A recent study highlights how forest thinning significantly influences the ability of forests to store or release carbon.
Thinning, the practice of removing some trees to promote the growth of others, is a common forestry management technique, but its effects on forest carbon dynamics vary greatly depending on the forest type.
The study carried out at the University of Helsinki examined the effects of thinning in two contrasting boreal forest types: an upland forest on mineral soil and a drained peatland forest. Researchers measured annual carbon accumulation rates and emissions from trees, forest floor vegetation, soil, and deadwood both before and after thinning. The results reveal that thinning triggers immediate changes in the forest carbon balance.
“Our findings emphasize the importance of adapting forest management practices to the unique characteristics of different forest types. While thinning can enhance tree growth and carbon uptake in upland forests, drained peatlands require careful management to avoid long-term carbon losses,” says Gonzalo de Quesada, doctoral researcher at the University of Helsinki.
Upland forest recovers quickly, while drained peatland faces long-term change
In upland forest, the rate of carbon accumulation in trees temporarily declined following thinning, but recovery was rapid. Increased light and space allowed forest floor vegetation, such as mosses and shrubs, to flourish and absorb more carbon. This shift transformed the forest from a temporary carbon source in the year of thinning back into a carbon sink by the following year.
– 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, because a large amount of carbon has been removed from the forest along with the biomass during logging, says Associate Professor Anna Lintunen from the Institute for Atmospheric and Earth System Research.
In contrast, the drained peatland forest, which was already releasing carbon prior to thinning, experienced increased emissions after tree removal. Slow tree growth combined with accelerated decomposition of harvest residues caused these forests to become even stronger net carbon sources one year after thinning.
Researchers also found that it may take decades for forest carbon stocks – the carbon stored in standing tree biomass and soil – to fully recover their pre-thinning levels in both forest types. In drained peatlands, the annual carbon stock increase, or carbon sink, remained negative throughout the study period, suggesting that such forests may struggle to regain their original carbon storage capacity after thinning.
"Understanding how different forest management practices affect carbon dynamics is essential, especially as Finland and other countries strive to balance timber production with maintaining forests as effective carbon sinks," summarises de Quesada.