According to a study carried out by the University of Helsinki, the University of Eastern Finland and Natural Resources Institute Finland, continuous-cover forestry disturbs the formation of carbon reservoirs in the soil less than conventional rotation forest management. Clear-cutting also reduces mycorrhizal fungi, which are important for the accumulation of stable carbon in the soil.

Even though most of the forests in the northern boreal zone are being managed, the effects of forest management on soil carbon are yet to be fully understood. In Finland, forests are usually managed in rotation. This means clear-cutting at the end of a rotation period that varies by tree species and growth zone. Interest in continuous-cover cultivation has increased in recent years. In this management option, forests are harvested and regenerated without clear-cutting.

“According to our findings, continuous-cover forestry and rotation forest management have different effects on the accumulation and storage of carbon in the soil. Clear-cutting reduced the amount of labile soil organic carbon, which decomposes easily. Forests under continuous-cover management contained more of this carbon type, and the amount was closer to the level of uncut forests. However, the management effects on the carbon stability observed in the relatively short term were less pronounced than expected, and there was no difference in the total soil carbon quantity between the management alternatives,” says Doctoral Researcher Eva-Maria Roth.

Roth investigated the amount and quality of carbon in the soil in two different experimental sites established by Natural Resources Institute Finland: spruce forest plots established in the winter of 1985–1986 in Vessari and pine forest plots established in the winter of 2010–2011 in Lieksa.

Mycorrhizal fungi key to carbon fixation
The researchers found that the fungal communities of forests have an important role in the amount of carbon stored in the soil and the stability of this storage. In continuous-cover forestry, the fungi in the soil, mycorrhizal fungi in particular, promoted the formation of organic matter and, consequently, carbon storage.

“After clear-cutting, the abundance and diversity of ectomycorrhizal fungi decreased markedly, which hinders the formation of stable carbon storages in the soil. The remains of fungi are important, as their quantity is strongly associated with the total carbon content of the soil. In practice, this means that after logging, a certain number of mature trees should be preserved in the area to maintain the fungal community and its ecological functions,” Roth says.

Long-term research needed
Empirical studies on continuously managed forests remain scarce, as the practice was in essence prohibited until 2014. According to Roth, long-term monitoring is needed to understand how different forest management practices affect soil carbon quantities.

“It’s important to monitor forest development in the long run to see the effects of clear-cutting over the years. At the same time, processes that affect the carbon cycle must be further investigated, as Finnish forests have in recent years transformed from carbon sinks into carbon sources as a result of intensive forestry,” Roth notes.