Air temperatures in the Northeastern U.S. are expected to increase 2.9-5.3 °C over the next century as is the frequency of winter soil freeze/thaw events. Climate warming over the last century has reduced winter snowpack, triggered earlier snowmelt, and lengthened the growing season resulting in earlier leaf-out and changes in microbial and exoenzyme activity. At present, little is known about changes to mycorrhizal communities in response to changes in the snowpack. The objective of this study is to determine how soil warming and cycles of freezing and thawing affect mycorrhizal community composition. Samples were collected at the Climate Change Across Seasons Experiment (CCASE) at Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. Samples were collected from replicate field plots that received one of three different soil temperature treatments: warming (+5°C above ambient), warming + freeze-thaw cycles (+5°C above ambient + 4 freeze-thaw cycles during winter), and control. DNA was extracted from 4 soil cores for multiple dates and 1 pooled root sample (10-20 root tips) per plot. DNA was amplified with the ITS1 and ITS9 primers by the Joint Genome Institute and sequenced on an Illumina Miseq platform. Sequences were quality checked, trimmed, and clustered at 95% similarity using the USEARCH program. Taxonomic identity was determined by blasting to the UNITE database using the RDP classifier. Functional guild was assigned using previously published taxonomic assignments of genera recognized in Index Fungorum.
Fungal communities on roots were significantly different among treatments (P< 0.0001). Root fungal communities also differed by plot and tree abundance (as stem count per plot). Fungal communities on roots did not differ across seasons (sample data: P = 0.053). However, fungal functional groups varied by treatment (roots: P = 0.015, soil: P < 0.001); the relative abundance of ectomycorrhizal species colonizing roots and soil were lower in the warmed and warmed + freeze/thaw plots relative to reference plots (roots: P = 0.015, soil: P < 0.001). By contrast, arbuscular mycorrhizal fungi did not differ significantly among soil temperature treatments. Instead, these fungi track shifts in arbuscular mycorrhizal host tree abundance, increasing in plots that have high abundance of these tree species (red maple, sugar maple, white ash; P = 0.006, F=4.402).