They found that during the mid-Holocene (8.2–4.2 ka BP, ka BP = thousand years before the present), precipitation in the region sharply decreased due to the poleward shift of the ...

Pederson, Daniel Stahle, David B. McWethy, and Cathy Whitlock for the PNAS research article, 'Dynamic treeline and cryosphere response to pronounced mid-Holocene climatic variability in the US ...