Drought and fire drove rapid changes in forest community composition in the northeastern United States

Background/Question/Methods

Climate change has been linked to changes in forest disturbance and community structure over the past several decades, with forest dieback and vegetation shifts predicted to continue during the next century. However, anticipating regional-scale changes in forest structure requires an understanding of how forests have responded to past climate variability and extreme events. Paleoecological records can provide these long-term perspectives, and are particularly useful when they are coupled with vegetation-independent paleoclimate records. For example, many pollen records from the northeastern United States record a shift from mesic forest taxa to xeric/fire tolerant taxa between 500-600 years ago. Although climate change has been suggested as a potential cause of this forest transition, the characteristics of the climatic and ecological changes have not been well described. To better understand the timing, dynamics, and causes of the forest changes at this time, we reconstructed the late-Holocene history of hydroclimate, fire occurrence, and vegetation from four bogs in Maine and northern New York using testate amoebae, charcoal, and pollen analysis.

Results/Conclusions

Our results reveal the occurrence of multiple, spatially coherent drought events during the past several thousand years. Times of widespread drought were associated with significantly higher probability of local fire events. Fires rarely occurred synchronously among the four sites, indicating that variability in fuel, ignition sources, and landscape characteristics likely led to spatially heterogeneous fires. However, the sole exception occurred at 550 yr BP when all four sites experienced both drought and fire. Our coupled climate, fire, and vegetation records directly link this widespread drought and fire event to the well-documented forest compositional change at this time. Mesic species, such as beech (Fagus grandifolia) and hemlock (Tsuga canadensis) declined rapidly in our records, while more drought and fire tolerant pines (Pinus spp.) and oaks (Quercus spp.) expanded and persisted at high abundance until European land-clearance. Although mesic forests of the Northeast have not experienced widespread drought and fire during historical times, our research indicates that these forests may be vulnerable to future changes in water balance and disturbance.