The influence of gap age on carbon and nitrogen biogeochemistry in old growth hemlock-hardwood forests of the Upper Great Lakes Region

Background/Question/Methods   In old-growth northern hardwood-hemlock forests, treefall gaps play an important role in biogeochemistry. After a gap forms, the microclimate changes markedly. In particular, solar radiation, soil temperature, and soil moisture are affected. These changes in microclimate can impact mineral weathering, microbial decomposition, and leaching rates. As the gap ages, the microclimate and biogeochemical cycling begin to resemble that found within the forest matrix. However, in northern hardwood-hemlock forests, it is not clear how long this process takes. Our objective is to investigate the impact of gap age on carbon and nitrogen fluxes and pools in the northern hardwood-hemlock forest. We identified 12 natural treefall gaps ranging in age from 0-12 years (as of 2008) and 4 control plots, located under closed canopy at the Huron Mountain Club in Upper Michigan. We analyzed the following fluxes and transformations of carbon and nitrogen in natural gaps and the adjacent forest: leaching losses from forest floor and soil, inputs through precipitation/throughfall, and transformations through mineralization and weathering. In addition, we measured the following nutrient pools: vegetation (plant biomass and coarse woody debris) and soil (soil organic matter, forest floor litter, and microbial biomass).

Results/Conclusions   During the first two years, percent cover and vegetation mass (herbs, sedges, seedlings) increased with gap age. In addition, all gap plots had significantly higher percent cover and vegetation mass (herb, sedges, seedlings) than the closed forest. Carbon mineralization (soil respiration) was similar across all gaps and control plots.