Spatio-temporal changes of vegetation coverage in Jingchuan County based on an analysis of TM imagery

Abstract

Declines in vegetation coverage can have adverse effects on the environment. Analyses of vegetation cover change are particularly important in semi-arid areas, where the results of such work can help to guide soil and water conservation efforts.In this study, based on Thematic Mapper(TM) imagery of semi-arid Jingchuan County, China, from 1987 to 2015, the spatio-temporal trends in vegetation coverage were analyzed with the Normalized Difference Vegetation Index(NDVI). The results showed that the vegetation coverage in Jingchuan County was high(38%–58%), and the vegetation fraction showed an overall upward trend in most areas. In particular, the average vegetation coverage in the towns/townships of Feiyun, Gaoping, and Yaodian was higher than50%, and these areas displayed obvious upward trends during the study period. The lowest average vegetation fraction of 38%–39%was detected in the towns/townships of Chengguan and Ruifeng; however, these areas also displayed upward trends during the study period. The vegetation coverage in the other towns/townships was in the range of 42%–49%. The increase in vegetation coverage in recent years was mainly due to the implementation of projects to convert farmland to forest and the development of orchards; notably, these are ashad slopes less than or equal to 2°over most of the terrain and the survival rate of vegetation after artificial planting was high.The Zhanglaosi farm, where the annual coverage was 48%, was one of the only places that showed a downward trend in vegetation coverage during the study period; this decrease could have been due to the loss of old orchard trees.Small areas along the river also displayed downward trends. The average vegetation coverage was different in areas with different slopes, and the coverage ranking from high to low for different slopes was as follows: 6°–15° (52% coverage), ≤2° (50% coverage), 2°–6° (47% coverage), >25° (42% coverage), 15°–25° (41% coverage).In regard to the inter annual variation, the correlation between the vegetation coverage and climate factors was assessed. The annual vegetation coverage was found to be significantly related to the annual rainfall. In particular, the annual vegetation coverage tended to increase with increasing amounts of rainfall.The relationship between the vegetation and annual average temperature was also positive but not significant.