南昌市植被覆盖度时空演变及其对非气候因素的响应

植被是陆地生态系统的重要组成部分,植被覆盖在空间上的差异是气候和人类活动交互作用的结果。随着城市扩张,人类活动的加剧及不合理的土地利用方式导致了很多生态问题,对植被覆盖有重大影响。基于地形调节植被指数的像元二分模型,利用3期landsat-5 TM影像图分析南昌市植被覆盖度时空演变特征,并结合DEM数据分析植被覆盖度及变化的地形梯度分异规律,利用3期土地利用图量化植被覆盖度变化对土地利用方式转变的响应。结果显示:1)研究区2001—2010年植被覆盖度从0.54下降为0.42,总体上呈退化趋势,2005年之后植被退化有所减缓;2)植被覆盖度的地形梯度变化显著。植被覆盖度与高程呈高度的正相关性,在坡度0—22°梯度带呈现较高的正相关,在坡度22—40°梯度带呈现较高的负相关。80%以上植被覆盖变化集中在海拔30 m以下、坡度4°以下的区域;3)植被覆盖度变化是地形与土地利用综合作用的结果。在平原低丘区,土地利用行为是植被覆盖变化的主导因素。城市的建设和扩张导致占用耕地、林地和草地,以及大面积的撂荒、伐林等土地活动对植被覆盖退化的贡献率为50%以上,是植被覆盖退化的主要原因,而退耕还林还草、废弃地复垦、后备资源开发为植被覆盖增加的主要原因。可为平原低丘区生态环境监测和构建环境友好型土地利用模式提供科学依据。

An analysis of the spatio-temporal variation in fractional vegetation cover and its relationship with non-climate factors in Nanchang City, China

Vegetation is an important component of the terrestrial ecosystem, and its spatio-temporal variations are determined by the interactions of various natural and human factors. Generally, climatic factors (e.g., temperature and precipitation) regulate the growth and spatial distribution patterns of vegetation across a landscape in the long term, whereas non-climatic factors, such as human activities, determine the changes in vegetation cover in the short term. The "returning farmland to lake" and "returning farmland to forest" projects implemented since 1998 have led to many ecological and environmental problems in Nanchang city. Previous studies have suggested that exploring the effects of non-climatic factors (e.g., topography and land use behavior) on vegetation cover can provide scientific bases and references for ecological and environmental monitoring in a low hilly plain region (e.g., Nanchang city). In this study, employing a dimidiate pixel model and three Landsat-5 TM images, we investigated the spatio-temporal variations in fractional vegetation cover from 2001 to 2010 in Nanchang city. The topography-adjusted vegetation index was used to derive the dimidiate pixel model. The fractional vegetation cover was calculated based on Landsat-5 TM images in 2001, 2005, and 2010. A digital elevation model (DEM) was used to investigate the effects of terrain factors on fractional vegetation cover. In addition, three periods of land use maps were used to characterize the response of fractional vegetation cover change to land use change based on the "overlap" and "zonal statistics" tools in ArcGIS. The results showed that:1) the mean values for fractional vegetation cover in Nanchang city were 0.54, 0.45, and 0.42 in 2001, 2005, and 2010, respectively. Overall, the fractional vegetation cover decreased from 2001 to 2010, and this trend was retarded after 2005. Spatially, locations with high fractional vegetation cover were clustered at the center of Nanchang city. Specifically, the fractional vegetation cover degenerated obviously in Xinjian County, Anyi County, and Jinxian County from 2001 to 2005. From 2005 to 2010, the fractional vegetation cover declined in Nanchang County, Jinxian County, and Xihu District and significantly increased in other counties, especially in Anyi County and QinShanhu and DongHu districts. 2) Relationships between fractional vegetation cover and terrain were significant. Specifically, the fractional vegetation cover was positively correlated with elevation and a slope gradient of 0-22 degrees and negatively correlated with a slope gradient of 22-40 degrees. More than 80% of the vegetation cover changes occurred in areas with an altitude of < 30 m and a slope of < 4 degrees. 3) The change in fractional vegetation cover resulted from the interactions of terrain and land use. Land use played a predominant role in altering vegetation cover, especially in the low hilly plain areas. Converting cultivated land, forest land, and grassland to developed land, abandoning cultivated land, and cutting forestland were the main causes of vegetation cover degradation (accounting for 50%). The policies of speeding up of urbanization in the low hilly plain area and the returning of farmland to forest and grassland, reclaiming land, and developing reserved land resources were the main causes for the vegetation cover increase.