Assessing the Physiological Responses of Regional Indian Forest Ecosystems to Climate Stressors using Quantile Regression

Vegetation plays a crucial role in the exchange of energy and moisture between land and atmosphere, acting as a link between the soil, water, and atmosphere continuum. Amid a changing global climate, the increasing frequency and spatial extent of droughts and heat-related extremes pose imminent threats to ecosystems. As compared to commonly measured air temperature, the surface temperature (Tsurf) of vegetation is a better indicator of physiological stress. Remote sensing of vegetation surface temperatures allows for a unique perspective on temperature effects on ecosystem function, thus presenting as a vital tool for monitoring ecological responses during periods of environmental stress, particularly at the canopy, regional, and continental scales.

Our analysis focuses on regional forest ecosystem health within the Western Ghats and Eastern Ghats regions, known for their unique biodiversity and consisting of tropical wet and semiarid eco-climate zones. Using gridded climate data and remote sensing datasets, we examine the impact of climate stressors on ecosystem-level vegetation response, specifically focusing on surface temperature. The investigation is organized around dominant vegetation types, outlining their distinctive reactions to specific climate stressors, including hot/dry conditions and their combinations. This approach involves not only examining mean/median changes but also assessing extreme quantiles, as these are likely to have the highest impact on vegetation responses. By examining the proximity of quantiles to physiologically relevant thresholds, specifically T50—the thermal threshold of photosynthetic decline, we establish a framework that connects the proximity of these thresholds to the diverse vegetation responses. This linkage is crucial for gauging the severity of the impact on vegetation health.

The results show that vegetation response to environmental stress differs among land cover classes, which can be related to different coping strategies. Particularly in semiarid regions, a strong relationship exists between Land Surface Temperature (LST) and Normalized Difference Vegetation Index (NDVI) anomalies in deciduous forests. However, upon closer investigation, a notable observation emerges: even during periods of below-average temperatures, NDVI anomalies persist. This occurrence is attributed to significant soil moisture deficits, indicating that water availability, or the lack thereof, strongly contributes to and drives vegetation anomalies in these regions. Additionally, we noted that under the influence of climate stressors, there were multiple instances where vegetation surface temperatures had reached critical temperature levels and were operating close to their physiological thermal tolerance thresholds. Such exposure to thermal stress often induces leaf senescence and can prove to be harmful, potentially accelerating mortality rates.

Our study contributes to understanding ecosystem-level vegetation response when exposed to critical environmental stress by providing a framework that underscores the relevance of physiological thresholds and goes beyond conventional mean-centric analyses.