Global change impacts on forest and fire dynamics using paleoecology and tree census data for eastern North America

KEY MESSAGE: The tree census, paleopollen, fossil charcoal, human population, and climate data presented here provide unique support for important anthropogenic influences on fire over the last 2000 years in the eastern USA. This includes multiple instances of climate fire anomalies that may be best explained by the role of human-caused burning. CONTEXT: The coupling of paleoecological and tree census data to address larger global change questions is a novel research approach to describe and ascribe recent vegetation dynamics vis-à-vis the climate versus disturbance debate. AIMS: The aims of the study are to (1) compile and compare pre-European settlement versus modern upland arboreal pollen and tree survey data from a large number of studies in various forest regions in the eastern USA, (2) analyze fossil charcoal dating back 2000 years for the northern versus central/southern tiers of the eastern USA, and (3) compare and contrast compositional and ecophysiological attributes for both datasets and temporal changes to known climate or disturbance phenomena to elucidate global change impacts and the drivers of forest change. METHODS: We analyzed paleoecological (pollen and charcoal) and tree census studies to compare protohistoric and modern vegetation assemblage for eastern North America, including the drivers of forest change. A total of seven forest types in the north and central regions of the eastern USA were used to co-analyze fossil pollen, fossil charcoal, and tree survey data. RESULTS: Disparities and consistencies existed when independently assessing witness tree and pollen records. Although forests north of the tension zone line (TZL) contained mostly Fagus, Pinus, Tsuga, and Acer witness trees, pollen records were dominated, as expected, by high-pollen-producing Pinus, Quercus, Tsuga, and Betula. Here, present-day pollen and tree survey data revealed significant declines in Fagus, Pinus, Tsuga, and Larix and increases in Acer, Populus, Fraxinus, Quercus, and Abies. South of the TZL, both witness tree and pollen records pointed to Quercus and Pinus domination, with declines in Quercus and Castanea and increases in Acer and Betula based on present-day data. Modern assemblages comprise tree genera that are increasingly cool-adapted, shade-tolerant, drought-intolerant pyrophobes. Paleocharcoal data from 1 to 1750 AD indicate a slight increase in burning in southern forests and stable levels in the north, despite the increasing cold associated with the Little Ice Age. The most significant increase in burning followed the dramatic increase in human population associated with European settlement prior to the early twentieth century. CONCLUSION: Post-1940, fire suppression was an ecologically transformative event in all datasets. Our analysis identifies multiple instances in which fire and vegetation changes were likely driven by shifts in human population and land use beyond those expected from climate alone.