Fire has been a preeminent force over much of the eastern United States for multi-millennia. As such, pyrogenic vegetation types dominated this region in pre-European times, including oak, oak-pine, and pine savannas/woodlands/forests and tallgrass prairies. Nation-wide fire suppression efforts began in earnest after particularly destructive fires in the early 1900s. Although subsequent structural and compositional changes were similar between the dry West and the humid East (increases in stand density and shade-tolerant species), the ecological consequences differed profoundly. While the conifer-dominated West experienced a precipitous rise in fire risk, the hardwood-dominated East generally did not. Instead, vegetation shifts from open lands to closed-canopy forests promoted cool and moist understory conditions and a concurrent shift in leaf litter from fire-promoting xerophytic species (oak, pine) to fire-suppressing mesophytic species (maple, beech, basswood). This caused fuel beds to be less receptive to fire. Mesophication refers to this positive feedback cycle, whereby microenvironmental conditions (cool, damp, and shaded conditions; less flammable fuel beds) continually improve for shade-tolerant, fire-sensitive competitors and deteriorate for shade-intolerant, fire-adapted species. This phenomenon unfolds most quickly and is most steadfast on rich mesic sites vs. more infertile drier sites.
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