Changing fire regimes and old-growth forest succession along a topographic gradient in the Great Smoky Mountains
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2015 Elsevier B.V. Patterns of past fire disturbance may be an important contributor to contemporary vegetation composition and structure in old-growth forests of the southern Appalachian Mountains. However, due to a lack of information on pre-suppression fire regimes, vegetation pattern in the region has been primarily attributed to variability in soils and climate. To assess the pre-suppression fire regime's role in shaping vegetation pattern, we characterized temporal patterns of tree establishment in an old-growth forest that experienced two centuries of frequent fire, followed by a century of fire exclusion. Forest plots were inventoried and cored to characterize age structure and composition in yellow pine, chestnut oak, white pine-oak, and cove forest communities on the south-facing slope of an old-growth watershed in Great Smoky Mountains National Park, Tennessee USA. We compared the timing and composition of tree establishment from the following disturbance periods: (1) frequent fire 1700-1909; (2) post fire 1910-1949; and (3) mesophication 1950-2000. Non-metric multidimensional scaling characterized successional change between the three age classes. Multivariate dispersion, species richness, and beta diversity were calculated for establishment in each disturbance period. We found distinct peaks in tree establishment in the yellow pine, chestnut oak, and white pine-oak stand types that occurred soon (<40. years) after fire cessation at the site. Xerophytic fire-tolerant species dominated establishment during the period of frequent fire; a mixture of xerophytic and mesophytic species established during the period immediately following the last major fire; and mesophytic, fire-intolerant species dominated establishment during the recent period of mesophication. Cohort recruitment was less clearly linked to fire suppression in the mesic cove stands; however "fire protected" cove stands exhibited different age structure and composition compared to cove stands adjacent to the frequently burned south-facing slope. Mean plot level species richness was greatest in the tree cohort that established soon after the last major fire; while beta diversity and multivariate dispersion were highest in the trees that had established during the frequent fire period. Tree establishment has generally shifted from shade-intolerant, drought-tolerant species to shade-tolerant, drought-intolerant species along the entire south-facing slope. Successional trajectory indicates a loss of yellow pine and chestnut oak communities as the xeric and sub-xeric sites convert to white pine and cove forest communities, which were formerly restricted to sub-mesic and mesic positions. Declines in beta diversity and multivariate dispersion within younger age classes indicate that in the absence of fire disturbance, community differentiation is declining along the topographic moisture gradient.