Natural Resources Institute Finland investigated the reasons for the decline in tree growth
The preliminary results of the 13th National Forest Inventory (NFI13) published in 2022 showed that for the first time since the 1970s, the growth of trees had begun to decline. At the same time, the felling of trees had increased, which was visible in the significant decrease in the carbon sequastration of forests, compared with previous years. At the request of the Ministry of Agriculture and Forestry, Natural Resources Institute Finland (Luke) investigated the factors contributing to the decline in the growth of trees.
At the time of the previous inventory (NFI12), the growth of the diameter of pines especially had been at a level significantly higher than usual due to environmental factors. The subsequent decline in the level of pine growth was affected by dry summers in the whole country in 2018 and in Southern Finland in 2019 and 2020. In Northern Finland, pine growth also decreased due to the strong flowering of trees in the summer of 2019, followed by abundant cone production in the next summer, which further weakened growth.
The ageing of pine forests beyond the phase of the fastest growth explained 20% of the observed decline of growth of 4.5 million cubic metres for all tree species.
At the time of the previous inventory (NFI12), the growth of the diameter of pines especially had been at a level significantly higher than usual due to environmental factors.
The investigation into the declining growth also revealed that the harvesting practices in forests has changed during this millennium. Thinning has increased, and excessive thinning has become more common to the extent that in the last years, a quarter of thinning operations have exceeded the recommendations given in the forest management guidelines.
The intensified thinning has not yet had any significant impact on tree growth at the whole country’s level, as the tree stock volume per hectare in growing forests has not decreased. However, at the level of forest stands, excessive thinning decreases tree growth for several years. If such felling practices continues for a longer time, the growth of forests will also decline at the national level.
Tree growth is connected with carbon sequestration
As the growth of trees is directly connected with the carbon sequestration in forests, Luke’s investigation of the reasons for the declining growth has led to further studies on how growth can be strengthened.
A report on the need for changes in the Forest Act and the best practices guidelines for sustainable forest management from the climate perspective was completed in April 2023. In addition, new scenario calculations of the role of forests and the land-use sector in reaching Finland’s carbon neutrality target for 2035 will be made during 2023.