Thinning Practices Affect Carbon Dynamics in Boreal Forests

Thinning, a forestry management technique involving the removal of some trees to promote the growth of others, significantly influences forest carbon dynamics. A study from the University of Helsinki examined how thinning affects two contrasting boreal forest types: an upland forest situated on mineral soil and a drained peatland forest. Researchers investigated the annual carbon accumulation rates and emissions from various components, including trees, soil, and deadwood, both before and after thinning.

The findings reveal immediate changes in the carbon balance of forests following thinning. According to Gonzalo de Quesada, a doctoral researcher at the University of Helsinki, “Our findings emphasize the importance of adapting forest management practices to the unique characteristics of different forest types.” While thinning appears to enhance tree growth and carbon uptake in upland forests, drained peatlands require careful management to prevent long-term carbon losses.

Upland Forests Show Quick Recovery

In the upland forest, researchers noted a temporary decline in carbon accumulation rates after thinning. However, recovery was swift. The increased light and space allowed forest floor vegetation, such as mosses and shrubs, to thrive and absorb more carbon. This transformation meant that the forest shifted from being a temporary carbon source in the year of thinning back to a carbon sink by the following year.

Anna Lintunen, an Associate Professor from the Institute for Atmospheric and Earth System Research, explained, “Carbon sink refers to the positive change in carbon storage per year. Carbon sinks often recover fairly quickly after thinning, but the recovery of total carbon stocks takes time due to the significant amount of carbon removed during logging.”

In contrast, the drained peatland forest, which was already releasing carbon before thinning, experienced increased emissions post-thinning. The slow growth of trees, combined with accelerated decomposition of harvest residues, resulted in these forests becoming stronger net carbon sources one year after thinning. Researchers highlighted that it could take decades for carbon stocks—comprised of carbon stored in standing tree biomass and soil—to return to pre-thinning levels in both forest types.

Long-Term Implications for Drained Peatlands

The study also indicated that in drained peatlands, the annual carbon stock increase, or carbon sink, remained negative throughout the observation period. This suggests that these forests may struggle to regain their original carbon storage capacity after thinning.

Understanding the impact of different forest management practices on carbon dynamics is crucial, especially as Finland and other nations strive to balance timber production with maintaining forests as effective carbon sinks. De Quesada summarized the importance of this research: “It is essential to understand how different forest management practices affect carbon dynamics.”

The comprehensive study is documented in the journal Forest Ecology and Management, providing valuable insights into sustainable forest management practices. This research is particularly relevant as global efforts intensify to combat climate change and enhance carbon sequestration in forest ecosystems.