Land Use/Cover Change And Its Environmental Effects In Sanjiang Plain In The Past 50 Years

Land resource is the living and development basis of human being. In the last few decades, human have been altering the earth's surface to produce food through agriculture activities. The acceleration of land use/cover change (LUCC) has spurred renewed concerns about the role of land use change in driving many environmental problems. Research on the causes and consequences of LUCC has been the focus of global environmental change studies. The west part of Northeast China locates in transition zone from semi-humid climate to semi-arid climate with many types of ecological landscape and is a typical area between agriculture and husbandry. Under the pressure of population growth and excessive land exploitation, such as over grazing and over reclamation in last century, the eco-environments that have been fragile become more vulnerable and unstable. This area is the typical and representative region of LUCC research in China.Using the tool of GIS with remote sensing images and other ancillary data on the basis of former studies, we analyzed the changing procession of land use/cover dynamic and it's characteristics according to the land use dynamic model, landscape pattern model and reclamation index, and then discussed the effect of driving forces, such as natural environmental change and social economical development, on land use/cover dynamic. At last we draw following conclusions:Firstly, there had been a great regional LUCC in Sanjiang Plain during the past 50 years. The area of agricultural land had increased about 4 times due to 3 large scale agricultural exploitations and subsequently continuous sporadic reclamation activities. Because of the large amount of reclamation activities,also occurred the great changes of other land use types.Secondly, the landscape pattern had also greatly changed during the past 50 years in Sanjiang plain. Most wetland, which was the background of landscape in 1950s originally, had been substituted by the crop land, a type of agricultural landscape, in large scale. The woods land, an important landscape type in this place, also changed dramatically in past 50 years: only the woods located on higher or bigger slope places were kept in better conditions relatively, and the place of the island-shape woods, which lain on the mesa or plain, were almost taken by the agricultural landscape. On the one hand, the landscape of grassland, which were of better reclamation condition for flat relief and fertile soil, had already been severely destroyed during the period of former exploitation in Sanjiang plain, on the other hand, because of the differences among exploitation periods and natural conditions, the modes and degrees of landscape dynamic were various in history.Thirdly, the LUCC reflected the interactional relationship between human and natural environment confessedly. Natural conditions were the basis or the prerequisite for land use dynamic driven by the social factors. Our analysis showed that: the land use change in Sanjiang Plain was closely related to the natural conditions, such as topography, slope, relief and soil condition etc, and for these reasons, it was on plain area, which has lower and flatter topography, good soil condition that the land use change were more acute in the beginning of exploitation. When the land resources in better conditions were to be exhausted, the land use in areas which were even unsuitable for reclamation also began to change rapidly. The most direct driving force of land use dynamic in Sanjiang Plain was agricultural reclamation as a result of both national food demand and dramatically increasing population mainly caused by migration. Population increment was the most direct and important factors for the area increment of crop land in Sanjiang plain. For the differences of natural condition, exploiting history and increasing mode of population in different periods and places, the relations between population and crop land were remarkably different.Fourthly, our analysis according to spatial distance model showed that: in macro-scope, wetland and woods land in Sanjiang Plain took gradually decreasing trend in past 50 years, and in different periods, the decreasing degrees of wetland and woods land were different, especially according the spatial characteristic of their locations to other types of land use. In past 50 yeas of reclamation, there exists a stable macro exploitation mode, and in fact it presented the macroscopical idea of reclamation, i.e., after construction ofroads and residential area as the forerunner, the cropland were reclaimed according to hydrological condition.At last, LUCC in Sanjiang Plain caused a series of regional eco-environmental problems, and it even threatened the regional agricultural production and ecological security. The main eco-environmental effects caused by LUCC in this place were as following: (1) ecological diversity in this region were lost, the genera of wild animals were degraded , succession of the botanic communities took place and as a result the net primary productivity of vegetation decreased;(2) after reclamation from wetland, the bulk density of soil increased, the porosity of soil reduced and the soil nutrient decreased dramatically;(3) after reclamation, the evaporation of wetland remarkably decreased, the runoff coefficient increased while the water table decreased with large scale plantation of paddy;(4) in past 50 years, the air temperature increased 1.2-2.3 centigrade in Sanjiang plain, this increasing trend was far more intensive than the one in the whole northeast China or the north part of Heilongjiang Province and the annual precipitation presented a decreasing trend in past 50 years in most places in Sanjiang plain;(5) after wetland reclamation, the carbon sink capability of wetland were weakened and the place was to change into carbon source, while CH4 flux deceased.