Dependence on Fire in Wood-living Insects: An Experiment with Burned and Unburned Spruce and Birch Logs

Several boreal wood-living insect species breed exclusively in recently burned forest. However, the reason for this dependence on fire is largely unknown. Here wood-living insects and other arthropods were sampled from burned and unburned logs of birch and spruce in a burned forest, together with unburned logs at a clearing and in an uncut forest, during two years of succession after tree death. Burned spruce logs hosted fewer beetles than unburned logs. Notably, bark-beetles and their associated fauna, responded negatively to fire-scorching of the logs while arthropods that feed on ascomycete fungi responded positively. Fire-scorched logs more often had visible ascomycete fungi, and lost their bark faster than unburned logs. However, despite this obvious effect of fire-scorching of the logs, the species composition in burned and unburned logs at the burned site was more similar than in unburned logs at the three different sites. A larger diversity of beetles, when measured with rarefaction, was found for fire-scorched logs. When sites were compared, birch logs had the most diverse fauna at the burned site and spruce logs in the uncut forest. Pyrophilous insect species were almost exclusively confined to the burned forest, but occurred in both burned and unburned logs. These species may be divided into two groups: (1) mycophagous species that need burned substrate per se because ascomycete fungi are favoured by burning, and (2) phloem-feeders and predators that are favoured by some habitat characteristic of recently burned forest rather than of burned wood.