Unearthing the effects of harvesting methods applied in continuous-cover forestry and rotation forest management on soil carbon storage

Boreal forests hold about 32% of the global forest carbon (C) stock and the majority of this C is stored in the soil. Forest management affects species composition, microclimate, plant growth, and litter production, and thus affects the soil organic carbon (SOC) storage. Hence, it is important to understand the effects of forest management practices on SOC storage and to adopt management strategies that protect SOC storage. We aimed to assess how two major forest management approaches differ in their impact on SOC quality and degradability to evaluate their effects on long-term SOC storage. Rotation forest management (RFM) based on clear-cut harvesting is the most common forest management practice worldwide. Continuous-cover forestry (CCF) as an integrated forest management approach has been suggested to enhance SOC storage. It uses repeated partial harvesting and retains a continuous tree cover. We present our recently published results from a field study in Ruunaa, Lieksa, eastern Finland. We compared the effects of logging methods applied in CCF and RFM on SOC storage and quality in boreal Scots pine (Pinus sylvestris) dominated forests ten years after the logging operations. We sampled gap-cuts as logging method applied in CCF, retention-cuts (20% of tree volume retained), and uncut mature forests and clear-cuts as two opposing stages of RFM. We tested the hypotheses: (1) colder microclimate and continuous litter input lead to higher SOC stocks in CCF plots than in clear-cuts and (2) more labile litter of grass- and herb-rich vegetation typical for clear-cut sites enhances SOC decomposition rates. We analyzed the SOC concentration and stock and modelled annual above- and belowground litter inputs based on stand characteristics (diameter at breast height, basal area, dominant tree height, understory species coverage). We used sequential chemical fractionation of organic layer samples and laboratory incubation to analyze the quality of SOC and its degradability under standardized conditions. To estimate the decomposition rate as impacted by the environment we incubated cellulose bags in situ. We assessed the impact of varying microclimate with field measurements of soil temperature and soil moisture. We analyzed the microbial biomass C pool with chloroform fumigation extraction. The SOC content and stock did not differ significantly between the treatments, despite the warmer microclimate and lower litter input recorded in clear-cut plots than in CCF plots. However, we detected differences in quality and degradability of SOC. Soils in clear-cut sites held lower proportions of labile SOC compounds than the other treatments. As hypothesized, decomposition rate was elevated in clear-cuts, but was equally high within the canopy gaps of gap-cuts. Accumulation of labile SOC due to cooler microclimate, combined with decreased decomposition rate – both found in uncut forests and retention-cuts – indicate a higher potential for future SOC accumulation in these treatments than in clear-cuts. Our study highlights that forest management affects the quality, degradability, long-term accumulation and storage of SOC. Thus, the chosen logging method can be an important tool in climate change mitigation and the forest management regime needs to be adapted accordingly.   Publication in Forest Ecology and Management [2023]: https://doi.org/10.1016/j.foreco.2023.121144