water resources, which results in the conflict between national economical water requirement and ecological and environmental water requirement. Hence, to solve the ecological environment deterioration problem in arid region, water resources must be rationally allocated. The key problem of water allocation is to determine ecological water requirement. As the cycle process of water in the soil-plant-air continuous system is very complex, it is hard to determine various water ins and outs accurately and precisely in water balance calculation. However, the effects of vegetation in various parts of hydrology cycle process and its response to different environmental variations make it very important to the relationship between vegetation and soil water in water budget process. What strategy would plant adjust to water stress under the ecological environment characterized by wind and sand and drought? How plants coexisting in the same place avoid excess competition for water? With the intensification of water resources development and utilization, what impacts would groundwater decline and surface runoff decrease have on plants? When the climate change result in the variation of precipitation quantity and form, different plants would have what kinds of responses? To answer these questions, it is essential to study the origins of plant water. In addition, researching how do plants make use of the limited water and responses to hydrological rhythm change have significance for maintenance of ecosystem stability.Under the background above, in our research, we choose Tamarix ramosissima Ledeb as the object, which is an excellent wind-breaking and sand-fixing shrub species widely distributed in the riparian of middle reaches of Heihe River..On the basis of Predeeessors’work and the adoption of stable isotope technique and advaneed Dynamax’s thermal banlence Probe method, we have a detailed analysis on about the water absorption, characteristics of stem sap flow, WUE, and water utilization, in order to clarify the response mechanism of T. ramosissima under different hydrology processes.The main study results are as follows:1. Hydrology processes and D and18O isotopic characteristics of riparian waterThe study area rainfall scarce and has concentrated features:rainfall commonly occur in every June-September, Heihe River water level and observation points during the underground water level fluctuation are more frequent; Because it’s the soil water content, with gas rainfall years and uneven distribution of different length for the rainy season, dry weather underground water level fluctuation gently; The rainy season by artificial storage effect, rainfall and water supply side effects, rivers and underground water original joints was changed, the response to go up drop in groundwater time lag effect, among them, at the end of June to mid August, the time lag of about three days, the late into the rainy season lag time reduced to less than one day. Two observation are nodded at different distances away from the river, because of make lateral velocity flow route by infiltration on the length of the larger impact, and at the same time, the differences of media types, and caused the soil infiltration coefficient vertical division exist, these factors, leading to the change of underground water level a response on vertical direction gradient river there.The LMWL was δD=8.01δ18O+1.51(r2=0.94, n=136), with smaller intercept. The d-excess went higher in winter and became lower in summer. The8D and δ18O values of precipitation presented an obvious seasonal variation:the isotopic composition of precipitation in rainy season in2008fell on the Local Meteoric Water Line(LMWL), its δD-δ18O relationship line was close to LMWL and a little lower than Global Meteoric Water Line(GMWL), and its intercept was approximately equal to that of LMWL. All of these showed that the water vapor originates from ocean and it belongs to monsoon rainfall. The isotopic composition of precipitation in dry season fell on the upper left of Local Meteoric Water Line(LMWL), its slope was far lower than that of LMWL and GMWL, and its intercept were larger than that of LMWL. These not only indicated that precipitation experienced evaporation and accumulation effect, but also implied that water vapor came from evaporation of local surface water and groundwater and transpiration of plant leaves. During observation, the isotopic composition of D and18O in river water presented an increase trend compare to the river water in Qilian mountain Pass. For one thing, river water, an open water system, underwent intense evaporation and caused heavy isotopes to accumulate; for another, precipitation water whose isotopic composition were more positive or water from upper reach reservoir recharged the river. In rainy season, there were positive correlation of3D and δ18O variation of between river water and precipitation, showing that rainfall was the main recharge source of river. Average values δ18O and δD in groundwater remained constantly, suggested that groundwater was recharge from Qilian mountain.Soil water content (SWC) varied under different site conditions. Rain had little impact on the deeper soil water potential but the surface soil. And the soil moisture of different soil depths in different sites varied significantly. The average values of δ18O in soil water samples increased as soil depths grew and the upper soil layer could be easily affected by environmental factors.2. Dependence of plants on groundwaterBy comparing theδ18O values of T. ramosissima stem water with those of various water resources, it was found that the Depth of water extraction by plants varied with the change of seasons. For T. ramosissima in NO.1site, When the groundwater level is low, T. ramosissima absorbs shallow soil water (15-35cm), while the groundwater level is high, it absorbs deep soil water (deeper than60cm), the root system of T. ramosissima is quite Plastic,which can help the Plant to adapt to different water capacity in different ground substanees. For For T. ramosissima in NO.2site, Throughout the growing season,the water utilization source for T. ramosissima was from shallow soil water (15-35cm). T. ramosissima in both sites did not take up rain water or river water during study period. T. ramosissima absorb soil water in a definite range of depth and its absorption depth was affected by groundwater level.3. WUE of T. ramosissima in riparianThe13C value of leaves was measured to evaluate the intrinsic water-use efficiency (WUE) of plant. The13C measurements indicated that all plants have higher in the is13C value (high WUE) in arid month (July) under low groundwater level, and lower13C value (low WUE), in wet months when groundwater rosing.4. Transpiration water consumption of T. ramosissima in riparianThe daily variation of sap flow velocity of T. ramosissima is wide-peak curve, and there is a phenomenon of "nap". It can be divided into four stages:rapid ascending period, stationary period, rapid descending period and nighttime period. The variation is mainly influenced by factors such as solar radiation, air temperature, relative air humidity, soil moisture and wind speed. There is nighttime water compensation in T. ramosissima. Nighttime sap flow still exits, but compared to daytime sap flow, the rate variation at night is slow, the curve is relatively smooth and the amount of sap flow is obviously lower. This is to compensate for the water deficit caused by strong transpiration during the day, and the nighttime sap flow rate in different soil water conditions are different. There is a difference sap flow velocity in T. ramosissima. In early July, the sap flow velocity are the highest in the grow season, when present a high soil water content, the sap flow velocity variation has positive correlation with solar radiation and air temperature. while that in middle August reach the lowest in the grow season with the sap flow velocity, the sap flow velocity variation has no significant correlation with environmental factors..In August and September, the sap flow velocity and sap flow capacity begin to increase with the drop of soil moisture, he sap flow velocity variation has negative correlation with soil moisture. |
PDF Full Text Request