Impact of salvage logging on stand structure and beetle diversity in boreal balsam fir forest, 20 years after a spruce budworm outbreak

•Dead wood was less abundant and diverse 20 years after salvage logging.•Beetle communities still differed 20 years after salvage logging.•Most indicator species were saproxylic and associated with unsalvaged stands.•Each silvicultural action after salvaging slightly differentiate beetle communities.•Short rotations could lead to dead wood rarefaction and erosion of biodiversity.

Spruce budworm outbreaks represent the most important natural disturbance in the boreal balsam fir (Abies balsamea (L.) Mill.) forest of Canada, killing trees over wide areas and thus generating large amounts of dead wood. This disturbance is part of the dynamic processes occurring within balsam fir forests, and its effects on biodiversity need to be understood in order to move closer towards sustainable forest management. We described stand structure and used beetles as biodiversity indicators to compare the ecological value of salvaged stands, managed afterwards with three different silvicultural treatments, with unsalvaged stands, 20 years after the last spruce budworm outbreak. Our experimental design involved five replicated stands of each of the four disturbance types for a total of 20 stands. A total of 3124 Coleoptera belonging to 47 families and 269 taxa were captured over 3 months of sampling. Results showed that habitat attributes still differed 20 years after a spruce budworm outbreak between unsalvaged and treated salvaged stands, such as more abundant and diverse dead wood in unsalvaged stands. Salvage logging and silvicultural treatments led to different beetle communities than in unsalvaged stands, even though abundance and richness did not change significantly. Most indicator species identified were saproxylic and associated with unsalvaged stands, suggesting salvage logging generated the largest changes to the original ecosystem while additional effects of the silvicultural treatments were less pronounced. This may result from the 67% reduction in downed dead wood volumes (e.g., 138 m3/ha in unsalvaged stands vs 46 m3/ha in managed stands). Even if this volume of dead wood is still relatively high, it is less diverse than in unsalvaged stands and includes almost no recent dead wood. Salvage logging affected saproxylic beetle communities, suggesting that the retention of naturally disturbed stands may help in implementing ecosystem management in boreal balsam fir forest and move closer towards sustainable management.