Examining Mechanisms of Vegetation Ecosystems Degradation Based on Water in Northern Farming-pastoral Zone:A Case Study on Wuchuan Country

The issues of ecological water requirements have already been a focus of research in the context of increasingly aggravated eco-environment. The ecological water requirement is defined as the water capacity needed by the ecosystem which achieves a certain ecology level or maintains a certain dynamic balance status. The ecological water requirement would be different for different regions and different ecological objectives. Water is the most critical ecological factor over the northern farming-pastoral zones in China. The interrelations and interactions between water and the eco-environment to a large extent dictate the environmental dynamics and status of the northern farming-pastoral zones. In the present work, the authors investigated the mechanisms of the degradation of ecosystems from a perspective of ecological water shortage, with the aim to provide meaningful reference for ecological restoration and reconstruction in the northern farming-pastoral zones. The vegetation ecological water requirements for the Wuchuan County, a typical dry and semi-arid agro-pastoral transitional zone located in the middle part of the northern ecotone and facing serious ecological degradation, were calculated with the Penman-Monteith equation. In addition, the status of ecological water shortage in Wuchuan County was analyzed in detail on the basis of the experimental data of vegetation ecological water consumptions. The results showed that the major reason for degradation of vegetation ecosystems in the northern ecotone was ecological water shortage. The average water consumptions of natural grassland, shrubbery land, arbor land and cropland in Wuchuan County during growing season in 2006 were estimated to be 150.3 mm, 193.4 mm, 213.3 mm and 158.6 mm, respectively. The smallest amounts of ecological water shortage were estimated to be 93.1 mm, 38.2 mm, 251.5 mm, and 96.6 mm, respectively. It was found that vegetation of the ecosystem continued to develop towards the direction of degradation. Reasonable adjustments for the structure of plant species and reasonable allocation for limited water resources were the urgent needs for local ecological restoration and reconstruction. As to the ecological water shortage which caused the ecological degradation in Wuchuan County, the rational allocation for the limited water resource is to increase water supplies for vegetation ecosystems and to make the ecological water consumptions achieve the minimum even the optimum demands for ecological water requirements. Due to combined effects of a variety of factors, the processes of ecological degradation are quite long and complex. To this end, more comprehensive studies are needed in the future.