Water Use Mechanism Of Typical Tree Species In Beijing Mountainous Areas

Plant water use is an important aspect of plant-water relations in arid and semi-arid regions, and is vital in understanding the adaptation of desert species to seasonal drought environment. Beijing mountainous area is the key ecological shelter for Beijing and an important part for the whole North China Plain, which has shallow and poor soil. The limited water resources also displayed seasonal distributions (dry season and wet season), which caused that the forest usually must endure the seasonal variations of soil availability. Anthropogenic climate change is likely to alter the regional precipitation patterns, through changes to the Earth’s energy budget (Houghton et al.2001), which would affect plant establishment, growth and reproduction. In this background, the plant-water relations became more and more complicated. At present, it is still not clear that how the forest vegetation in Beijing mountainous areas adapt to the shortage of water resources and seasonal drought environment conditions, so it is necessary to research plant seasonal water uptake and conclude the water use mechanism of forest vegetation. In this study, Quercus variabilis L. and Platycladus orientalis L., as the typical species in Beijing mountains area, were selected, for systematical studies of their water use patterns and mechanism. By field investigation and observation about hydrology, soil, vegetation and terrain factors, and combined with the field sampling and stable isotope analysis of rainfall, soil water, plant water and groundwater, the dynamic variation regularity of isotopic characteristics of water cycle component in different time scales were analyze. Rainfall infiltration process and the migration way were analyze by the combination of soil water isotope compositions and soil water content; the source and pattern of plant water use were analyze by the combination of soil water isotope compositions and plant water isotope compositions; the mechanism of plant water adaptation to seasonal drought, was analyze, by the combination of plant physiological and ecological characteristics and plant water use patterns. The major results were as follows:(1) The rainfall isotope characteristics showed unique variation regularity, due to the seasonal variation of water vapor source, and strong local recycle water vapor resulted from Terrain conditions in the study area. large differences existed in the process of typical precipitation event, the slope and intercept of meteoric water line, and annual average of rainfall isotope value in the study areas, comparing with the Global, China and Beijing areas.(2) In the two forest lands, the soil sensitive areas in response to precipitation focused on the surface soil layers (0-30cm); rainfall move in the soil profile by the way of replacement in P. orientalis L. stand; but in Q. variations L stand, rainfall moved by the way of diffusive mixing in the dry season, and the way of replacement in the wet season.(3) The distribution of groundwater isotopic compositions concentrated near the local meteoric water line in the study area, suggested that the groundwater in Beijing mountain area was recharged by local precipitation.(4) The water use pattern of the two species showed similar characteristics within the year, but the water uptake depth of Q. variations L is shallower than P. orientalis L. In rainy season, the water use depth concentrated in surface soil (0-10cm), the main water use areas of two species are similar in the prophase and metaphase stages of dry season, and with the worsening drought, main water sources progressively increased to the deeper depth. And the two species can not use groundwater as their water source.(5) In the dry season, the response of P. orientalis L to rainfall was more sensitive to rainfall pulse than Q. variations L, result in the difference of precipitation threshold requirement, and the minimum threshold is 9.2mm; in rainy season, Q. variations L is more sensitive to rainfall because of the effect of litter layers, and the contributions of litter layers to plant water use progressively decreased with the extension of rain free period.(6) In dry season, the root distribution and soil water content were all the limiting factors of plant water use, but there were not leading factor between them.