The boreal forest is the largest forest biome and an important terrestrial storage for carbon, which is mainly stored as soil organic carbon (SOC). Although boreal forests are largely managed, the management effects on SOC storage and quality are not yet completely understood. The dominant management regime is rotation forest management (RFM), which includes clear-cut harvesting. Continuous-cover forestry (CCF) operates without clear-cutting and currently draws attention as an alternative. However, empirical studies in Finland are still sparse as CCF was politically discouraged until 2014.
In this dissertation, I aimed to shed light on the effects of CCF on SOC stabilization compared to RFM. We sampled Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) forests in central and eastern Finland. The study included RFM treatments (pre- and post-harvest), an uncut control, and various CCF harvesting treatments, adapted to the light demand of the dominating tree species.
In the first sub-study I assessed SOC storage and above- and below-ground litter inputs in pine forests as affected by clear-cuts, retention-cuts, gap-cuts, and uncut forests. SOC quality was assessed by analysing fractions with different chemical stability and short-term incubation in the laboratory. I found the SOC stocks to be similar for all treatments, despite warmer microclimate and decreased litter inputs in clear-cuts. However, clear-cut sites featured lower amounts of labile SOC compounds and higher in-situ decomposition rates.
In the second sub-study I assessed the forest management effects on root density and SOC pools with different stability in spruce forests. Root density was marginally higher in uncut plots than both stages of RFM. Stability indicators reacted ambiguously to the forest management treatment. Like in the first sub-study, the labile SOC pool was reduced after clear-cutting. Uneven-aged CCF plots showed an enrichment in 15N which may indicate an increased role of ectomycorrhizal fungi in soil organic matter formation.
In the third sub-study I analysed the effects of the management regimes on the soil fungal community and functional guilds. I found that clear-cutting altered the community composition and the ecological functionality of soil fungi. Specifically, the abundance, diversity, and richness of ectomycorrhizal fungi declined and their ratio to saprotrophic fungi was lower. Mature stands―uneven-aged and even-aged―also featured different community structures, but their functionality was similar to uncut forests, which highlights the functional redundancy in the fungal community.
This dissertation provides new information on the effects of CCF on the potential long-term storage and stabilization of SOC compared to RFM. Although the short-term effects of clear-cutting on SOC stocks are mitigated over time as SOC accumulates again when the next stands regrow, clear-cutting affects root litter inputs, the quality of SOC, and the fungal community, and RFM may thus cause a risk to decrease the SOC storage in the long term.