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 special importance for parasitic fungi, which may significantly shape Arctic vegetation composition. Here, we studied two contrasting plant parasitic fungi’s occurrences (biotrophic Exobasidium hypogenum on vascular plant Cassiope tetragona, and necrotrophic Pythium polare on moss Sanionia uncinata) 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. While bryophyte growth in general may be influenced by increased soil moisture and reduced competition from vascular plants, Pythium 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.

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1. Plant-soil feedbacks (PSF) play a substantial role in determining plant performance in native and foreign habitats. Yet, PSF strength may be altered by climatic parameters, creating synergies. Here, we assess performance of alpine grassland species in their native and in foreign soils in an experiment including different climates. 2. Using soil and seeds sampled from six alpine grassland sites spreading in a gradient from Northern Africa to Northern Europe, we compared plant performance in home soil and in five foreign soils, as well as home and foreign climates (simulated temperature and photoperiod in growth chambers). 3. We found that despite a high variability in plant performance between sites, plants generally performed better in their home soil and home climate, than in foreign soil or in foreign climate. However, an interaction between soil and climate effect caused this better performance in home soil to occur only when in foreign climate. Similarly, performance improved in home climate only when plants were also placed in foreign soil. 4. Synthesis: In contrast to predictions from the literature, no benefit from growing in foreign soils are indicated. At least on the short term, climate change alone is not suggested to affect native grassland plant performance. However, when introduced to a habitat with a similar climate to their native habitat, plants may perform as well as in their native range, but when introduced both to a new soil and climate, plants will do poorly. This finding sheds light on the interactive effect of climate and soil origin, as well as the possible success of plant introductions under a changing climate.

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