Multiple lines of evidence point to climate change as the driving factor suppressing tree regeneration since 1970 in Sinking Pond, a 35-hectare seasonally flooded karst depression located on Arnold Air Force Base near Manchester, Tennessee. Annual censuses of 162-193 seedling plots from 1997 through 2001 demonstrate that the critical stage for tree survival is the transition from seedling to sapling and that this transition is limited to shallow (less than 0.5 meters) ponding depths. Recruitment of saplings to the small adult class also was restricted to shallow areas. Analysis of the spatial and elevation distribution of tree-size classes in a representative 2.3-hectare area of Sinking Pond showed a general absence of overcup oak saplings and young adults in deep (ponding depth greater than 1 meter) and intermediate (ponding depth 0.5-1 meter) areas, even though overcup oak seedlings and mature trees are concentrated in these areas.
Analysis of tree rings from 45 trees sampled in a 2.3-hectare spatial-analysis plot showed an even distribution of tree ages across ponding-depth classes from the 1800s through 1970, followed by complete suppression of recruitment in deep and intermediate areas after 1970. Trees younger than 30 years were spatially and vertically concentrated in a small area with shallow ponding depth, about 0.5 meter below the spillway elevation. Results of hydrologic modeling, based on rainfall and temperature records covering the period January 1854 through September 2002, show ponding durations after 1970 considerably longer than historical norms, across ponding-depth classes. This increase in ponding duration corresponds closely with similar increases documented in published analyses of streamflow and precipitation in the eastern United States and with the suppression of tree regeneration at ponding depths greater than 0.5 meter indicated by tree-ring analysis. Comparison of the simulated stage record for Sinking Pond with the ages and elevations of sampled trees shows that prolonged (200 days or more per year) inundation in more than 2 of the first 5 years after germination is inversely related to successful tree recruitment and that such inundation was rare before 1970 and common afterwards.
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Tree-regeneration and mortality patterns and hydrologic change in a forested karst wetland--Sinking Pond, Arnold Air Force Base, Tennessee