By Dr. Mac Callaham, USDA Forest Service Southern Research Station

Presented on October 24, 2023


Abstract: Most forests in the eastern USA are thought to have experienced frequent fires (3-5 year return intervals) in the time period between glacial retreat about 15,000 years ago, up until the time of European colonization. The high frequency of fires likely came about through climate-driven changes in vegetation, with the species composition being influenced by frequent ignitions from both natural (lightning) and human sources. Since the time of broadscale clearing for agricultural purposes, and logging, the frequency of fires has diminished such that most landscapes in the east have been effectively fire-free for many decades. One consequence of this fire exclusion has been the gradual accumulation of organic material in the forest floor (hereafter referred to as duff), which on one hand represents an important storage pool for carbon and nutrients, but which also represents available fuel when ignitions occur under certain environmental conditions. Our recent work suggests that when wildfires occur under very dry conditions, the consumption of duff (and fine roots therein) can result in significant post fire mortality and/or decline of overstory trees – sometimes several years after the fires actually occurred. This pattern of delayed mortality was particularly pronounced for oak-dominated stands, and counterintuitively, was not observed in stands dominated by mesic species such as maple and tulip poplar (where thick duff layers do not form). As such, restoration of oak stands might be improved if restoration treatments specifically targeted duff reduction as an objective. Evidence from one restoration study in the mountains of western North Carolina, where prescribed fire has been applied 4 times over 20 years in combination with mechanical removal of shrubs, suggests that duff reduction can be achieved with repeated management interventions.  However, to date, this has failed to effect much change in species composition of the overstory, and that tree species composition in the recruitment age-class was only slightly shifted away from mesophytic species. Taken together, these results suggest that more aggressive treatments (possibly involving selective harvest and/or herbicide applications) may be necessary if managers have the objective of reversing species composition shifts away from oak-dominated stands.