Burning of logged sites to protect beetles in managed boreal forests

Published source details Toivanen T. & Kotiaho J.S. (2007) Burning of logged sites to protect beetles in managed boreal forests. Conservation Biology, 21, 1562-1572.
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Summary

Boreal forests of northern Europe intensively managed for timber production are typically clearcut when harvested. As a result, natural disturbances such as forest fires are rare and the volume of dead wood has decreased. These changes have had a profound negative effect on species that depend on dead wood. Sympathetic modifications of forest management methods might enhance the survival of such species. This study, undertaken in southern Finland, assessed whether burning of logged sites and leaving trees (i.e. retention trees) benefited saproxylic, rare, and red-listed beetle species, and how long the burned sites remained suitable for these species.

Study sites: The study was undertaken in within a 12 × 7 km area around Evo, southern Finland (61ºN, 25ºE) where intensive forest management has been practiced since the early 20th century. Studies were undertaken at 40 logged sites (20 burned soon after logging) on which 2–62 harvestable trees/ha were left standing. The sites had been logged from 1 to 16 years previously. Burned and unburned sites (geographically intermixed) were represented by all of the time periods since burning (except for 4-year-old burned sites and 14-year-old unburned sites). The burned sites were logged the winter preceding burning, with burning conducted in June or July. The average area of the burned sites was 5.5 ha (range 2–9 ha) and that of unburned sites 3.1 ha (range 0.8–9 ha).

There were on average 18.8 (SD 12.4) large retention trees (diameter >20 cm)/ha at the burned sites and 25.0 (SD 15.0) large retention trees/ha at the unburned sites. Their density was similar at burned and unburned sites. The average age of the retention trees was 124 years (range 85–146 years). The dominant retention trees were Scots pine Pinus sylvestris (55%) and birch Betula spp. (32%), with some Norway spruce Picea abies and aspen Populus tremula. Most retention trees (82%) at the burned sites had died during the fire or within a few years of it, the proportion of dead retention trees at the unburned sites was 22%. The proportion of dead retention trees was not correlated with the age of the site among the burned or among unburned sites.

Beetle sampling and grouping: Beetles were sampled with window-flight traps (two crosswise-set transparent 40 × 60 cm plastic panes with a funnel and container below with saline water and detergent to preserve the beetles). Five traps were set at random locations within a 1 ha area at each study site. Trapping was undertaken from 20 May to 20 July 2002, which covered at least part of the flying season of most beetle species in the region.

Most beetles (99.6%) were identified to species. Exceptions were females of Philhygra and Euplectus (Staphylinidae) identified to genus only, and a few Atomaria, Corticaria, Cryptophagus, Gyrophaena and Leiodes that could not be identified reliably.

Total abundance and species richness of beetles were recorded, with species classified into five groups: saproxylic, nonsaproxylic, rare saproxylic, rare non-saproxylic, and red-listed species. Species recorded in <25 squares of a 100-km² area in Finland were classified as rare. Species considered critically endangered (CR), endangered (EN), vulnerable (VU), or near-threatened (NT) in Finland were classified as red-listed species.

Over the trapping period 697 beetle species (23,843 individuals) were recorded; 594 species(12,809 individuals) occurred at the burned sites and 485 (11,034 individuals) at the unburned sites. The number of species and individuals decreased with time since logging. The difference between burned and unburned sites increased with the number of retention trees but this effect of burning was only significant if there were 12 or more retained trees/ha.

The number of beetle individuals was positively affected by burning, but the number of retention trees had no effect. The effect of burning was significant only if there were 18 or more retention trees/ha.

A total of 291 saproxylic species (10,884 individuals) were recorded, of which 257 (6,178 individuals) occurred at burned sites and 199 (4,706 individuals) at unburned sites. Their abundance and species richness were positively affected by burning, but the effect was not significant when there were fewer than about 15 retention trees/ha. The species richness of saproxylic beetles decreased with time since logging at both burned and at unburned sites.

Importantly, species groups unlikely to persist in ordinarily managed forests (rare saproxylic and red-listed beetles) benefited strongly from burning and tree retention.

Conclusions: Burning of logged sites and leaving an adequate number of retention trees may be useful in the conservation of disturbance-adapted species and can be used to improve the environmental quality of the matrix surrounding protected areas. Unfortunately, sites remained high-quality habitat for only a short time; thus, a continuum of burned areas must be ensured.

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http://www.blackwell-synergy.com/doi/abs/10.1111/j.1523-1739.2007.00808.x