Litter Flammability of 50 Southeastern North American Tree Species: Evidence for Mesophication Gradients Across Multiple Ecosystems

Abstract

Widespread fire exclusion and land-use activities across many southeastern United States forested ecosystems have resulted in altered species composition and structure. These changes in composition and structure have been implicated in positive fire-vegetation feedbacks termed “mesophication” where fire spread and intensity are diminished. In forests and woodlands, inherent flammability of different species is the mechanistic driver of mesophication. To date, there has been limited work on documenting the high diversity of flammability among species in the region, limiting the ability to differentiate among species to restore fuels that sustain fire regimes. Here, we coalesce disparate flammability data and add missing species across the spectrum from species that facilitate fire (so called “pyrophytes”) to those that dampen fire (so called “mesophytes”). We present data on 50 important tree species from across the southeast, all burned using identical laboratory methods. We divide our results for four dominant ecosystems: Coastal Plain uplands, oak-hickory woodlands, Appalachian forests, and bottomland forests. Across ecosystems, the most flammable species were American chestnut (Castanea dentata), a suite of pines (Pinus palustris, P. elliottii, P. serotina, and P. rigida), several oaks (Q. laevis, Q. falcata, Q. margaretta, and Q. alba), and sourwood (Oxydendrum arboreum). At the mesophytic end, the least flammable species were Tsuga canadensis, Acer rubrum, and several other hardwoods previously implicated in mesophication. Each of the four ecosystems we studied contained species that spanned the pyrophytic to mesophytic gradient. These data fill in some key holes in our understanding of southeastern fire adaptations, but also provide context for restoration decisions and fire management prioritization efforts to restore and sustain fire-prone ecosystems of the region.

Multivariate roles of litter traits on moisture and flammability of temperate northeastern North American tree species

“…A. rubrum is commonly implicated as a strong mesophyte throughout eastern North America (Nowacki and Abrams 2008;Kreye et al 2013;Kane et al 2021;McDaniel et al 2021). Similarly, a prior study examining flammability in southeastern US species has identified Nyssa sylvatica as being a strong mesophyte (Varner et al 2021). The inclusion of Betula alleghaniensis as a strong mesophyte confirmed our expectations, but our study is the first to report this finding.…”

“…There remains a debate over the flammability and ecological roles of Carya spp. in eastern North American woodlands and forests (Pile-Knapp et al 2021;Varner et al 2021). Our data suggest that they function more as a mesophyte, but the clear dichotomy between these two coarse categories is likely more nuanced for this genus and other species (Keeley and Zedler 1998;Varner et al 2021).…”

“…Oaks were highly flammable across all species. Q. alba, a widely dispersed species in eastern North America, was a clear pyrophyte based on its rapid drying and high flammability, as has been found by others (Kane et al 2021;McDaniel et al 2021;Varner et al 2021). The role of fire in northeastern North American oak ecosystems has been well established (Brose et al 2014), and long-term fire exclusion has been implicated in the loss of oaks, particularly Q. alba, in the region (Abrams 2003;Nowacki and Abrams 2008).…”

Sustaining eastern oak forests: Synergistic effects of fire and topography on vegetation and fuels

“…Fuel beds are also an important component of sustaining oak ecosystems with fire. Oak leaf litter decomposes slowly, providing a continuous fuel bed with a low bulk density that burns readily when sufficiently dry (Dickinson et al, 2016; Kreye et al, 2013; Nowacki & Abrams, 2008; Varner et al, 2021). Fire exclusion decreases flammability, because litter from mesophytic species decomposes more rapidly and has chemical properties that make their litter less likely to support combustion (Dickinson et al, 2016; Kreye et al, 2018).…”

FLAMITS: A global database of plant flammability traits

“…non‐flammable, fast‐flammable, and hot‐flammable species; Pausas et al., 2017). Similarly, plant flammability measurements allow understanding the fire‐vegetation feedbacks that underlie mesophication processes or fire‐driven biological invasions (Murray et al., 2013; Varner et al., 2021). In addition, laboratory tests permit a better control of the conditions during ignition (e.g.…”