In historically fire-dependent upland oak forests of the eastern United States, anthropogenic fire exclusion is likely a major cause of increased dominance of shade-tolerant, fire-sensitive tree species (i.e. mesophytes) and simultaneous oak regeneration failure as part of a hypothesized feedback loop termed mesophication. The increased abundance of mesophytes will likely alter leaf litter and fuelbed properties and fire behavior when coupled with decreases in oak abundance.
In this study we examine individual leaf traits and moisture dynamics of two upland oaks (Quercus stellata and Q. falcata) and three hypothesized mesophytes (Liquidambar styraciflua, Carya spp. and Ulmus alata) as well as the flammability and moisture dynamics of four litter mixtures (0, 33, 66, 100% mesophyte litter). We implemented in-field experimental burns of litter mixtures in northern Mississippi and measured flame height, percent consumption, heat index, and rate of spread. We then combined these metrics into an index of flammability using principal components analysis and conducted in-lab soaking and drying experiments of mixed litter and single species litter. We measured leaf structural traits including curl, area, and perimeter for each species and assessed trait differences using a PERMANOVA.
Results/Conclusions
Flammability significantly decreased as the proportion of mesophyte leaf litter increased (R2=0.59, P<0.001), and litter mixtures containing 100% Quercus leaf litter were ~ 4x as flammable as those containing 100% mesophyte leaf litter. The mixed-species moisture dynamics experiment revealed that initial moisture content increased with increasing proportion of mesophyte leaf litter (R2=0.87, P<0.001). In single species experiments, the most extreme mesophyte, U. alata, gained ~ 1.5x more moisture than Q. falcata, which gained the least. These differences in moisture retention are potentially driven by leaf characteristics that differ among species (P<0.001). These results indicate that fuelbed flammability and fuel moisture are altered by changes in species composition and explain a potential mechanism whereby fire-sensitive species create self-promoting conditions that are less favorable for oak regeneration.