Trädens utveckling efter att de har lämnats som naturhänsyn : tillväxt, mortalitet och strukturell förändring 6-18 år efter avverkning

Abstract: Today green tree retention, together with leaving dead wood, is a common practice with the aim to reduce negative impacts on biodiversity and environment due to clear-felling. To better understand whether this conservation practice also makes a long-term difference, it is important to know what happens to these trees after harvest. Few studies have made an assessment of what happens to retention trees over a longer time period. In this study I inventoried retention trees that were left at clear-cuts six, twelve and eighteen years ago. I carried out a re-inventory of 57 retention patches (divided into free-standing tree groups, small swamp forests and edge zones) and free standing dead trees, spread over 25 previous clear-cuts in central Sweden. The first inventory was made by the landowner (Bergvik skog; formerly Stora Skog/Stora Enso Skog). Tree growth and mortality inside the retention patches have been investigated by estimating the amount of dead wood, removed trees and survival of trees. The change of standing dead wood outside patches was also investigated. The results showed that the annual tree mortality was 9.8% in six years old retention patches and 1.8% in those twelve years old. There were no differences in the changes of volumes of living trees or dead wood between the age groups (six, twelve and eighteen years). This shows that the changes in volume primarily occur in the first six years and this period might therefore be considered the most sensitive. The volume of trees that died post-harvest in the six to eighteen years old retention patches was on average 45.7 m3ha-1. The main reason was wind (67%). There was more dead wood in exposed retention patches compared to those that grew more protected (p=0.007). On the other hand, I found no relationship between mortality and the edge:area relationship of the retention patches. Retention trees had also been removed after harvest. On average, 14.9 m3ha-1 dead and living trees were removed from the retention patches after six to eighteen years, and 28% of these trees were alive while the rest (72%) was a direct loss of dead wood. In this study I also show that trees were removed more often if there were seed-trees in the clear-cut. Furthermore, the loss of trees was higher in swamp forests, followed by tree groups and edge zones (p=0.002). The number of standing dead trees outside the retention patches decreased, but there were no differences between the age groups. Tree retention therefore contributes to a continuity of live trees in the new generation of forest, and an increase of dead wood in the forest landscape. The number of trees surviving and the volume of dead wood depend on the location of trees in the clear-cut, amount removed and what the trees endures during the first six years after harvest.