Mimicking natural disturbances of boreal forests: the effects of controlled burning and creating dead wood on beetle diversity

Published source details Toivanen T. & Kotiaho J.S. (2007) Mimicking natural disturbances of boreal forests: the effects of controlled burning and creating dead wood on beetle diversity. Biological Conservation, 16, 3193-3211.
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Summary

The young successional stages of boreal forests are an important habitat for many saproxylic species. These habitats are formed by disturbances such as fires and are characterized by large volumes of dead, sun-exposed, wood. Today, such early successional woodland patches of natural origin are very rare in Fennoscandia. Summarised here are results of a large-scale field experiment undertaken to create this habitat type, which included burning and creating different volumes of decaying wood on timber-harvested sites. The main aims were to determine i) how burning and increasing the volume of dead wood affected beetle diversity and species assemblages, ii) to study the effects on the occurrence of saproxylic, rare or red-listed beetles and iii) to evaluate whether these restoration practices are useful in coleopteran conservation.

Study area: The study was undertaken within 24, 2-ha plots located within a 25 x 15 km area in the vicinity of Evo (61º11'N, 25º0'E), southern Finland. All plots were c.80-year-old managed forest dominated by Norway spruce Picea abies (on average 93% of standing tree volume) with some birch Betula spp. and Scots pine Pinus sylvestris. The initial volume of standing wood in plots was 251.9 ± 64.8 m³/ha (mean ± SD) and dead wood 17.3 ± 13.7 m³/ha, with volumes similar between plots. The dead wood consisted mostly of logging waste i.e. < 20 cm diameter logs and cut stumps.

Treatments Treatments were fire (burning vs. no burning) and harvesting with creating dead wood (four levels: partially harvested with three levels of dead wood and unharvested). Each treatment was replicated three times, randomized among the plots.

In February and March 2002, 18 plots were harvested so that the volume of standing retention trees (i.e. trees left unharvested) was 50 m³/ha. Six plots were left unharvested. On the harvested plots, cut down trees were left spread throughout the plot to achieve volumes of large-diameter (>20 cm) dead wood of 5, 30 or 60 m³/ha, each level applied to six plots.

In mid-June to beginning August 2002, half of the plots (3 of each harvesting and dead wood combination and 3 unharvested plots) were burnt using the traditional Finnish ''horseshoe'' technique.

At harvested plots, the fires were mainly ground fires with occasional jumps to the canopy. The volume of standing dead trees after burning in plots with cut down wood was: 24.1 ± 17.2 m³/ha at 5 m³/ha cut wood; 14.8 ± 21.6 m³/ha at 30 m³/ha cut wood; and 43.8 ± 31.0 m³/ha at 60 m³/ha cut wood.

In unharvested plots, fires were less intense and large patches did not burn at all; the volume of standing dead trees after burning was 1.4 ± 2.3 m³/ha.

Beetle sampling: Between 10 May-10 September 2003, beetles were sampled with flight-intercept traps (two crosswise-set, transparent 40 × 60 cm plastic panes with a funnel and container with saline water and detergent to preserve the beetles). Five traps were set at random locations in each plot. Most beetles (99.9%) were identified to species. Exceptions were females of Philhygra and Euplectus (Staphylinidae) mostly identified to genus only, and a few Acrotrichis, Atheta, Atomaria, Epuraea and Leiodes that could not be identified reliably.

At burnt plots, 40,576 individuals of 655 species were recorded. At unburnt plots 15,455 individuals of 532 species were recorded. There were 223 species that occurred at burnt plots only and 100 that occurred at unburnt plots only.

The species richness and abundance of both saproxylic and non-saproxylic beetles were increased by burning and harvesting but the volume of dead wood left on harvested sites had no recorded short-term effect (sampling undertaken only about 1-year after treatments applied) on species richness or abundance.

Harvesting with creating dead wood had a general positive effect, but there was an interaction between the factors: harvesting with creating dead wood increased the number of individuals among unburnt plots but had no effect among burnt plots. Among unburnt plots, there were more individuals at harvested plots than at unharvested ones but the volume of dead wood on the harvested plots had no effect on beetle abundance.

Rare species, especially saproxylic ones, and also red-listed and pyrophilous species preferred burned sites. Burning and harvesting also resulted in different species assemblages.

Conclusions: The authors conclude that in this study, fire was successfully used to create beetle-rich habitat patches in boreal forests managed for timber production, and facilitated the recovery of declining species. Long-term monitoring is needed to clarify the effects of such restoration practices, in particular those of creating dead wood without using fire.

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