New Publication! Boreal trees naturally regenerate well after prescribed burning with retention

The first publication of my "Let it Burn!" project is now available in Forest Ecology and Management! Open access at: https://doi.org/10.1016/j.foreco.2026.123765

What did we find?

A graphical summary of our study and key results. Each of the eight treatments was replicated three times across the landscape in the Ilomantsi and Lieksa regions of Eastern Finland. The density, height, diameter, and vigour of naturally regenerated trees were measured across all sites by species 11 years after the areas were burned. We found that natural regeneration after harvesting met commercial guidelines (2000 stems per hectare) even with high levels of retention. Prescribed burning offered further benefit for pioneer species, where were found in higher densities on burned sites. Image: N. Kokkonen

We found that natural tree regeneration (i.e. not artificially seeded or planted) met or exceeded commercial standards after prescribed burning and clearcutting with moderate levels of green tree retention. Pioneer species that establish after disturbance, including Scots pine and silver birch, flourished after burning, while Norway spruce (a species that grows later in forest development) did better on unburned areas where it remained after harvesting. Compared to these harvested areas, natural regeneration was smaller and sparser in protected areas that were burned for restoration with no harvesting, showing that a new generation of young trees may be slow to establish after restoration burning.

What does this mean?

These results mean that current forest management practices on dry upland Scots pine sites can be updated to better meet biodiversity objectives, while still achieving commercially-acceptable regeneration. New practices include retaining higher amounts of mature trees during harvesting, prescribed burning after harvesting, and reducing site preparation that disturbs the soil. The higher costs of burning and decreased harvesting volume due to retention trees can be partially offset by relying on natural regeneration and eliminating the need to scarify the soil to improve regeneration. The overarching aim of these practices is to increase biodiversity and habitats for threatened species in the Fennoscandian boreal forest.

Young tree regeneration on a burned site where the stems of retained mature trees are still visible. Here, we can see that remaining logs are trees are charred indicating the past fire history. Scots pine and silver birch sapling dominate the site with a few disturbance-adapted forbs and shrubs. These young trees will create the future forest on the site, while the standing dead trees and logs create habitat for dead wood-loving (i.e. saproxylic) and fire-dependent (i.e. pyrophilic) species, many of which are red-listed in Fennoscandia. Photo: N. Kokkonen

Why would be leave trees in the forest only to burn them?

For restoration.

Fennoscandia is known for its forests. It is often considered to have some of the last wilderness in the EU. However, this is countered by the history of meticulous forest management in this region- made famous by a certain American president who claimed that Finns rake their forests. While this is not true, it does evoke an image not far from the truth - there is very little wood left in the forest unutilized by the forest industry. This clean and economically-optimized forest has lead to a situation where the number of red-listed species has grown; animals, insects, fungi, and other life that relies on dead and decaying wood have nowhere to go. To make this worse, fire - a once common natural disturbance - has been effectively excluded from the Fennoscandian boreal forest for decades now meaning that burned habitats, which many species specifically rely on, have disappeared from the landscape. If we can modify forest practices to increase biodiversity and habitats for these threatened species, while still supporting forest harvesting, then we will achieve a forest matrix that is able to support our society as well as our environment.

An example of a Scots pine forest that has been thinned and managed to optimize economic output, leading to a decrease in species and structural diversity. Photo: N. Kokkonen

What did we do?

We examined the quality and amount of natural tree regeneration 11 years after prescribed burning and tree retention treatments in a replicated large-scale landscape experiment. On each site, a systematic grid of plots was established where the density, height, diameter, health, and species of each tree was recorded. A total of 24 sites were distributed across the landscape in the Ilomantsi and Lieksa regions of Eastern Finland. Half of these were burned, while the other half remained unburned. Each site was either clearcut, harvested with retention (10 m³/ha or 50 m³/ha), or left uncut, as shown in the first image in this blog post.

Using this extensive survey information, we summarized and statistically analyzed differences between treatments in terms of species composition, density, size, and health of the natural regeneration using both multivariate and univariate methods.

Further details can be found in the original research article in Forest Ecology and Management at: https://doi.org/10.1016/j.foreco.2026.123765