Long-term fire exclusion has altered ecological function in many forested ecosystems in North America. The invasion of fire-sensitive tree species into formerly pyrogenic upland forests in the southeastern United States has resulted in dramatic shifts in surface fuels that have been hypothesized to cause reductions in plant community flammability. The mechanism for the reduced flammability or "mesophication" has lacked empirical study. Here we evaluate a potential mechanism of reduced flammability by quantifying moisture retention (response time and initial moisture capacity) of foliar litter beds from 17 southeastern tree species spanning a wide range of fire tolerance. A k-means cluster analysis resulted in four species groups: a rapidly drying cluster of eight species; a five-species group that absorbed little water but desorbed slowly; a two-species group that absorbed substantial moisture but desorbed rapidly; and a two-species cluster that absorbed substantial moisture and dried slowly. Fire-sensitive species were segregated into the slow moisture loss clusters while fire-tolerant species tended to cluster in the rapid drying groups. Principal-components analysis indicated that several leaf characteristics correlated with absorption capacity and drying rates. Thin-leaved species with high surface area : volume absorbed the greatest moisture content, while those with large, curling leaves had the fastest drying rates. The dramatic shifts in litter fuels as a result of invasion by fire-sensitive species generate a positive feedback that reduce the windows of ignition, thereby facilitating the survival, persistence, and continued invasion of fire-sensitive species in the uplands of the southeastern United States.

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Pyrophytic oak landscapes across the central and eastern United States are losing dominance as shade-tolerant, fire-sensitive, or opportunistic tree species encroach into these ecosystems in the absence of periodic, low-intensity surface fires. Mesophication, a hypothesized process initiated by intentional fire exclusion by which these encroaching species progressively create conditions favorable for their own persistence at the expense of pyrophytic species, is commonly cited as causing this structural and compositional transition. However, many questions remain regarding mesophication and its role in declining oak dominance. In the present article, we review support and key knowledge gaps for the mesophication hypothesis. We then pose avenues for future research that consider which tree species and tree traits create self-perpetuating conditions and under what conditions tree-level processes might affect forest flammability at broader scales. Our goal is to promote research that can better inform restoration and conservation of oak ecosystems experiencing structural and compositional shifts across the region.

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. 2018. Mesophytic litter dampens flammability in fireexcluded pyrophytic oak-hickory woodlands. Ecosphere 9(1):e02078. 10. 1002/ecs2.2078 Abstract. Fire exclusion in eastern North American Quercus-Carya woodlands has resulted in overstory compositional changes, linked to altered fuel composition, structure, and ultimately, altered fire regimes. These compositional changes have been implicated in a dampening effect on fire behavior in formerly fire-prone ecosystems, the positive feedback termed "mesophication." Several proposed mechanisms are likely involved in this process; however, few have been examined. We evaluated the potential mechanism of dampened forest floor flammability through changes in litter composition in a southeastern U.S. oak-hickory forest undergoing mesophication. Laboratory drying and burning experiments revealed that increasing the relative contribution of litter from fire-sensitive mesophytic tree species (Liquidambar styraciflua, Ulmus alata, and Cornus florida) increased moisture retention and diminished litter flammability. Litterbeds composed of ≥66% mesophyte litter gained the most moisture following fuelbed saturation and were consistently wetter than pyrophyte litter following 12 and 24 h of drying. Flammability metrics decreased with increasing contribution of mesophyte litter with reductions most pronounced under more moist conditions. Under dry conditions, where litter moisture did not differ across compositional treatments, mesophyte litter was less flammable than fuelbeds that contained any pyrophyte litter. The combined effects of moisture retention and less flammable litter reveal an important synergistic impact that may result with increased dominance of fire-sensitive mesophytic species. Restoration efforts in longunburned eastern hardwood forests may require altering stand composition to limit mesophytic litter input or developing burn prescriptions that intensify surface fire behavior in less flammable fuels.