Rapid recovery of depressed Norway spruce after selective harvesting in a drained peatland forest

According to a new study, the carbon sequestration of suppressed spruce was recovered during the following growing season after a harvest option. This is great news from a climate change perspective. The conditions for practicing continuous cover forestry in fertile drained peatland forests are also good in terms of tree growth, as the growth period of slow stem diameter, sc. The “liberating effect” turned out to be relatively short-lived.

Continuous cover forestry has been proposed as an alternative to even-age rotation-based forest management in drained peatland forests. Based on previous research, avoidance of ditch network maintenance, mitigation of water quality and climate impacts, and better economic profitability have been considered the advantages of continuous cover forestry in spruce-dominated peatland forests.

On the other hand, continuous cover forestry has been questioned in relation to tree growth, especially as after selective harvesting, spruce trees released from their suppressive position relative to dominant trees tend to grow more slowly for several years, delaying their growth even in an otherwise suitable growth environment.

“The isotope analyzes carried out show that the carbon dioxide sequestration of the suppressed trees immediately accelerated during the next growth period after the harvest selection, even if the stem diameter growth recovered more slowly,” says Professor Aleksi Lehtonen from the Institute Natural Resources of Finland. Luke).

As a consequence of selective harvesting, the growth area of ​​suppressed firs was greatly increased, although this varied between individual trees. Based on the study, the change in photosynthesis of the trees reflected the altered light conditions.

The results led the researchers to determine that the suppressed trees’ increased carbon sequestration was directed elsewhere than at the stem during the years following harvest selection, likely to strengthen the tree’s root system.

The slow growth period of the suppressed trees lasted less than five years. During the five years after selection harvest, the relative change in stem diameter growth of suppressed trees was greater than that of dominant trees when comparing the period before harvest to the period after.

In the study, tree stem diameter growth was measured in both suppressed and dominant trees before and after selection harvest. In addition, the ratio of stable isotopes (¹³DO/¹²C) measured from tree rings in both the selection harvest area and an unharvested control area. This isotope ratio allows a review of the relationship between photosynthetic rate of trees and stomatal conductance, i.e., photosynthetic rate of trees compared to water use. The study was carried out in Lettosuo in Tammela.

The results of the survey were published in Forest Ecology and Management.