Interannual Variation of NDVI, Precipitation and Temperature during the Growing Season in Langtang National Park, Central Himalaya, Nepal

Abstract

Vegetation is an essential component of terrestrial ecosystem, and its responses to the climate change has been recognized as a key indicator for monitoring global climate. This study analyses the temporal and spatial changes of Normalized Difference Vegetation Index (NDVI) in the Langtang National Park (LNP), Nepal, during 2000 - 2018, using the MODIS 16-day NDVI product, and the ERA-5 precipitation and temperature reanalyzes. Regression models were applied to estimate temporal trends in NDVI, and Pearson correlations between NDVI and climatic variables (i.e., temperature and precipitation) were employed to assess vegetation responses to climate change. Average annual NDVI increased significantly (0.002yr-1, p = 0.001), and the average growing season (AGS: April-October) NDVI also increased significantly (overall, 0.0023yr-1) including in spring (April-May, 0.003yr-1) and autumn (September-October, 0.002yr-1). During summer (June-August), NDVI increased by 0.002yr-1 (p > 0.05). Temperature and precipitation both increased significantly during the growing season, and significant increases in NDVI during spring in LNP indicate high levels of photosynthesis, biomass accumulation and productivity. The NDVI relative change ratio (RCR) was 12.79% during the last 19 years in LNP. The spatial distribution of NDVI increased by 30% (p < 0.05) of the area during growing season. Overall, during the AGS, 66% of the study area showed a positive correlation with temperature, of which 9.09% was significant. Positive correlation was observed between temperature and NDVI, and negative correlation between precipitation and NDVI, in the forest, shrubland, grassland and agriculture vegetation types. In the AGS, NDVI was positively correlated with temperature, but weakly related to precipitation. The results demonstrated that increasing temperature promotes vegetation growth. Quantifying the spatial response of NDVI to temperature and precipitation will support further studies on conservation and on vegetation responses to climate changes across this Himalayan national park.