Forecasts of forest responses to climate variability are governed by
climate exposure and ecosystem sensitivity, but ecosystem model
projections and process representations are under-constrained by data at
multidecadal and longer timescales. Here, we assess 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 highest in simpler models and higher than observations,
suggesting that interactions among biodiversity, demography, and
ecophysiology processes dampen the sensitivity of forest composition and
biomass to climate variability and change. By integrating ecosystem
models with observations from timescales extending beyond the
instrumental record, we can better understand and forecast the
mechanisms regulating forest sensitivity to climate variability in a
complex and changing world.