Land Models Likely Underestimate the Impact of Future Atmospheric Dryness on European Tree Growth

Abstract Understanding how the land carbon sink will be altered by climate change is critical for projecting future atmospheric CO 2 concentrations under different emission pathways in Earth System Models (ESMs). Most land models assume that woody growth, the main driver of land carbon storage, is driven by photosynthetic carbon supply. In reality, growth is often constrained by physiological processes limiting cell division independent of carbon availability, like reduced turgor pressure. Rising vapor pressure deficit (VPD) under climate change reduces turgor pressure and suppresses actual growth more strongly than photosynthesis, suggesting ESMs may overestimate future terrestrial carbon uptake. We compare simulations of the Community Land Model (CLM) to a statistical model built on dendrometer observations, showing that CLM underestimates decreases to growth under elevated VPD by a factor of 2–3. This highlights the need for land models to represent physiological constraints on tree growth to improve projections of future terrestrial carbon uptake.