Tree growth determines the forest carbon balance at the landscape level in managed boreal forests. Carbon emissions from the soil due to decomposition processes are relatively less significant; within ten years of harvesting, a clear-cut area accumulates more carbon than it releases. This was concluded in a comprehensive study based on the survey of 50 different forest stands. The results have been published in the journal Global Change Biology.
In the northern coniferous forest belt, forests are significant carbon sinks, making them important in climate change policy. However, it is unclear how much carbon dioxide these areas can sequester from the atmosphere, a topic of much debate. Current knowledge is largely based on studies conducted in Canada and Russia, while knowledge regarding managed Nordic forests is based on only a few isolated studies in different locations.
SLU (Swedish University of Agricultural Sciences, Sveriges Lantbruksuniversitet) Professor of Forest Landscape Biogeochemistry Matthias Peichl (Matthias Peichl) has led the most extensive field study to date on the carbon balance in Sweden's northern forests. Over three years, researchers studied both carbon emissions from soil and carbon uptake from vegetation in 50 different forest stands three times. In this way, they were able to determine the carbon balance for forest stands and the forest landscape as a whole. Forests of various ages – from clear-cuts to older forests – located in the Svartberget Experimental Park in Vindeln were studied. In total, the study covers a forest landscape of 68 km2.
The results obtained provide in-depth knowledge on several currently relevant issues. Among other things, they show that limestone mountain ridges become carbon sinks within ten years of being a carbon source, and that carbon dioxide emissions from soil decomposition processes remain more or less-than-constant regardless of forest age.
“After harvesting, we do not see evidence of what is called a ‘carbon bomb’. This result applies to managed boreal forests. It is possible that nutrient-richer forests in a warmer climate might react differently,” says Matthias Peichl.
The study also shows that forest floor vegetation has a significant impact on the carbon balance in clear-cuts during the first ten years. Dwarf shrubs and grasses, which take root quickly, initially take up more carbon than trees.
The researchers hope that this study can be used as a solid baseline to compare the carbon balance in current forestry practices with other management options, such as continuous-cover forestry or leaving forests to grow without intervention.
Matthias Peichl (Matthias Peichl) points to another important conclusion stemming from the extensive study. The researchers observed large differences even between forests of the same age.
“In other words, local results are difficult to scale up to the landscape level. This can lead to biased conclusions,” says Matthias Peichl.
Results in brief
A limestone mountain becomes a carbon sink within ten years.
Forest floor vegetation has a significant impact on the carbon balance in the first decade. Initially, dwarf shrubs and grasses take up more carbon than trees.
Carbon emissions from soil are generally constant across all forestry stages; they are not a factor that influences carbon balance dynamics.
Differences between forest stands depend largely on tree age, as growth rates vary with age. At the landscape level, the carbon balance is determined by the ratio of old forest to young forest.
Because tree growth dictates carbon balance dynamics, forest management has a major impact.
The study also included several sites with old, managed forests. The older forests continue to absorb carbon, corresponding to about half the absorption volume of a middle-aged forest.
https://onlinelibrary.wiley.com/doi/10.1111/gcb.16534