Fire as a regeneration filter: contrasting effects of heat and smoke on Arctic seed germination.

Jeronimo Vazquez-Ramirez , Margherita Tognela, Natasha de Vere, Barbara Gawlak, Julia Kemppinen, Daniel Kępski, David Kniha, Simone I. Lang, Petr Macek, Maija-Susanna Sujala, Otso Suominen, Anne Tolvanen, Brandon Samuel Whitley, Sergey Rosbakh

The rapid warming of the Arctic is increasing the frequency, intensity, and spatial extent of fires. Because fire has historically been rare in this region, most Arctic plant species are unlikely to have evolved traits that confer tolerance to fire, and the consequences for early life-history stages such as seed germination remain largely unknown. Here, we experimentally tested the effects of two key fire-related cues, heat shock and smoke exposure, on the germination traits of 25 widespread Arctic plant species. Seeds collected across the Arctic were subjected to four treatments: (i) high-heat (110 °C for 4 min, simulating surface fire exposure); (ii) low-heat (50 °C for 10 min, simulating soil seed bank conditions); (iii) smoke water produced from Arctic plant biomass; and (iv) a combination of low-heat and smoke water. We analysed germination responses at three levels: overall, across functional groups (forbs, graminoids, and woody species), and at the species level. High-heat treatment almost completely inhibited germination, reducing the final percentage of germination from ~78% to ~2%, with only three species showing any germination, two of them graminoids. Low-heat treatment produced no overall or functional group changes in final germination but reduced germination speed and germination synchrony in 20% of the species. Smoke water did not alter the final germination percentage but accelerated overall germination speed by ~9–11%. Together, these results suggest that most Arctic species are not adapted to survive the temperatures experienced at or near the soil surface during fires; however, buried seeds may remain viable after low heating and be stimulated by smoke, creating an ecological filter that favours species with persistent soil seed banks. As Arctic fires become more frequent and intense, seed-based regeneration will likely play a growing role in shaping post-fire recovery and will influence the diversity and composition of plant communities.