Glaciers retreat-induced gravelization encroached alpine hillside grassland: A case study in Qilian Mountain, China

Abstract

The current global warming is accelerating deglaciation in polar regions, directly or indirectly threatening terrestrial environment and conservation. During deglaciation, a climate warming-induced secondary disaster − gravelization encroachment in alpine hillside grassland appears as a threat of alpine grassland ecosystem, however, this phenomenon remains largely overlook. Here, we mapped in detail gravelization in alpine hillside grassland by a representative glacier as a case study, and aggregated all available surface reflectance Landsat 5 TM and Landsat 8 OLI_TIRS images covering study area between 1987 and 2019 based on Google Earth Engine. Annual cell-specific Normalized Difference Vegetation Index (NDVI) values were calculated for the Maximum Value Composites, and cell-by-cell non-parametric Sen’s slopes were computed. The high resolution (0.5 m) remote sensing images taken in 2008 and 2019 were interpreted to validate Sen’s slopes analysis of NDVI. We found that glaciers retreated as average annual temperature increased from the period between 1987 and 2019, with a total melting area ratio of 65.02 %. More importantly, after glacier retreat, the gravel debris fragments accelerated sliding, covering and reducing the alpine grassland area. NDVI value at the grassland − gravel debris interface showed a decreasing trend, especially after 2000. Gravel debris expansion caused the loss of 12.98 ha of alpine grassland between 2008 and 2019. Overall, as a consequence of the current climate change, the gravel debris covered area replaced the glacier and grassland gradually, giving rise to this emerging surficial process. Gravelization encroachment in alpine hillside grassland has profound implications for alpine grassland ecosystem that should be paid more attention in future studies.