Mid Sweden University

A major problem within forestry is winter browsing on young Scots pine (Pinus sylvestris) by European moose (Alces alces). Moose are selective foragers and make foraging decisions on many scales, for example, they avoid trees with high levels of plant defensive compounds, which reduce digestibility. Bark extract from Norway spruce (Picea abies) is known to have high levels of such defensive compounds. The aim of this study was to evaluate the effects of Norway spruce bark extract as a repellent to European moose. Spruce bark was extracted in a conventional way with ethanol as the solvent, creating a solution that was applied to the apical leader of young pine trees in planted forests. In two field trials in northern Sweden (winter 2017–2018 and winter 2018–2019), the apical leader of young pine trees (height 1–2.5 m) were sprayed with bark extract from Norway spruce. A total of 5247 pine trees were included in the study. The field trials showed that spruce bark extract lowered the number of browsed apical leader of pine trees, and directed moose to browse less valuable lateral shoots, and neighboring trees. During the first field trial, apical leaders were treated every second meter along transects with a concentration of the spruce bark extract of 2.8% by dry weight. The level of browsed apical leaders changed from 15.1% to 6.8% of all apical leaders. During the second field trial, apical leaders were treated in circular plots, in which all main stems inside the circle were treated with a concentration of the spruce bark extract of 5.0% by dry weight. The level of browsed apical leaders changed from 19.5% to 4.7% of all apical leaders. Hence, results show that spruce bark extract (a non-toxic forest byproduct) can function as an efficient repellent. The bark extract can control browsing to a level that is considered acceptable – generally a maximum of 5% of all apical leaders browsed every year. This would result in approximately 7 out of 10 trees, in treated small scale areas, having unbrowsed apical leaders when the trees reach a height at which the apical leaders are safe from moose browsing. 

Bark beetles can cause epidemic outbreaks and kill millions of cubic meters of economical and ecologically important forests around the world. It is well known what attracts and what repels different species of bark beetle, and these chemical cues can be used to protect trees and catch the beetles without using pesticides. Applying this knowledge, we investigated the use of push–pull strategies with trap logs along susceptible edges of a Swedish boreal spruce forest. The repellents (push) used were non-host volatiles (NHV) attached to tree trunks at the forest edge, and the attractants (pull) was a commercial aggregation pheromone attached to trap logs. The aim was to test whether the Ips typographus catch could be significantly increased by combining a push–pull system with traditional trap logs, thereby providing additional protection. The experiment was performed over two years and included the main flight period of I. typographus. The study sites were clear-cuts that had been harvested the preceding winter, and sun-exposed forest edges of mature spruce were targeted for protection. A full factorial setup was used comprising two treatments (repellent and attractant) and a control. Seven replicates of the trap logs were used, three during the first year and four during the second. The number of established I. typographus maternal galleries per square meter of log surface was used as the response variable. The trap logs captured large numbers of I. typographus, at an average density of 353 and 169 maternal galleries per m2 during year 1 and year 2, respectively, over all treatments. Based on the catch data, with a sufficient number of trap logs, the risk of tree mortality at forest edges may be reduced and we recommend its general use. However, we did not see any significant effect of either the repellent or the attractant on the density of maternal galleries. Hence, we cannot recommend the addition of chemical cues to improve the efficiency of trap logs. Although trap logs are efficient in capturing bark beetles and hence may protect forest edges, it does not imply that they can provide protection on a larger scale. In line with other studies, we hence recommend that forest management to target nature-based solutions that strengthen the resilience of forest stands, by using mixed forest stands and resistant plant species, and nurture habitats for natural predators of I. typographus.