Does clear-cut harvesting accelerate initial wood decomposition? A five-year study with standard wood material

Coarse woody debris (CWD) serves a variety of ecological functions in forests, and the understanding of its decomposition is needed for estimating changes in CWD-dependent forest biodiversity, and for the quantification of forest ecosystem carbon and nutrient pools and fluxes. Boreal forests are often intensively managed, so information is needed on the effects of timber harvesting on wood decomposition, and the factors controlling the decomposition process. Therefore, decomposition of standard wood stakes of Scots pine, loblolly pine, and aspen were monitored in an uncut forest and in an adjacent clear-cut in Finland. Stakes of each species were placed horizontally on the top of the surface organic layer, at the organic layer–mineral soil interface, and vertically in the mineral soil to depth of 20cm in both the uncut forest and in the clear-cut. Five stakes of each tree species were taken every year from each stake location for five years. Mass loss of wood stakes from all three species was greater in the clear-cut than in the uncut forest during the five-year decomposition period, losing an average 59.8% of their mass in the clear-cut, which was greater than mass loss by both pines (19.8±3.0SE%) and aspen (43.3±5.1SE%) in the uncut forest. Aspen wood stakes decomposed faster than both Scots and loblolly pine stakes in the uncut forest during the whole study period, but after two years there were no differences between the three species in the clear-cut. In the uncut forest, mass loss of stakes on the surface of the organic layer was 6–10% faster than those at the mineral soil interface or in the mineral soil. In contrast, mass loss of stakes, placed on the top of organic layer in the clear-cut was 32–35% lower than those deeper in the soil probably due to low moisture conditions at the soil surface. Wood stake mass loss was positively correlated with the sum of soil temperature degree days (r⩾0.94). In the uncut forest mass loss was positively correlated with wood stake N accumulation, indicating that N availability was also a factor in decomposition before harvesting. Our study indicates that wood decomposition in this boreal forest is more sensitive to increased soil temperatures and N availability after clear-cut harvesting than found in earlier studies.