Release of retained oaks in Norway spruce plantations. A 10-year perspective on oak vitality, spruce wood production and ground vegetation

Delphine Lariviere,Emma Holmström,Jörg Brunet,Jan Weslien

doi:10.1016/j.foreco.2020.118670

Abstract

This study explores the decade-long effects of release cutting around old retained oaks (Quercus robur L.) in a Norway spruce (Picea abies L. Karst) stand that was 33 year old when thinned. The impacts on both nature conservation values and spruce wood production were evaluated in a randomized block design. To release oaks from competition, stems of Norway spruce were cut around 33 oaks, in three different treatments: high release (HR), medium release (MR) and no release (NR). Trees within a circular sample plot (15 m radius from the oak) were measured at time of treatment and 10 years after. The treatment effects on stand development, oak vitality and understory vegetation were evaluated after ten years, using tree diameter, height measurements, oak crown and tree structure estimates as well as ground vegetation surveys. Release cutting did not impact spruce production within the sample plot, and given that there were no other obvious sources of spruce suppression in the stand, we speculate that release cutting has little to no impact at the stand scale. Oak crowns in the control plots (NR) became smaller after ten years, while the crowns expanded and colonized the gap in the release treatments. Simultaneously, the amount of dead wood in the crown increased among oaks in the control treatment, indicating dieback. Cover and species richness of vascular plants in the understory were significantly higher in the HR and MR treatments compared to NR. These results suggest that the creation of relatively wide gaps (greater than 2 m) around retained oak crowns is one efficient approach to maintain their conservation values in a spruce-dominated stand on a longer time frame. This will allow oaks to expand their crowns, increase their vitality and increase species richness and diversity of plants under the canopy. The economic loss of creating large gaps instead of no gaps may be negligible since the overall spruce production was not affected within 15 m of each oak.

Highlights

In northern Europe, intensive forest use and expansion of coniferous plantations has led to dramatic changes in the structure and composition of boreal and semi-boreal forests (Anderberg, 1991, Lindbladh et al, 2014)
The basal area (m2ha− 1) for the Norway spruce adjacent to the focal oaks (15 m radius) was not significantly different among treatments (HR, medium release (MR), no release (NR)) directly after release cutting (Table 1)
The arithmetic mean of Norway spruce Diameter at breast height (DBH) was smaller in NR compared to high release (HR) and MR (Table 1)

Summary

Introduction

In northern Europe, intensive forest use and expansion of coniferous plantations has led to dramatic changes in the structure and composition of boreal and semi-boreal forests (Anderberg, 1991, Lindbladh et al, 2014). The retention of older deciduous trees, or retention forestry, is a practice used in clear-cut systems which are applied in most European forests (Vanha-Majamaa and Jalonen, 2001, Gustafsson et al, 2019, Gustafsson et al, 2020). This approach to harvesting retains important structures, safeguards habitat continuity and positively affects biodiversity compared to clear felling (Gustafsson et al, 2010, Fedrowitz et al, 2014, Gustafsson et al, 2020)

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