Late Holocene drought, fire, and vegetation in northeastern North America inferred from peatland archives

Abstract Recent and projected increases in global temperatures, associated changes in regional moisture balance, and changes in the frequency of extreme climatic events, have raised concerns about the potential for rapid ecosystem changes. The paleoecological record is rich with examples of ecological change, and provides a long-term perspective that includes evidence of climatic extremes and ecological dynamics that exceed historical variability of the past few centuries. Although paleoecology has often focused on millennial-scale changes, examining records of past climate and ecological dynamics at high-resolution, particularly across time periods of rapid ecosystem or climatic changes may provide critical insights into ecosystem responses to climate change at timescales relevant to management and adaptation efforts. In this dissertation, I developed high resolution records of hydrological, fire, and vegetation history from peatlands in Maine, to 1) examine the linkages between hydroclimate and fire occurrence in the humid Northeast, 2) understand the drivers and spatiotemporal dynamics of an abrupt forest compositional change that occurred 500 and 600 years ago, and 3) use site-based records collectively to infer the timing of widespread climate extremes over the past 3000 years. Results indicated that drought intervals significantly increased the probability of regional wildfires, indicating strong climatic control of fire in this humid region. The most severe and widespread drought of the past 3000 years occurred between 500 and 600 yr BP, when all three studied peatlands recorded both drought and fire. A synthesis of forest pollen data from this time period showed eastern hemlock (Tsuga canadensis) and American beech (Fagus grandifolia) both rapidly declined in much of the Northeast, while more drought tolerant pines (Pinus spp.) and oaks (Quercus spp.) increased and persisted in higher abundance until European settlement. Coupled analysis of pollen, fire, and paleohydrology from the same peat cores clearly demonstrated that this vegetation shift was in response to the drought and associated fires. A comparison of extreme dry periods and wet periods developed from three peatland records indicated that dry periods were synchronous across multiple sites, however only 67% of wet periods were synchronous among multiple sites. The most severe and prolonged dry periods were centered at 550 Cal yr BP, 800 Cal yr BP, and 2300 Cal yr BP and the time period from 400 to 1000 Cal yr BP contained the most extreme events (i.e., dry or wet phases). While ecologically significant droughts and widespread fire have not been common in the northeastern USA during the past century, this research clearly highlights the potential vulnerability of the region to future drought and fire impacts. Results also demonstrate the utility of coupled records of fire and climate in understanding regional fire-vegetation-climate dynamics, and demonstrate the usefulness of combining multiple hydroclimate records to assess regional climate and ecological history.