Characterising the vegetation-rainfall relationship in the Northeast Himalaya, India

Ecosystems are complex systems and are characterised by positive and negative feedbacks between the abiotic and biotic components. The response of an ecosystem to its environment can be determined by examining state diagrams, which are plots of the state variable as a function of the environmental driver. For instance, tree cover as a function of rainfall is widely used to characterise vegetation patterns. Previous studies have shown that tree cover shows bimodal distributions for intermediate rainfall regimes in Africa and South America. In this study, we construct a vegetation state diagram - by plotting vegetation cover as a function of mean annual rainfall - for Northeast India, which is part of the Eastern Himalaya and the Indo-Burma biodiversity hotspot. We use remotely sensed satellite data of Enhanced Vegetation Index (EVI) as a proxy for vegetation cover. We obtain Mean Annual Precipitation (MAP) from the CHIRPS data (Climate Hazards Group InfraRed Precipitation with Station data). We find that EVI increases monotonically as a function of MAP in the range 1000-2000 mm, after which it plateaus. The 1000 to 2000 mm MAP corresponds to the vegetation transitional zone (1200-3700 m), whereas >2000 MAP region covers the greater extent of the tropical forest (<1200 m) of NEI. In other words, we find no evidence for bimodality in tree cover or vegetation states at coarser scales in North Eastern India. Our characterisation of the state diagram for vegetation in northeast India is important to understand response to ongoing change in rainfall patterns.