Retaining trees for conservation at clearcutting has increased structural diversity in young Swedish production forests

Abstract

Retaining trees for conservation at final harvest is becoming increasingly common within forestry globally, especially connected to clearcutting. The main action is to leave single living and dead trees, tree patches and buffer strips, to benefit biodiversity and to enhance ecosystem functioning. We present the first national analysis of effects on structural components from applying the retention approach. In Sweden retention forestry has been practiced large-scale for about 25years, prescribed by the law and a requirement in certification standards. By analyzing data from the Swedish National Forest Inventory we found that the volume of dead trees (⩾100mm in diameter; single trees and trees in patches<0.02ha; data for larger retention patches not available) in stands 0–10years old increased about 70% during the period 1997–2007, with a current average level of 8m3ha−1, and with a larger increase rate in this age class than in other forest ages. Retained living trees (⩾150mm in diameter; single trees and trees in patches<0.02ha; data for larger retention patches not available) decreased in quantity from 1955 until the early 1980s, with lowest levels of about 5ha−1 (excluding Pinus sylvestris, commonly used as a seed tree) and then increased, approximately reaching the 1950s level by 2007, with about 15treesha−1 on average. Large-scale application of the clearcutting practice is the probable cause of the decrease, whilst retention actions are the likely explanation for the increase during the last decades. Our study clearly shows that young forests have become structurally richer since the introduction of the retention approach in forestry. However, comparatively low amounts of dead wood in forests 0–10years old compared to what is available in old forests imply loss at harvest and studies of possible mechanisms to explain this are needed. Our results can indicate possible changes in other parts of the world, where the retention approach has been introduced more recently.