Abstract
The Southwest China Karst, the largest continuous karst zone in the world, has suffered serious rock desertification due to the large population pressure in the area. Recent trend analyses have indicated general greening trends in this region. The region has experienced mild climate change, and yet significant land use changes, such as afforestation and reforestation. In addition, out-migration has occurred. Whether climate change or human-induced factors, i.e., ecological afforestation projects and out-migration have primarily promoted forest restoration in this region was investigated in this study, using Guizhou Province as the study area. Based on Moderate-Resolution Imaging Spectroradiometer (MODIS) Normalized Difference Vegetation Index (NDVI) data, we found general greening trends of the forest from 2000 to 2010. About 89% of the forests have experienced an increase in the annual NDVI, and among which, about 41% is statistically significant. For the summer season, more than 65% of the forests have increases in summer NDVI, and about 16% of the increases are significant. The strongest greening trends mainly occurred in the karst areas. Meanwhile, annual average and summer average temperature in this region have increased and the precipitation in most of the region has decreased, although most of these changes were not statistically significant (p > 0.1). A site-based regression analysis using 19 climate stations with minimum land use changes showed that a warming climate coupled with a decrease in precipitation explained some of the changes in the forest NDVI, but the results were not conclusive. The major changes were attributed to human-induced factors, especially in the karst areas. The implications of an ecological afforestation project and out-migration for forest restoration were also discussed, and the need for further investigations at the household level to better understand the out-migration–environment relationship was identified.
Highlights
Karst developed on soluble rock is an ecologically vulnerable environment that is subject to rocky desertification under high population pressure [1]
More than 89% of the forests have experienced an increase in the annNDVI, and among which over 40% were statistically significant at a 90% confidence level
Compared with the normalized difference vegetation index (NDVI) trends from 1982 to 2003 [38,39], the forest greening trends during 2000–2010 observed in this study has spread westward and northward, and browning trends were only found in small patches in the south and east regions
Summary
Karst developed on soluble rock is an ecologically vulnerable environment that is subject to rocky desertification under high population pressure [1]. Forest is generally the dominant land cover type. Once forest is destroyed in a karst region, it is difficult to restore through natural succession, and rocky desertification usually follows due to the unique hydrological and geological conditions. The satellite-based normalized difference vegetation index (NDVI) is widely used for monitoring and characterizing variations in vegetation cover [3,4], productivity [5], and phenology [6,7] from local to global scales. Based on a time series of NDVI observations and on model predictions, many studies have reported general greening trends (increasing trends in vegetation indices) in the northern mid–high latitudes [8,9], which have mainly been attributed to the warming climate [10,11]. Zhou et al [13] found a persistent increasing trend in growing-season
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