Impact of Elevated CO2 on Plant Water Stress and Vegetation Productivity in India

Global warming poses significant threats to ecosystems, primarily due to increasing atmospheric CO₂ levels. While literature has enhanced our understanding of carbon and water cycle interactions, a critical question remains: how will plants respond to the changing climate? The present study addresses this gap by investigating the critical soil moisture threshold, which signifies plant water stress. Using the Weather Research and Forecasting model coupled with the Noah-MP land surface model, we conducted three simulations over India, the second highest contributor to global greening, for the period 2004-2018: (1) Varying CO₂, (2) Fixed CO₂ at 2004 (low CO₂), and (3) Fixed CO₂ at 2022 (high CO₂). We identified the critical soil moisture threshold as the point during drydown where vegetation productivity begins to decline due to decreasing soil moisture and increasing vapor pressure deficit, indicating when plants experience stress. Our findings reveal that critical soil moisture threshold has decreased in response to enhanced water use efficiency by plants under elevated CO₂, reflecting variations in plant physiology. Despite this, vegetation productivity has declined under elevated CO₂ conditions. This can be attributed to the two-way carbon-climate feedback: while increased atmospheric CO₂ enhances plant carbon gain by regulating physiological responses such as altering stomatal conductance, it also acts as a radiative forcing agent, driving temperature increases, altering precipitation patterns, and reducing the effectiveness of ecosystems as carbon sinks. This warming effect, coupled with soil moisture deficit and atmospheric aridity, explains the reduction in vegetation productivity. Our study highlights that although plant physiological alterations in response to elevated CO₂ are significant, they are insufficient to counteract the warming and drying impacts. Thus, both feedback mechanisms must be considered when analyzing plant responses to changing climate conditions.