The effects of two different silvicultural methods on the greenhouse gas emissions and evaporation in a nutrient-rich peatland forest were studied in a dissertation by Mika Korkiakoski, Finnish Meteorological Institute. The study found differences between even-aged and continuous cover forestry. The climate impact of Finnish forest management can be reduced by favoring partial harvesting, which is typical for the continuous cover forestry.
As a result of clear-cutting, carbon dioxide emissions increased significantly in the peatland forest due to the removal of carbon sequestration, the destruction of undergrowth and the decomposition of logging residues. Emissions were very high even four years after harvest. The soil in the clear-cut area changed from a methane sink to its source as the water table depth rose closer to the ground surface. Nitrous oxide emissions also increased significantly and accounted for a significant share of total post-harvest, climate-warming emissions. At the partial harvest site, only carbon dioxide emissions increased significantly, but also significantly less than at the clear-cut site. Carbon dioxide emissions from partial harvesting were only 25 % of the clear-cut emissions. The smaller increase in emissions is due to the fact that the photosynthetic carbon dioxide uptake of the undergrowth improved and there were fewer logging residues than at the clear-cut site.
Evaporation and water table depth also influence emissions
After partial harvesting, the increase in water table depth was about half of the increase at the clear-cut site, although partial harvesting removed pines, which accounted for 70% of the forest stand. This is explained by the fact that the evaporation of the remaining stand, especially birches, was more efficient than that of pine, which kept the water table depth relatively low. In the Finnish forest management, the height of the water surface can be adjusted by the number of trees and their species. A sufficiently low water level ensures that the growth of the forest is not affected and thus large nutrient loads are avoided. On the other hand, too low a water level enhances the decomposition of peat, which increases carbon dioxide emissions.
Partial harvesting is the more climate friendly option to clear-cutting
According to the study, the direct climate impact of peatland forest management can be reduced by avoiding clear-cutting and favoring partial harvesting. With partial harvesting, even a significant part of the forest can be removed, while the climate and water impact will be considerably smaller than clear-cutting. There are still open questions. The study only covers the first four years after harvesting and the results do not directly indicate whether continuous cover forestry is a more climate-friendly option than even-aged forestry. “This study provides valuable information on the immediate climate impact of harvesting. The information can be utilized in practical forest management recommendations, in the development of peatland models and as part of a broader study of peatland forest management”, concludes Mika Korkiakoski.
The dissertation can be found from Helda.