The Water Physio-ecology Mechanism Of Hippophae Rhamnoides L. Subsp. Sinensis Decline Plantation

As excellent Multiple Purpose Tree in arid semi-arid areas, and with powerful feature of cloning, Hippophae rhamnoides L. subsp. Sinensis has been playing a significant role in the ecological environment building and forestry industry development. However, some plantations have suffered a large-scale declination even death in recent years due to a negative balance of soil moisture caused by the continuous drought and the forest growth. There are many reports about decline phenomenon and pests but few about water mechanisms of decline. After experiments on different site types and with different soil moisture conditions thanks to different irrigation intensities, this paper tries to analyze the relationships between soil moisture and population density, population structure, clone breed ability, biomass allocation, photosynthesis, Malondialdehyde and reactive oxygen species, to reveal the water physio-ecology mechanism of decline plantation in drought stress in the perspective of ecology and physiology (biochemistry). It could enrich the study of clone plant and forest decline and provide basis for plantation design, forest management and recovery of species. And the main results and conclusions are introduced as below.(1) Drought press is proved the leading cause of the decline of planted Hippophae rhamnoides L. subsp. Sinensis on Mu Us Sandland and Loess Plateau, on Mu Us Sandland, the natural forest on lower site can live more than 50 years and maintain population increase or stability by gap regeneration and edge dispersal, while those on windward slope decline at 8a. Though natural forests possess more soil moisture, organic matter and nitrogen than plantation, it is the soil moisture that has been selected as a factor of the stepwise regression equation and has the determination coefficient of 85%. The field trials further show that Hippophae rhamnoides L. subsp. Sinensis grows faster and has better fertility when was irrigated by water of 900mm-1050mm,3.0-3.5 times more than the normal rainfall. On the Loess Plateau, the soil moisture decreases as well as the population stability and growth rate on northern slope, western slope, the top, eastern slope and southern slope in turn. The stability and growth rate of population decrease in turn. The forests of 8a keep growing on northern and western slope and maintain stable on the top area and on the eastern slope while those on the southern slope have show decline. Therefore, under the circumstance of lack of enough rainfall, it is the soil moisture on the forested area that decides the trend of community succession.(2) Compared with natural forest, Hippophae rhamnoides L. subsp. Sinensis possesses less soil moisture, nutrient content, productivity, biodiversity and lower fertility and shorter life expectance of ramet. Drought press leading to the decline of reproductive ability and growth is the fundamental cause of the population decline by interrupting the process of self restoration. The vary of soil moisture in different sites leads to the change of fertility and growing ability, and thus affects the population stability and life expectance of ramet. In Mu Us Sandland, natural forest shows advantages in reproductive ability, productivity, life expectance and population stability if compared with plantation. But the plantation forests on sand-dune performs better than those on the winward slope, on depression better than on the flatland in terms of reproductive ability, productivity and population stability. The field trials further show that Hippophae rhamnoides L. subsp. Sinensis grows faster and has better fertility when was irrigated by water of 900mm～1050mm,3.0-3.5 times more than the normal rainfall. On the Loess Plateau, the reproduction and production of the population are positively related with the soil moisture. So under the circumstance of lacking rainfall, drought press will lead to population decline if the soil moisture in forested area fails to meet the requirement of growth and reproduction of Hippophae rhamnoides L. subsp. Sinensis.(3) Under different water conditions Hippophae rhamnoides L. subsp. Sinensis must balance the survival against reproduction through biomass allocation and adjustment to maintain the persistence of clone and stability of population. The population growth and reproduction ability correlate positively with the biomass investment of the population and modular, biomass allocation rates among tree trunk and organ of cloning, while correlate negatively with biomass allocation rates between branch and vertical roots. The biomass investment of the population and stem, biomass allocation rates among tree trunk and organ of cloning have an inverted U-shaped relationship with the irrigation amount, while the biomass allocation rates between branch and vertical roots show a U-shape relationship with the latter. Therefore, with proper water condition, Hippophae rhamnoides L. subsp. Sinensis would performance better in population growth and reproductive ability if it invests more biomass in population and its stem and allocates more to the tree trunk and organ of cloning. Under the circumstance of drought and lack of water, the population would allocate more biomass to the growth of vertical roots to obtain more water and nutrient to maintain the survival and keep growing, which would surely decrease the population growth and fertility.(4) Through changing the water conditions of plant tissues, the irrigation would affect the photo-physiological and water physiology characteristics, of which changes would influence the population biomass investment and allocation status. The population then relatively forms certain behaviors to adapt to the water supply, which is to maintain the persistence of cloning and population stability by balancing between survival and reproduction. Under high moisture conditions, the increasing of soil moisture in plant tissues would lead to a rise in photosynthetic rate and decrease in MAD and reactive oxygen species radicals, the population invest and allocate more biomass to the organs of cloning, which the whole process will benefit the productivity and reproduction. Therefore, population with higher plant height and intensive distribution would be good for the persistence of cloning and population stability. In contrast, low moisture conditions would depress the productivity and fertility. It is concluded that the population with short plants and scattered distribution would help the survival and growing rates but negatively affect the persistence of cloning and population stability.