The microbial community is primarily responsible for regulating carbon and nutrient cycles in the soil.  In particular, the way bacteria and fungi respond to forest harvest practices may influence long term site productivity.  Microbial communities have shown shifts following clearcuts; however, it is unknown how smaller scale cuts may influence the microbial community.  The objective of this study was to determine how group selection harvests influence the abundance and structure of the microbial community of a northern hardwood forest.  We measured soil microbial communities after the creation of forest canopy gaps (simulating group selection harvests) and mechanization within plots (simulating ground disturbance following logging). The experiment was carried out in the Flambeau River State Forest, Wisconsin, which contains northern hardwood forests that are representative of the Great Lakes states forest region in age structure and composition.  Samples were collected in May and August in both pre-treatment (2006) and post-treatment (2007) time periods. Microbial biomass and microbial community structure were assessed using lipid analysis.

Results/Conclusions

Microbial biomass differed between May and August of the pre-treatment year but the differences were not explained by microclimate or the implementation of the treatments.  The ratio of gram-positive to gram-negative (Gm+:Gm-) bacteria was significantly greater in harvested gaps than control and mechanized plots for August post-treatment.  The relative abundance of three Gm+ bacterial indicators was greater in harvested gap treatments than control and mechanized treatments during August of the post-treatment year. Small harvests of 200-350 m² influenced microbial community composition even though the total microbial biomass was not changed. Soil microbial communities are affected by many aboveground and belowground factors and therefore we should not expect that the responses will be direct or simplistic.  Total biomass is one way we can look at changes in the belowground community; however, microbial biomass pared with microbial community structure provides a better indication of change through time and treatment.