Abstract
In western coniferous forests adapted to frequent and low to moderate intensity fires, climate change, long-term fire suppression, and changing land use patterns have increased the frequency, severity, and season length of wildfires. Soil fungi in these forests are important for carbon storage, nutrient cycling, and rhizosphere processes that can affect plant nutrition and regeneration. Although prescribed fire and mechanical thinning are being used to re-establish frequent and low to moderate intensity disturbances in these forests, little is known about how these forestry management treatments impact soil fungi. To assess whether prescribed fire and mechanical thinning alter soil fungal diversity, I compared fungal communities in unmanaged forest stands with stands that experienced prior applications of prescribed fire and/or mechanical thinning. I used high-throughput DNA sequencing of the Internal Transcribed Spacer region (ITS1F) to quantify the richness and abundance of fungi in 144 soil samples collected from 12 forest stands at the Blodgett Forest Research Station in the western Sierra Nevada, California. Beginning in 2001, each forest stand was randomly assigned to one of four forestry treatments: prescribed fire only, mechanical thinning only, mechanical plus fire, and control. Fire only treatments were applied in 2002, 2009, and 2016. Mechanical only treatments occurred in 2002 and 2017. For the mechanical plus fire treatment, both methods were applied in 2002 and in 2018.
Species richness of soil fungi did not differ among the four forestry treatments. In contrast, the relative abundance of distinct taxonomic groups differed. Composition of soil fungi also differed among forestry treatments. Fungal taxa in fire only and mechanical plus fire treatments differed in composition from the unmanaged control. Similarly, the mechanical only treatment differed in fungal composition from the fire only and mechanical plus fire treatments. Across the forestry treatments, 41 fungal genera were either significantly enriched or depleted in their abundance compared to the control. Mechanical thinning plus fire had the most differentially abundant genera, with 19 enriched and 8 depleted. Fire only stands had 6 enriched and 8 significantly depleted fungal genera compared to the control, whereas the mechanical only treatment had 4 genera that were enriched. My results indicate that low intensity prescribed fire and mechanical thinning modify the composition and relative abundance of soil fungi in mixed coniferous forests of the western Sierra Nevada. Such changes in the composition of soil fungi in these forests may have important implications for forestry management in the face of shifting fire regimes.