Forest responses to last‐millennium hydroclimate variability are governed by spatial variations in ecosystem sensitivity

Forecasts of future forest change are governed by ecosystem sensitivity to climate change, but ecosystem model projections are under‐constrained by data at multidecadal and longer timescales. Here, we quantify ecosystem sensitivity to centennial‐scale hydroclimate variability, by comparing dendroclimatic and pollen‐inferred reconstructions of drought, forest composition and biomass for the last millennium with five ecosystem model simulations. In both observations and models, spatial patterns in ecosystem responses to hydroclimate variability are strongly governed by ecosystem sensitivity rather than climate exposure. Ecosystem sensitivity was higher in models than observations and highest in simpler models. Model‐data comparisons suggest that interactions among biodiversity, demography and ecophysiology processes dampen the sensitivity of forest composition and biomass to climate variability and change. Integrating ecosystem models with observations from timescales extending beyond the instrumental record can better understand and forecast the mechanisms regulating forest sensitivity to climate variability in a complex and changing world. Although forecasts of forest responses to 21st‐century climate change are often highly sensitive to the magnitude of climate exposure. However, our study shows that both observations and models of change in forest composition and biomass from the last millennium indicate that ecosystem responses are strongly governed by ecosystem sensitivity rather than climate exposure. Ecosystem sensitivity in models was higher than observations and highest in simpler models, suggesting biodiversity, demography and ecophysiology dampens the sensitivity of forest composition and biomass to climate variability and change.