Litter trait driven dampening of flammability following deciduous forest community shifts in eastern North America

Global change has contributed to substantial vegetation shifts across many ecosystems through altered fire-vegetation feedbacks, but the direction, strength, and mechanism of these processes vary considerably. Most prior studies have focused on fire-vegetation feedbacks that enhance community flammability with less emphasis on examples of feedbacks resulting from dampened flammability. Widespread fire exclusion in many deciduous forests and woodlands of eastern North America has caused substantive changes in overstory species composition. These changes have led to feedbacks that are hypothesized to decrease community flammability (i.e. mesophication) to the detriment of fire-dependent “pyrophytes”. Mesophytic species, such as maple (Acer spp.), are implicated in these fire regime changes and have extensively invaded many oak (Quercus spp.) landscapes across the region. To date, only limited circumstantial evidence is available in support of dampened flammability as the mechanism contributing to mesophication. Here we use laboratory experiments to determine whether red maple (A. rubrum) and sugar maple (A. saccharum) could reduce forest floor flammability through the input of their leaf litter. Wetting, drying, and burning characteristics of three pyrophytic oaks, white oak (Q. alba), northern red oak (Q. rubra), and chestnut oak (Q. montana), and the two mesophytic maples revealed the strong potential for red maple to reduce community flammability. Red maple absorbed the greatest amount of water, was the slowest to dry, and was the least flammable among these species while white oak was the most resistant to water absorption, the fastest to dry, and the most flammable. Northern red oak, chestnut oak, and sugar maple were intermediate in their moisture dynamics and flammability. The moisture and flammability responses of the species examined were a result of their corresponding leaf traits, with drying rate and curling having the strongest influence. Our findings provide substantial evidence that vegetation shifts resulting from an altered fire regime could result in dampened litter flammability that further limit fire and reinforce on-going changes.