Assessing ecosystem restoration alternatives in eastern deciduous forests: The view from belowground

Both structural and functional approaches to restoration of eastern deciduous forests are becoming more common as recognition of the altered state of these ecosystems grows. In our study, structural restoration involves mechanically modifying the woody plant assemblage to a species composition, density, and community structure specified by the restoration goals. Functional restoration involves reintroducing dormant-season, low-severity fire at intervals consistent with the historical condition. Our approach was to quantify the effects of such restoration treatments on soil organic carbon and soil microbial activity, as these are both conservative ecosystem attributes and not ones explicitly targeted by the restoration treatments, themselves. Fire, mechanical thinning, and their combination all initially resulted in reduced soil organic C content, C:N ratio, and overall microbial activity (measured as acid phosphatase activity) in a study site in the southern Appalachian Mountains of North Carolina, but only the effect on microbial activity persisted into the fourth post-treatment growing season. In contrast, in a similar forest in the central Appalachian Plateau of Ohio, mechanical thinning resulted in increased soil organic C, decreased C:N ratio, and decreased microbial activity, whereas fire and the combination of fire and thinning did not have such effects. In addition, the effects in Ohio had dissipated prior to the fourth post-treatment growing season. Mechanical treatments are attractive in that they require only single entries; however, we see no indication that mechanical-structural restoration actually produced desired belowground changes. A single fire-based/functional treatment also offered little restoration progress, but comparisons with long-term experimental fire studies suggest that repeated entries with prescribed fire at intervals of 3-8 years offer potential for sustainable restoration.