Chestnut blight, chainsaws and water from the sky: Multiple interacting disturbances drive forest dynamics in eastern North America

Background/Question/Methods Over the last 500-years, deciduous forests of eastern North America have been concomitantly influenced by a variety of factors.  A large body of work has focused on the role of fire in these ecosystems- our goal has been to seek a broader context for this single disturbance process.  We summarized data from scarred tree cross-sections to illustrate the dynamics of fire in the region.  We then explored a number of alternative ecosystem drivers.  We used data from the North American Drought Atlas to generate a 500-year climate reconstruction for the region.  The last 100 yrs are examined more finely through PRISM data.  GIS based analysis of land cover change was conducted using county-level U.S. decennial censuses.  A literature review allowed assessment of forest dynamics associated with the loss of American chestnut (Castanea dentata) and population dynamics of major wildlife species. 
Results/Conclusions

The fire scar data indicates a peak of burn frequency every 6-7 years from ca. 1880-1930 that is followed by a period virtually void of fire across the landscape.  This changing fire regime was simultaneous with myriad other changes.  For instance, our reconstructions indicate that there have been seven significant multi-year droughts in the last 500 years, yet none has occurred in the last 100 years.  The most extreme droughts were centered on the years 1565 and 1630.  PRISM data over the last century supported this finding, indicating a 100-year trend of cooler (-4-5° C) and wetter (3-15%) growing season conditions.  Land-use has also been highly dynamic in the recent past.  Forest cover in eastern North America was approximately 50-75% in 1850, but after a period of intense land-utilization (ending ca. 1930) that value had dropped to <5% across large portions of the region.  This broad land-use dynamic surely influenced forest dynamics.  Many other factors, including the loss of American chestnut and dramatic changes in wildlife population numbers, must also be drivers of ongoing forest dynamics.  We propose a “multiple interacting disturbances hypothesis” for forests in the EDF which explicitly recognizes these complex and interacting forces.