Climate impacts on forest production and biodiversity occur at many scales, from seasonal to millennial. Sediment cores from a series of lakes from Minnesota to Nova Scotia, analyzed at high temporal resolution, provide a basis for understanding long-term climate impacts on the conifer-hardwood forests of eastern North America. Many of the changes in the forests observed in survey data from the 20th century are continuations of trends begun centuries ago. During the past 4000 years, a long-term cooling in the area has caused “borealization” of the forests of the region. In the Common Era (past 2000 years), century-scale climate variability had a small but measurable impact on the vegetation of south-eastern Canada and northeastern US. The impact of centuries-long climate variability, such as the Little Ice Age (~CE1450-1850) and Medieval Warm Period (~CE1850-1300) is seen as temporally and spatially consistent, changes in the composition of the forests across the region. Until the European colonization, fires were rare toward the east, but more common in the Great Lakes area. Climate impacts are affected by landscape type as well as the interaction of fire and climate. These results can be used when managing forests to a sustainable or “pre-European” state. Climate variability of decadal to centennial scale is superimposed on global warming (as it was on neoglacial cooling) and affects the production and biodiversity of forests. Antecedent and contemporary land use, and changes in the fire regime induce more variability when trying to model the spatio-temporal evolution of the forests.