Could canopy, bark, and leaf litter traits of encroaching non-oak species influence future flammability of upland oak forests?

Reviewing Fire, Climate, Deer, and Foundation Species as Drivers of Historically Open Oak and Pine Forests and Transition to Closed Forests

“…Fire exclusion as a primary driver provides a unifying process to explain conversion to fire-sensitive species and transition to dense eastern broadleaf forests following western expansion of settlers and increased human densities (Figure 3; Gleason, 1922;Bromley, 1935;Day, 1953;Neumann, 1985;Abrams, 1992;Lorimer, 1993;Brown, 2000;Brose et al, 2001;Peterson and Reich, 2001;Williams, 2005;Arthur et al, 2012;Hanberry et al, 2018a,b;Stambaugh et al, 2018). Importantly, the transition to closed forests of fire-sensitive species is exacerbated by the resulting positive feedbacks whereby these species further reduce the flammability and abundant establishment of fire-sensitive species, which results in difficulty in applying prescribed burns and removal of fire-sensitive species (Mitchell et al, 2009;Ryan et al, 2013;Stambaugh et al, 2015;Dickinson et al, 2016;Kreye et al, 2018;Babl et al, 2020). Beyond the loss of overstory oak and pine dominance, there are consequences for regional biodiversity with the corresponding losses of a diversity of herbaceous plants and vertebrate and invertebrate animals linked to open ecosystems and their diverse understory now monopolized by trees in the absence of fire (Hanberry and Thompson, 2019).…”

“…Herbaceous vegetation provides continuous fine fuels that ignite easily and spread more readily than litter, shrubs, or coarse woody fuels. Historically dominant and fire-resistant oaks and pines supplemented this community flammability via their litterfall (Mitchell et al, 2009;Dickinson et al, 2016;Kreye et al, 2018;Babl et al, 2020). We expect that for historical fire regimes and fire exclusion to be a driver, historically dominant fire-tolerant oaks and pines should decrease in composition and fire-sensitive species should increase in composition with associated increases in tree densities.…”

“…Herbaceous vegetation provides fine fuels that ignite more readily than coarse woody fuels, increasing fire frequency. Additionally, low severity fire filters species composition, favoring fire-tolerant tree species of pines and oaks, which also contribute flammable fuels, including leaf litter (Kreye et al, 2013(Kreye et al, , 2018Babl et al, 2020). Exposure to sun and wind promote ignition conditions by curing vegetation and spreading fire.…”

“…Closed forests moderate the environment, maintaining humidity and blocking sun and wind. Tree density in closed forests is sufficient to suppress flammable fine vegetation and the litter of fire-sensitive tree species often is not flammable (Kreye et al, 2013(Kreye et al, , 2018Babl et al, 2020), and further controls herbaceous vegetation.…”

“…The absence of frequent surface fires has been offered to explain loss of oaks and pines and relative success of maples (Acer) and other fire-sensitive tree species (Abrams, 1990;Gilliam and Platt, 1999;Nowacki and Abrams, 2008). 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). Conversely, the climate hypothesis (e.g., McCarthy et al, 2001;Pederson et al, 2015) posits that dry climate in the past favored drought-tolerant oak and pine species and increased tree mortality, resulting in open forests, while recent pluvials, or increases in precipitation, have allowed drought-sensitive species to become competitive and establish at great tree densities.…”