Upland oak forests of the eastern United States are shifting dominance towards shade-tolerant, fire-sensitive species (i.e. mesophytes). This shift is hypothesized to be driven by anthropogenic fire suppression and lead to mesophication, a positive feedback loop where mesophytes create a cool, moist understory, reducing forest flammability and promoting their own proliferation to the detriment of pyrophytic, shade-intolerant oak. There are few empirical studies identifying mechanisms of mesophication, and these studies have yet to consider potential mesophytes other than red maple (Acer rubrum). To address this issue, we sampled canopy, bark, and leaf litter traits of five hypothesized mesophytes (A. rubrum, A. saccharum, Carya glabra, Fagus grandifolia, and Liriodendron tulipifera) and four upland oak species (Quercus alba, Q. coccinea, Q. montana,, and Q. velutina) across a gradient of sizes (20-60 cm DBH) in western Kentucky. We also measured understory environmental variables (light and temperature) and fuel characteristics (loads, moisture, and bulk density) to determine how species-specific physical traits impact forest floor flammability.
Results/Conclusions
Canopy area, volume, and leaf area were 100, 200, 85% greater in mesophytes, respectively, compared to oaks. Understory light intensity and temperature were significantly lower under American beech and sugar maple when compared to chestnut oak. Oak bark thickness and roughness were 100 and 150% greater, respectively, when compared to mesophytes. These differences may explain a 20% increase in fuel moisture one and two days post-rainfall under mesophytes crowns. Mesophytes, with the exception of beech, lost 8% more litter mass after 12 months and their leaves were flatter, thinner, and had reduced leaf area compared to oaks. The fuel mass under mesophytes and oaks was similar, but there was 18% less oak litter under a mesophyte canopy. Differences in litter quality and decomposition may reduce future forest flammability as the less pyrophytic and more labile litter from mesophytes could dampen surface fires and potentially reduce fuel loads. This study provides evidence that some mesophytes may make fire less likely and impactful through canopy, bark, and litter traits that create a cooler, humid, and less flammable microclimate. Fire suppressing traits of mesophytes are essential to understand and may help determine when prescribed fire is an appropriate tool for upland oak conservation.