The contribution of the Coweeta Hydrologic Laboratory to developing and understanding of long-term (1934-2008) changes in managed and unmanaged forests
Long-term records from USDA Forest Service Experimental Forests and Ranges (EF&Rs) are exceptionally valuable scientific resources and common ground for research in natural resource management. Coweeta Hydrologic Laboratory, Southern Appalachian Mountains in western North Carolina, is one of 82 EF&Rs located throughout the United States and Puerto Rico. Since its establishment in 1934, the wealth and breadth of scientific knowledge gained from Coweeta Hydrologic Laboratory research has provided both public and private land managers information on forest land management and has added to the knowledge base of natural resource science. We described the early watershed research at Coweeta and used long-term measurements and inventories (from 1934 to 2008) to: (1) explore the influences of large-scale disturbances and vegetation responses on ecosystem processes and (2) assess the long-term and short-term impacts of an exotic, invasive species on a southern Appalachian deciduous forest. We focused on changes in vegetation patterns influenced by natural and managed disturbances and then described the linkages between long-term vegetation measurements and water yield and quality responses. For natural disturbances, we used a network of over 900 permanent vegetation plots established in reference watersheds and unmanaged areas; first measured in 1934 and a subset re-measured in 1969–1972, 1988–1993, and 2003–2008. For the managed disturbances, clearcuts and species conversion experiments, vegetation was measured in permanent plots within treated watersheds before and after treatment. By the time of the 1934 survey, the forest had only 10 years to recover from logging before chestnut blight (Cryphonectria parasitica) induced mortality of virtually all remaining Castanea dentata trees. With the loss of C. dentata as the dominant species, Acer rubrum and Quercus montana became the dominant species and Liriodendron tulipifera and Tsuga canadensis increased in coves and along riparian corridors. T. canadensis is currently threatened by another invasive species, hemlock woolly adelgid (HWA, Adelges tsugae), with 33% tree mortality between 2003 and 2008. Smaller-scale disturbances, such as drought and windthrow, have created canopy gaps and the disturbance agent, gap size, and species-specific demography (dispersal, survival, growth, and mortality) affected colonization and recruitment. In general, the composition of species recruiting into canopy gaps was a reflection of the vegetation already in place at the time of disturbance, because regeneration occurred primarily through saplings or sprouts. In managed forests, where gaps were much larger (9–59 ha), early successional species recruit and become established and the long-term vegetation patterns are different than in unmanaged forests. The vegetation data provide a description of changes in forest structure and composition through time and space, and they have been used to develop process-based models to scale-up to the catchment level. Long-term climatic, hydrologic, biogeochemical, and vegetation databases coupled with process-based ecohydrology and ecophysiology models are essential to understanding broader- and more complex environmental issues such as climate change, carbon cycling, atmospheric deposition, and water supply and quality.