Hypothesis
Assimilating tree-ring-width chronologies 'online' into an atmospheric climate model would outperform the simpler 'offline' approach in reconstructing past climate.
What failed
In perfect-model experiments with pseudo tree-ring data, the online approach did not outperform the offline one despite its considerable extra implementation complexity.
Broken assumption
Cycling proxy information back into the model (online assimilation) improves paleoclimate reconstructions from tree rings.
Why
The paper reports that tree growth's nonlinear response to temperature and soil moisture degrades the time-averaged EnKF, that the skill loss is sensitive to the growth-rate function used for limiting factors, and that error reduction is modulated by the model's yearly internal variability.
From the published paper: Acevedo et al. Climate of the past, 2017. ↗CC BY