Carbon Stocks in Vermont's Managed Forests: Complex Interaction of Soil Properties, Land-Use History and Earthworms.

Many present-day northeastern USA forests were cleared for agriculture in the 1700s and 1800s, with associated long-term loss of soil carbon. After reforestation, net gains in carbon have occurred but these gains may be affected by earthworm invasions. All earthworm species common to New England were introduced from either Europe or, more recently, Asia. Some sites likely have a long legacy of earthworm populations but, if not, invasion by exotic earthworms can cause considerable change to both forest soils and vegetation structure. In addition, the impact of recent, “second-wave” invasions by Asian earthworms is not yet clear. We have been monitoring 18 managed forest stands in Vermont to be able to determine long-term changes in carbon stores. In addition to measuring carbon with depth into the C horizon, we have documented land use history dating back to colonial times, determined earthworm species and density, measured tree species and site metrics, and measured a suite of soil chemical parameters. We also determined carbon distribution in soil microaggregates in a subset of sites. Prior land use in the 18 monitored plots included cultivation, pasture, farm woodlot and, surprisingly, possibly iron mining. Higher earthworm species diversity correlated with reduced forest floor depth, higher mineral soil carbon, and greater stability (microaggregate-protected) of that carbon. Sites with the highest worm density and species richness had a history of more intense agricultural land use (although not all former agricultural sites had earthworms). There were also positive interactions between exchangeable calcium pools and earthworm density, and between elevation and carbon in the forest floor. With only 18 sites, it is difficult to establish statistically robust relationships. Present-day carbon stores appear to be a complex interaction of land-use history, site location, earthworm history and soil chemistry.