Connecting the Carbon and Water Cycles through Vegetation

Socioeconomic growth in India has resulted in a substantial increase in carbon dioxide (CO₂) emissions. Despite this, India emerges as the second-largest contributor to global greening, as revealed by remote sensing datasets. These conflicting factors pose a unique challenge in understanding the variability of atmospheric CO₂ and its implications for global warming. The present study aims to address this research gap by presenting the first analysis of climate controls on carbon flux variability in India. Our key objectives are (1) to identify the climate drivers influencing the variability of vegetation productivity in agriculture-dominated India and (2) to understand the implications of increased plant growth on water availability by analyzing the CO₂ fertilization effect. Unlike previous studies, we have not used simplistic estimates like partial correlation for causality; instead, we employed a recent tool, PCMCI, designed explicitly for detecting causality. In contrast to global studies, we find no causal connection between terrestrial water storage and vegetation productivity. Our results suggest that precipitation plays a significant role in the Indian region rather than deep groundwater, due to its immediate impact on shallow-rooted vegetation. Our findings highlight the significance of land use, land cover, and distinct irrigation practices— aspects often overlooked in current land surface models. Furthermore, we are investigating the response of soil moisture to CO₂ fertilization via two pathways: increased leaf area index (LAI) and enhanced water use efficiency (WUE) using state-of-the-art CMIP6 simulations. We are evaluating whether WUE can ameliorate plant water stress, especially when the LAI can counteract its impact by increasing transpiration. The present study adopts a holistic approach to demonstrate the critical interaction and feedback between climate controls, vegetation, and CO₂ fertilization, thereby significantly improving our understanding of land-atmosphere interaction.

Keywords: Climate controls, CO₂ fertilization, Soil moisture, Vegetation productivity, Causal discovery