Could canopy, bark, and leaf litter traits of encroaching non-oak species influence future flammability of upland oak forests?
Shade-tolerant, fire-intolerant tree species are expanding in historically oak-dominated landscapes in the central and eastern U.S. Once established, these species are hypothesized to accelerate their own expansion through canopy, bark, and leaf litter traits that decrease forest flammability, consequently hindering the growth and survival of pyrophytic, shade-intolerant upland oaks (Quercus spp.). To better understand how canopy, bark, and leaf litter traits associated with flammability differ between oaks and common competitors, we quantified these traits in an upland oak forest in western Kentucky for four oak species and five non-oak species varying in shade and fire tolerance. Compared to oaks, American beech (Fagus grandifolia Ehrh.), red maple (Acer rubrum L.), and sugar maple (A. saccharum Marshall.) had: (1) wider, deeper canopies, traits associated with shadier, cooler understory conditions and higher fuel moisture; (2) thinner, smoother bark, traits that increase fire susceptibility, yet produce higher stemflow volume and potentially moister fuels near the tree’s bole; and (3) leaf litter with a higher specific leaf area and surface area:volume ratio, traits linked to higher fuel bed bulk density and fuel moisture. Hickory (Carya spp.) and tulip poplar (Liriodendron tulipifera L.) traits were generally similar to that of oaks. Our findings show that non-oak tree competitors commonly found in upland oak forests display canopy, bark, and leaf litter traits often associated with low flammability, but that the number and array of nonflammable traits varies widely by species and sometimes changes with tree size, leading to a gradient of traits and potentially fire dampening abilities. If these species continue to expand, reduced flammability could limit prescribed fire effectiveness in upland oak restoration.