Tree growth became a topic of concern in the spring, when the flash estimates for the 2021 greenhouse gas inventory indicated that the land use (LULUCF) sector had turned from a carbon sink into a source of emissions. What this meant in practice was that emissions from fields were larger than the carbon sink of forests.

“It does seem that the sink decreased last year, and there are two underlying reasons for this. First, felling volumes exceeded those of the few previous years. Second, the greenhouse gas inventory calculations were updated, and new growth measurements were introduced, which contributed to the decrease in the sink,” says Kari T. Korhonen, Senior Researcher at Natural Resources Institute Finland.

“Since fluctuation in annual growth is normal, this finding is not in itself concerning. It would be different if we could detect a more permanent decline in growth due to environmental factors.”

Growth rate determined by the width of the annual growth ring

In the national forest inventory, volume growth is determined by measuring trees on permanent sample plots every five years. The difference between these measurement results is the volume growth, which is converted into annual growth by dividing the measurement interval by the number of growing seasons. In turn, the annual growth rate is estimated based on samples bored from sample trees, indicating annual fluctuations in the ring width.

“A decrease in the growth rate was detected for pine and birch, but we have yet to determine what caused it. One factor could be the increased thinning intensity detected in the inventory.”

According to Korhonen, this is partly due to snow damage in Northern and Eastern Finland. When removing damaged trees, thinning must sometimes be carried out too intensely or in stands that would not have been thinned in normal circumstances.

“After intense thinning, recovery is slower than normal, and this can lead to a decrease in the width of the annual growth ring.”

Graafi, joka kuvaa puun läpimitan kasvun vaihtelua.
Index of growth in pine in southern Finland 1990-2021. Index value 100 is the average growth level, from which the effect of tree size and age on growth has been removed. When measuring the growth of a stand in a single year, the variation is large.

The carbon sink is not constant

“The age structure of forests is a big factor causing changes in tree growth. In young forests, growth is fastest in 21–40-year-old trees and is still relatively fast in 41–60-year-old trees. Growth then begins to slow down,” says Korhonen.

From a nationwide perspective, the age structure of forests has gradually changed. Northern Finland has a large post-war generation of forests that are more than 60 years old, which are no longer at the best growth stage. The results of the previous inventory already showed a decline in tree growth in the area. While growth continued to increase at the national level, it was doing so at a slower pace.

“When we have a lot of young forests that grow well, they function as a strong carbon sink. Felling is also at a low level, as no regeneration felling is carried out in young forests. As forests become older, their growth slows down, and there are more opportunities for felling. We are now moving to this stage, and this will decrease the carbon sink.”

Climate benefits from wood products

According to Korhonen, forest use and climate goals can be combined by using forests, as they provide renewable natural resources. This offers climate benefits, but it also requires us to accept that forest use leads to fluctuations in the size of the carbon sink.

“I believe we should look at things holistically. Forests produce wood products that are based on renewable natural resources and do not generate emissions in the same way as products based on fossil raw materials.”

“It does not make sense to restrict production based on renewable natural resources while generating emissions through a fossil economy. Our focus should be on reducing emissions instead of on restricting the use of renewable natural resources to strengthen carbon sinks.”

Good forest management strengthens the carbon sink

If a forest owner strives to maximise the carbon sink of their forest, Korhonen recommends traditional methods that comply with good forest management practices: using the tree species most suitable for the site type and performing forest management work on time.

“For example, a delay in thinning has negative impacts. It results in increased natural removal, tree death and the ensuing release of carbon. When thinning is carried out on time, growth improves and recovers more quickly. In addition, thinning does not have to be too intense, which in turn causes a longer-lasting decline in tree growth.”

“Initial measures must also be taken care of in forest regeneration. Soil preparation ensures considerably better initial development for seedlings. Even if it may have detrimental impacts on the carbon sink when it is carried out, good seedling growth will compensate for this.”

This article was published in issue 3/2022 of Metsä Group’s Viesti magazine.
Text Armi Purhonen
Photo Vastavalo/Kimmo Rampanen