Response Of Grassland Vegetations Distribution To Climate Change In Loess Plateau

Taking into account frontier problems that the eco-environmental construction of the Loess Plateau involve in grassland science, ecology and environment sciences, depending on the summarization of previous relevant research achievements, taking grassland vegetation distributions and their climatic responses of the meadow, typical, desert and scrub grasslands distributing in the loess Plateau as its subjects, and proceeding from the angle of grassland vegetations and climatic variations against different longitudes in the loess Plateau, the study collected relevant data of 105 counties by field survey and sampling, lab and statistic analysis and scientific deduction and probed into temporal and spatial water and nutrient variations during the degeneration and rehabilitation of the different types of grasslands according to the data, with the purpose of revealing climatic response and spatial variation patterns of indicator species in different vegetation zones, putting forward regulating measures for rehabilitating different vegetations under the different climatic zones in the loess plateau, and providing basic data and theoretical basis for sustainably and stably improving the vegetation productivity as well as scientific and theoretical guidance for maintaining the ecological balances, human beings-nature harmony in the loess plateau and carrying out eco-environment construction projects for West China. The main results and conclusions of the study were as follows:(1) The study investigated the tendencies of major climatic factors of typical regions and different indicator species of the grasslands in the loess plateau. It revealed that in the past fifty years the annual rainfall of the semi arid regions of the loess plateau generally tended to decline with its decrements totaling 40.31 mm, averagely 7.9 mm every 10 years, and in the latest ten years, its annual rainfall amounted to only 392.85mm, reducing 47.86mm by 10.86% compared with its annual rainfall in the past fifty years, so that the grassland vegetation damage reached its extreme; in the latest thirty years, its average annual temperature increased 1.2℃, averagely 0.4℃every ten years. It appeared obvious that because of global climatic drying, the typical indicator species varied in their roles and that spread into the other grasslands so as to cause some dominant species to gradually become companion ones.(2) The study revealed the characteristics in soil water variations of the different grasslands in the loess plateau. In the loess plateau, where the longitude ranged within 100°～ 114°, the 0～100cm soil water contents of the meadow, typical, desert and scrub grasslands varied within17.28～24.68%, 11.7～21.84%, 5.5～17.0% and 13.96～21.26%, respectively; the different soil layers of the grasslands presented different soil water variations: the 0～30cm soil presented a soil water content that varied rapidly because of the rainfall, thus a layer where the soil water was quick to change ; 30～60 soil suffered water deficiency arising from water use because of the influence of the rainfall intensity and the soil bulk weight, thus a layer that suffered water deficiency ; The soil below 60～100cm deep was slow to recover its water because of the soil water reservoirs, thus a layer where the soil water was slow to recover.(3) The study examined soil nutrients of the grasslands in the loess plateau. In the loess plateau , the soil nutrients of the different grasslands ranked in the order of meadow grassland>typical grassland>scrub grassland>desert grassland ; Because the different grasslands had different grazing-prohibiting periods and degeneration degrees, their vegetation growths and soil nutrients significantly differed, all the four steppes increasing their soil nutrients as their grazing prohibiting periods prolonged and decreasing their nutrients as their degeneration degrees and soil depths increased.(4) The study probed into the productivities and climatic responses of the different grasslands in the loess plateau. In the loess plateau, the rainfall increased as the longitude increased from west to east; and the rainfall increased from 410mm to 660mm as the longitude increased from 100°to 114°, the productivities of the four grasslands correspondingly tended to gradually increase and appeared extremely significantly correlated with the rainfall variation and that the effects of the rainfall on the productivities were additional. This corresponded with the growths and water requirement patterns of the grasslands, which indicated that within the longitude range of the loess plateau, water was the primary driving factor affecting the grassland productivity.(5) The study revealed typical species and their response characteristics to climatic changes in the loess plateau. In the recent years, because of the influences of global climatic changes, the grassland species appear sensitive and their distributions are gradually expanding. The Bothriochloa ischcemum keng-represented four typical species of the meadow grassland had transformed from edificator species into typical grassland dominant species. The ten species of the typical grassland could be divided into three types, gramineae-dominant xerophytes and mesoxerophytes, which distributed in both the meadow and typical grasslands; Compositae dominant xerophytes and mesoxerophytes, which expanded from the typical grassland into the meadow and desert grassland; mesoxerophytes over which legumes and other plant families dominated , which were not only the edificator species of the typical grassland but also expanded into the desert grassland, which indicated that the climatic drying got intensified. The four typical species of the desert grassland were not only its own edificator species, but also expanded into desertifying grassland areas and mixed communities composed of such extreme xerophytes as Artermisia Dalailamae. Among the seven species of the scrub grassland. such species suitable for growing on the meadow grassland as Sophora davidii had expanded into the typical grassland and become its dominant species; among the species that grew on the typical grassland, such species as Caragana korshinskii had expanded into the desert grassland and become its dominant species; Periploca sepium that grew on the desert grassland had expanded into the ecotone between the typical and desert grasslands and appeared as the companion species or occasional species of the ectone. All The above described species distributions could be used as indicators or important markers for the climatic changes and that provided scientific basis for researching on climatic changes, species diversity and distributions of China.(6) The study found it out that in the loess plateau, the grasslands appeared obvious to change their distributions. Analysis according to the geographical distributions and floral elements and environmental conditions and in combination with the survey and research data and literatures about the vegetations in the loess plateau indicated that the tendency of modern climate tending to warm and dry as well as repeated human vegetation damages caused the border line of the forest grassland to recede south, the borderline of the typical grassland to continue expanding, moving south and directly invading into the forest grassland, the desert grassland to continue moving south into the typical grassland, so that currently the forest grassland, typical grassland and desert grassland presented obvious mosaic vegetation distributions in the loess plateau.