Strong impact of groundwater on long-term photosynthesis

Plants can access underground water reserves to sustain their activity, releasing moisture into the atmosphere—a critical survival mechanism during drought. Understanding the role of groundwater in regulating photosynthesis is thus key for predicting land-surface processes. However, the impact of groundwater on terrestrial ecosystem productivity remains poorly quantified, particularly when compared to well-known factors like aridity. Here, we use satellite observations of solar-induced fluorescence as a proxy for photosynthesis, together with model estimates of water table depth and aridity, quantified by the moisture index with reanalysis data, to investigate the relationship between groundwater and photosynthesis. Using causality-guided explainable machine learning (Causal Shapley values), we demonstrate that groundwater plays a crucial role in determining spatial patterns of photosynthesis, with varying importance across ecosystem types, and that its effect is comparable to aridity. We show that the relative importance of groundwater accounts for 48 to 101% of the effect attributed to aridity in modulating forest photosynthesis across the contiguous USA. The relative importance of groundwater compared to the aridity remains substantial in savannahs and shrublands (30-58%), grasslands (22-42%), and croplands (15-32%). Our findings highlight the key role of groundwater in driving ecosystem long-term productivity.