Jeffrey M. Kane, Jesse K. Kreye, Raul Barajas-Ramirez, J. Morgan Varner, 2021




Flammability of maple and oak litter is explained by drying rates, supporting a mechanistic explanation of forest mesophication.

White oak litter quickly recovers flammability following saturation.

Leaf traits explain and promote the continued persistence of red maple (long associated with mesophication).

The rate of forest mesophication due to reduced litter flammability will depend on the species present.

In this study, the authors use an experimental approach to demonstrate a mechanism of forest mesophication based on the leaf litter characteristics of different tree species. Red maple is frequently considered important in forest mesophication; here the authors experimentally demonstrate the strength by which this species dampens litter flammability due to its leaf traits. Most previous flammability studies have used only oven-dried litter, but these experiments compared litter flammability under various wetting and drying treatments, which can represent a wider range of field conditions.

More than a century of fire exclusion and intensive forest practices in eastern U.S. forests has resulted in the replacement of relatively open, fire-adapted communities by shade-tolerant species at higher densities; most commonly, with maples (Acer spp.) replacing oaks (Quercus spp.). Microclimate mesophication from these structural and compositional changes has been attributed to forest floor dampening as mesophytic species replace sun-loving, pyrophytic ones; for more information on this process see Nowacki and Abrams (2008). Mesophication reduces the flammability of formerly oak-dominated forests, but important evidence is lacking for understanding mechanisms of moisture dynamics and litter flammability.

The authors collected undecayed leaf litter of five tree species: two maples (red maple, A. rubrum and sugar maple, A. saccharum) and three oaks (white oak, Q. alba; red oak, Q. rubra; and chestnut oak, Q. montana). Species were selected to represent a gradient of flammability levels, with white oak representing the most pyrophytic and red maple representing the most mesophytic. Laboratory-dried leaves were weighed and measured for thickness and depth of edge curl. A subset of leaves was then separately wetted, pressed flat, and dried before measuring blade length, width, and area, and calculating leaf volume and surface area-to-volume ratio. A series of drying and weighing yielded moisture drying curves and response times (gaining and losing moisture) for each species’ fuel beds. Established burn trial methods using three drying treatments determined flammability, maximum flame height, flaming time, smoldering time and percent litter consumption for each species. This was repeated seven times for each species and treatment combination.

A comparison of litter flammability in white oak (Quercus alba, left) and red maple (Acer rubrum, right) under laboratory conditions in the Cal Poly Humboldt Wildland Fire Lab. Samples in this photo were burned under similar methods used in the study, except that these samples were burned simultaneously and under ambient litter moisture conditions (somewhere between the oven-dried and 24-hour drying treatments used in the study). (Photo: Jeffrey Kane)


Results indicated that litter characteristics of red maple and other strong mesophytes related to leaf traits can sufficiently dampen ignition and spread of fire and promote their persistence. Oaks and maples as groups differed for all leaf traits, with oak leaves 5 – 8 cm longer and containing 60 to 120% lower saturation moisture content than maples. Species-level trait differences were greatest between white oak and red maple; the other species’ leaf traits were mostly intermediate. White oak was least absorbent, dried fastest, and most flammable, while red maple was most absorbent, dried most slowly, and was the least flammable of all tested species. Litter drying rates and leaf curling best explained flammability differences; species with greater leaf curl and shorter response times were more persistently flammable between drying treatments. Importantly, this study demonstrated that flammability differences were more pronounced, especially for maximum flame height, flaming time, and percent consumption, in the 24 hour and 16 hour drying treatments following saturation compared to oven-dried conditions (see figure below).

Comparison of litter flammability characteristics associated with species and drying treatment. Drying treatments include: Dry = burn trials with litter burned under oven-dried conditions, 24-hr and 16-hr = burn trials with litter burned after 24 h and 16 h of drying following saturation, respectively. Error bars represent standard error.

Although sugar maple leaf traits resembled red maple, it had similar flammability to northern red oak and chestnut oak that persisted across the multiple drying treatments. Sugar maple is known to be less tolerant of flooding and wet conditions than red maple, and the authors suggest that its role in forest mesophication merits further study. The intermediate drying and flammability for northern red oak and chestnut oak was not surprising, due to their varying degrees of shade and fire-tolerance. The authors noted their surprise that saturated white oak litter became as flammable as dry oak litter just 16 hours after saturation.

The substantial differences in moisture dynamics and flammability between white oak and red maple suggests that the strength or rate of forest mesophication depends on the species present. This study showed that litter traits reinforce these changes and explain a primary mechanism of forest mesophication, independent of other factors that contribute to vegetation change. In areas where low flammable species that readily absorb and retain moisture (e.g., red maple) replace highly flammable species that dry quickly (e.g., white oak), vegetation shifts will exhibit a more rapid or stronger fire-vegetation feedback effect and mesophication. The intermediate flammability of sugar maple, chestnut oak, and northern red oak suggests that the strength of feedback effects in areas with these species would likely be less than red maple. However, invading species of intermediate flammability such as these can promote mesophication if they have slower drying rates and lower flammability than the species being replaced.


Nowacki, G.J. and M.D. Abrams. 2008. The demise of fire and “mesophication” of forests in the eastern United States. Bioscience 58 (2):  123-138.