Spatio-temporal variation of vegetation carbon storage and density in karst areas of Northwest Guangxi based on remote sensing images

The southwest karst area in China could be a key carbon sink after appropriate ecological engineering. Vegetation carbon pool has been noted to be more critical for carbon budgets of ecosystems than shallow soils which have limited carbon saving capacity. However, due to the unique nature of geological settings of karst regions, spatial distributions of vegetation in these regions usually follow three-dimensional characteristics of discontinuity and fragmentation. This has resulted in difficulties in obtaining the belowground portions of vegetation, which have in turn caused a general lack of attention to this issue. Thus vegetation carbon pool has become a limiting factor in the assessment of regional vegetation carbon savings. In this study, we attempted to reveal the spatial distribution characteristics and differentiations of vegetation carbon savings in typical karst regions by analyzing remote sensing images. The spatio-temporal variation of vegetation carbon storage and density in typical karst areas of Northwest Guangxi in China was analyzed using remote sensing images of 1990, 2000 and 2005 in combination with elevation and weather data in geographic information system (GIS) environment. The results showed that vegetation carbon storage and density in the study area increased for 1990-2005. Vegetation carbon storage increased from 1.03×10⁸ t in 1990 to 1.41×10⁸ t in 2000 and then to 1.63×10⁸ t in 2005. The corresponding carbon density was 14.82 t·hm^-2 in 1990, 20.38 t·hm^-2 in 2000 and 23.49 t·hm^-2 in 2005, consistent with vegetation carbon density in Sichuan (18.47 t·hm^-2) and Jiangxi (25.38 t·hm^-2) Provinces of China. With regards to spatial distribution of carbon storage and density, high values were noted for the west or high-elevation (elevation > 500 m) regions and low values in the east or low-elevation (elevation < 500 m) regions of the study area. The carbon density was 15 22 t·hm^-2 in most of the western counties of the study area while it was 8 15 t·hm^-2 in most of the eastern counties of the study area in 1990. While an explicit increase was noted in the trend of change in spatial distribution of carbon storage and density in low-value areas (the eastern part of the region), slight decrease or increase was noted in high-value areas (the western part of the region). Vegetation carbon density obviously increased in typical karst areas with carbon storage to vegetation ratios of 45.54% in 1990 and 51.99% in 2005. The study suggested that vegetation carbon storage and density in cluster and depression regions obviously increased. Also ecosystem conditions accordingly improved due to policy initiatives regarding prevention of rocky desertification such as ecological migration and returning farmlands to forests. This had clearly benefited by enhancing vegetation carbon storage. This paper showed that assessments based on remote sensing images could provide the scientific basis for reducing uncertainty in estimating carbon stocks in karst regions and carbon balance in terrestrial ecosystems. It provided the basis for scientific reference regarding beneficial evaluation and adaptive regulation of rocky desertification in karsts in a comprehensive manner.