Abstract
Historically open oak and pine savannas and woodlands have transitioned to closed forests comprised of increased numbers of tree species throughout the eastern United States. We reviewed evidence for and against a suite of previously postulated drivers of forest transition focused on 1) change in fire regimes, 2) increased precipitation, 3) increased white-tailed deer densities, and 4) loss of American chestnut. We found that fire and fire exclusion provide a parsimonious mechanism for historical dominance by open forests of fire-tolerant oak and/or pine species and subsequent transition to closed forests with fire-sensitive tree species that fill the vertical profile. Based on statistical tests, increased precipitation during the past century was within historical ranges and thus fails to provide an explanation for forest change; additionally, precipitation variability is incongruent with tree traits (i.e. both drought-tolerant and drought-intolerant species have increased and decreased) and patterns of tree establishment. Similarly, current deer densities fail to provide a statistical relationship to explain tree densities at regional scales, species trends are unrelated to deer browse preferences, and both historically open forests and contemporary closed forests contained high deer densities. Functional extinction of the American chestnut had localized impacts but was not abundant compared to oak or widespread enough in distribution to match forest transitions throughout the eastern United States. Although Euro-American settlement affected many processes, not all changes were consistent enough to cause transitions in forest composition and structure that generally trailed westward expansion by Euro-American settlers. Evidence about these drivers continues to mount and we recognize the need for further research and continual re-evaluation of drivers of historical forests and forest change due to importance for understanding and management of these ecosystems.
Highlights
We reviewed primary components of forest transition hypotheses, covering fire and fire exclusion, drought and increased precipitation, change in land use, reduced and recovered deer populations, and loss of American chestnut, by provided supporting and contradicting evidence based on expected patterns and realized outcomes
Prior to European contact, eastern North America was dominated by open forests of savannas and woodlands with surface fire as a frequent disturbance, occasional precipitation increases in space and time, and high deer densities
Contemporary eastern ecosystems consist of closed forests with infrequent fires, occasional precipitation increases in space and time, and high deer densities
Summary
During the last century or more, open upland oak (Quercus) and pine (Pinus) savannas and woodlands across eastern North America (Figure 1) have transitioned to dense closed forests comprised of diverse tree species (e.g., Nowacki and Abrams, 2008; Fralish and McArdle, 2009; Hanberry and Nowacki, 2016; Hanberry and Abrams, 2018; Hanberry et al, 2018a,b). The concept of mesophication (Nowacki and Abrams, 2008) conveys the measurable state transition from fire-tolerant oak and pine species, and their associated open forest structure, to fire-sensitive species of dense forests as surface fire is excluded and forests become more resistant to surface fires, due in part to replacement of herbaceous vegetation (i.e., fine fuels for frequent, low severity fire) by increased number of trees with poorly flammable litter (Kreye et al, 2013; Dickinson et al, 2016; Babl et al, 2020). American chestnut had a limited distribution, generally most abundant in the Appalachian Mountains (Hanberry and Nowacki, 2016), whereas state transitions to closed forests of fire-sensitive species occurred within and far outside of the range of American chestnut; loss of American chestnut would not affect forest composition and structure across the eastern US. Much of the growing space gained initially by co-occurring oaks was claimed by fire-sensitive species in the longer term (Mackey and Sivec, 1973)
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