Dynamic Changes In Vegetation Coverage Using 1990-2013 Landsat Time Series

Vegetation coverage is an important indicator that has usually been used to analyze and monitor the state of plant growth. Working on vegetation cover degree of landscape pattern change and its dynamic evolution research has gigantic or important theoretical and practical value about analyzing a region of the evolution of surface vegetation and the factors that caused the change of the vegetation distribution.In this paper, Landsat-5 TM and Landsat-8 OLI image was taken as research object in September,1990,2000 and 2013 in Hefei. Normalized difference vegetation index (NDVT) and dimidiate pixel model were used to estimate vegetation coverage. The vegetation coverage of Hefei city was divided into 5 levels (non-vegetation coverage, low vegetation coverage, moderate vegetation coverage, moderate-to-high vegetation coverage and high vegetation coverage) by the method of equal interval division based on the characteristics of the study area, and then made the results into the distribution map of vegetation coverage. The qualitative and quantitative analysis of the dynamic change of vegetation coverage and the change of landscape pattern was carried out based on transition matrix method, difference method and landscape pattern index method.8 evaluation indexes (PLAND, NP, PD, AREA-MN, COHESION, CONTAG, SHDI and SHEI) were selected to analyze the variations characteristics of the area, quantity and spatial landscape pattern of vegetation with different coverage degree in Hefei, which was finished in FRAGSTATS 4.2.1. The qualitative analysis of the distribution of vegetation coverage in Hefei found that, the spatial distribution of vegetation coverage is centered on the main urban areas and divergence from inside to outside form 1990 to2013. And the vegetation coverage of main urban was not high, while it was high in outside. At the same time, reducing of vegetation was increasingly evident in some regions with active economy; the northern and eastern of the study area had evident vegetation degradation.2. From 1990 to 2013, the vegetation coverage in Hefei was increasingly decreased. And from 1990 to 2000, vegetation degradation was severe. For example, the decreased area of high vegetation coverage was the largest and it accounted for 15.04%. While the conditions of vegetation began to improve slowly from 2000 to 2013. The areas of low vegetation coverage and middle vegetation coverage had decreased in different degrees, but middle-high and high vegetation coverage had increased oppositely. The analysis of transition changes found that, the degradation areas of high vegetation coverage were 2480.5 km2 from 1990 to 2013, and it had more 1196.58 km than areas of the evolution. And the areas of middle vegetation coverage degenerated into middle-high and high vegetation coverage by 778.77 km2 and 1094.06km2, respectively. These data indicated that the vegetation coverage of Hefei had deteriorated in the past 23 years.3. From 1990 to 2000, the total number of patches was reduced from 833488 to 554933, while diversity index and evenness index were on the rise. This showed that the landscape diversity of vegetation is relatively good in Hefei, and combining the changes of spread degrees between 1990 and 2013 indicated that landscape showed a clear gathering trend on the whole and had good natural connectivity.4. The landscape patterns’variation of vegetation coverage in Hefei was mainly affected by natural factors, social and cultural factors, policies of government. Furthermore, the natural factors mainly include temperature and rainfall. The annual average temperature showed a clear upward trend, the annual precipitation showed a downward trend in nearly 23 years, which accelerated the loss of surface water and increased the degree of drought in the region. Then the growth of vegetation and the change of vegetation coverage were affected. The social factors include population, economy and changes of land cover types. The total population increased by a large margin from 1990 to 2013, which increased from 3.8642 million to 7.611 million, and housing and transportation land area increased significantly. The above factors resulted in the bad changes in vegetation condition and the vegetation cover degree changing. The policy of returning farmland to forest has greatly increased the change of forest land area, which makes the area of moderate-to-high vegetation coverage and high vegetation coverage increased in 2002.