Spore deposition by ectomycorrhizal and polyporous fungi depicts timber harvesting impacts and retention patch effectiveness in a coastal British Columbia montane forest

Conservation of the vast fungal diversity found in forests is a global priority, and one recommended practice is retention forestry to provide habitat refugia within harvested landscapes. A potential limitation with retention systems, however, is the fragmentation stress associated with individual scattered trees or small patches. We used fungal DNA from passive spore deposition by two indicator guilds (wood-decay polyporous and symbiotic ectomycorrhizal fungi [EMF]) to test the efficacy of a retention forestry approach. The study layout included large retention strips ( $$\:\sim$$ 0.9 ha) embedded in 30-year-old cutovers, alongside a remnant old-growth stand, in a coastal montane coniferous forest of British Columbia. After three years of monitoring across 27 plots, we detected a total of 246 EMF and 180 polyporous species. Many fungal species were relatively uncommon, often detected in only one or two plots. Polyporous fungi richness declined with timber harvest, and the community composition of both guilds were altered in cutovers and retention strips compared to an old-growth remnant. Approximately two dozen indicator species were found to be limited in distribution to the retention strips and old growth, underscoring the long-term effectiveness of these large refugia in retaining interior-forest species. The deposition patterns also illustrate the limited dispersal range of these taxa as their spore DNA was rarely detected in the adjacent cutovers at 50 m away. As a monitoring tool, the comprehensive, fine-scale census provided by passive spore deposition highlights an efficient and effective approach in evaluating conservation endeavours.