Evidence for the close climatic control of New England vegetation history

Sediments from lakes in the northeastern United States (New England) document climatic changes over the past 15 000 years that may, in turn, explain the long-term history of regional forest development. A rise in New England temperatures similar to14 600 yr BP (calendar years before present) coincided with the initial increase in spruce (Picea spp.) populations after deglaciation. Later temperature fluctuations correlated with changes in spruce forest composition until 11600 yr BP, when evidence for a shift to warm, dry conditions agrees with the replacement of spruce by pine (Pinus spp.) populations. Raised lake levels indicate increased moisture availability by 8200 yr BP when mesic hemlock (Tsuga canadensis) and beech (Fagus grandifolia) populations replaced the dry-tolerant pines. Cooler-than-modern temperatures, however, persisted until 6000 yr BP and appear to have limited the expansion of hickory (Carya spp.) populations. Similarly, moisture-dependent chestnut (Castanea dentata) populations did not increase until similar to3000 yr BP, when moisture availability rose to modern levels. Pathogens played a key role in a dramatic decline in hemlock populations around 5400 yr BP, but the decline also corresponds with low lake levels indicating that moisture availability may have been a factor in the decline and recovery. Our analysis, therefore, demonstrates that changes in both the abundance of existing taxa and the arrival of new taxa closely correlate with independently documented changes in climate. Temperature trends do not explain all aspects of the vegetation history, but when both temperature and moisture balance are considered, we see that forest composition consistently changed within centuries or less of climatic changes.