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In the eastern US, upland oak forests maintain dominance through adaptations that facilitate their persistence in fire-prone environments while simultaneously contributing to a feedback loop that promotes fire. This loop can create a seemingly stable steady state where oak forests self-replace following fire disturbances; however, abrupt alterations to the natural fire regime via anthropogenic fire exclusion may cause a rapid transition to an alternative state by allowing tree species that are fire-intolerant and shade-tolerant to outcompete oaks in the absence of fire. This new state is hypothesized to be driven by a mesophication process, which is a positive feedback loop between (1) anthropogenic fire exclusion, (2) the spread of fire-sensitive, shade-tolerant tree species (i.e., mesophytes) that create cool, moist conditions in the forest understory, and (3) a subsequent reduction in flammability. Thus, without fire disturbances, mesophytes may foster their own proliferation through a variety of mechanisms, while increasing vulnerability and decreasing resilience of upland oak forest. While mesophication is commonly cited as a major cause of changes in forest dominance, few studies to date have explicitly tested the mechanisms by which mesophytes create this hypothesized feedback loop. Here, we discuss ways that mesophytes may alter forest flammability through their canopy, bark, and leaf traits and provide recent data from work in northern Mississippi and Kentucky illustrating these mechanisms.
