The Lethal Factors And Survival Strategy In Haloxylon Ammodendron Germination And Ecesis Process

Water is the primary limiting factor of desert ecosystem that affects every link of the ecosystem process. Xerophytes show different adaptation strategies under drought stress. At the transition between desert and oasis, the groundwater table falled significantly as a result of overexploitation and global climate change in the near20years, and the vatiations in groundwater will lead to adapative adjustment in water use strategies of the plants, furthermore the changes of the plant community. The study was carried out to evaluate the effects of the remarkble changes in groundwater and precipitation on the survival of Haloxylon ammodendron, which is a native dominant desert shrub species, and find the lethal factors during the seedling stage.Experiments were carried out at at the southern boundary of the Gurbantunggut desert in the growing seasons from2008to2013. The first experiment employed a nested design with snow and rainfall as the two manipulated parameters, and nine treatments including three snow treatments:triple snow, normal snow and no snow, plus three rainfall treatments:normal rainfall, no rainfall and double rainfall were applied. At each treatment plot, soil moistures, numbers of H. ammodendron seedlings and epidermal plants were monitored every5days from March to May on2008to determine the lethal factors of H. ammodendron seedlings in humid rainy period of the Gurbantunggut desert. In the second experiment, growth characterics and survival statues of first-year H. ammodendron seedlings, meteorological factors and soil moisture were monitored through the whole growing season of2009. The third experiment carried out during the growing seasons from2010to2013. The plant height, root length and biomass of H. ammodendron were monitored to determine the survival strategies of H. ammodendron under various precipitation amounts.Results showed that most of the H. ammodendron seedlings death happened in seeding stage. There was no significantly difference of the germination of H. ammodendron seeds under three snow treatments. However, the amount of epidermal plants germinated was significantly correlated with snow amount. This can be due to competition for water between H. ammodendron seedling and epidermal plant, e.g. at treatments with good water conditions, the survival ratio of H. ammodendron seedling was low; on the other hand, at treatments with poor water conditions, because of low competition from epidermal plants, the survival ratios of H. ammodendron seedlings were high. Namely, at the very early stage, H. ammodendron seedling was at disadvantage in competition for water with early spring epidermal plants. As a result, because of competition for water from epidermal plants, good water condition does not necessarily result in high survival ratio of H. ammodendron seedling. In conclusion, this integrated study had advanced our understanding of the H. ammodendron regeneration at very early stage. Soil and atmospheric drought is the major cause of death in H. ammodendron seedling at dry, hot period in Gurbantunggut desert. When the soil moisture content as low as0.82percent, seedling mortality rates have increased dramatically, regardless of the degree of drought in the air; when the soil moisture content was higher than1.25percent, the seedlings could tolerate drought in the air, the seedlings death mortality rate was governed by soil moisture content at layer soil. When soil moisture content was at0.82-1.25%, the extent of drought in air governed the seedling mortality rate fluctuations. We may conclude that:when the soil moisture content at up layer was less than1.25percent, the atmospheric conditions governed the seedling mortality; otherwise, the soil moisture governed. The efficient morphological adjustment is the key factor of H. ammodendron to maintenance photosynthesis and acclimation to variation in precipitation. Diebacks of the assimilating branches in drought period could maintain the balance between water supply of roots and water consumption of the assimilating organs. This morphological adjustment ensured the photosynthetic ability of survival organs. Furthermore, more photosynthate assigning toward roots could ensure H. ammodendron survival during drought and recovering when water conditions become better. The purpose of the change of root morphology and plasticity, adjust of photosynthetic organ area and biomass accumulation, the change of photosynthate distribution between root and cap with various precipitations is to maintain the dynamic balance between root surface area and assimilating branches surface area.This study shows that seedling is the weakest stage in life cycle of H. ammodendron, and always committed highest death percentages. H. ammodendron showed efficient self-adjustment and adaptation abilities in acclimation to variation in water deficit condition. These were mainly achieved by changing the morphology, which were reflected in initiatively maintaining balance between the root surface area and assimilation organs area, intercoordination between photosynthetic organs and assimilation organs, and constitute the optimal strategy of water and carbon acquisition together. Researches on response characteristics and adaptation strategy to global changes provide critical theoretical basis for predicting dynamic changes of desert ecosystem.