Spatio-temporal Trends in Seasonal Vegetation Photosynthetic Activity Across Uttarakhand Himalayas, 2000-2014

Abstract:

Himalayan mountain system in the Indian sub-continent are among the most ecologically sensitive environments and are also a repository of biodiversity and ecosystem services. Over the last few decades, land transformation related to exploitative land uses is among the main drivers of changing vegetation cover and productivity in western Himalayas. In a region where field based research is challenging due to heterogeneous relief and high altitude, quantifying changes in vegetation productivity using remote sensing can provide essential information regarding trends in vegetation cover and its linkages with Land Use Land Cover (LULC) dynamics. We conducted seasonal trend analysis (STA) on MODIS NDVI time-series data (2000-2014) over Uttarakhand Himalayas and examined spatio-temporal patterns in vegetation trend and its association with altitudinal gradient and LULC dynamics. In STA the first step determines the annual mean and seasonal NDVI patterns and the second step analyzes the non-parametric trend in magnitude and timing of the annual mean and seasonal NDVI cycle. In total 3286.82 km² (6.9% vegetated area of Uttarakhand) showed significant trend (p<0.01) in mean annual greenness. While areas <800 m elevation showed dominant negative trend in mean annual greenness, area between 800-1600m showed mostly positive trend and majority of areas >1600 m were characterized by negative trend in mean annual greenness. Considering LULC characteristics, majority of intensively cultivated irrigated croplands in the Himalayan foothills as well as areas around growing urban centers showed widespread negative trend in mean annual greenness, which was contrastingly different from rainfed cultivation areas that showed dominant positive trend. Negative trend in mean annual greenness was observed to be consistent with increasing altitude and particularly in closed needle leaf forests and alpine shrublands except areas where human impacts has led to mixed patterns. Trends in the annual seasonal timing of NDVI indicated an earlier green-up for most parts of the Uttarakhand Himalayas. These results are somewhat surprising and are partially in agreement with previous studies that suggest increasing brownness in Himalayan vegetation based on analyses of comparatively much coarser spatial resolution NDVI time-series.