Increased snow and cold season temperatures alter High Arctic parasitic fungi – host plant interactions

In the Arctic, fungal mycelial growth takes place mainly during the cold season and beginning of growing season. Climate change induced increases of cold season temperatures may, hence, benefit fungal growth and increase their abundance. This is of particular importance for parasitic fungi, which may significantly shape Arctic vegetation composition. Here, we studied two contrasting plant parasitic fungi’s occurrences (biotrophic Exobasidium hypogenum Nannf. on the vascular plant Cassiope tetragona (L.) D. Don., and necrotrophic Pythium polare Tojo, van West & Hoshino on the moss Sanionia uncinata (Hedw.) Loeske) in response to increased snow depth, a method primarily used to increase cold season temperatures, after 7–13 years of snow manipulation in Adventdalen, Svalbard. We show that enhanced snow depth increased occurrences of both fungi tested here and indicate that increased fungal infections of host plants were at least partly responsible for decreases of host occurrences. Although bryophyte growth, in general, may be influenced by increased soil moisture and reduced competition from vascular plants, Pythium polare is likely enhanced by the combination of milder winter temperatures and moister environment provided by the snow. The relationships between host plants and fungal infection indicate ongoing processes involved in the dynamics of compositional adjustment to changing climate.