Niche construction mediates climate effects on recovery of tundra heathlands after extreme event

Coastal sites did have a higher total biomass in extreme event plots. However, the biomass reg- istered in extreme event plots was so small compared to control plots that the overall effect of climate had apparently negligible biological significance on the time scale of this study, thus confirming our hypothesis. Though plant trait values in extreme event plots differed from those in control plots, they were still typical of a slow growing late succession community [36] and belonged mostly to dwarf shrubs. A possible explanation for the near absence of grasses and forbs is that faster growing species, like A. flexuosa and C. neglecta were not present in high enough tiller density as to facilitate rapid regrowth [20] or that the low amount of germin- able seeds in the seed bank and seed rain restricted the recruitment possibilities and hence veg- etation recovery after disturbance [45]. However, the speed of recovery appeared to stagnate after four years and was not promoted by the addition of a large amount of seeds belonging to fast growing species, suggesting that it was not the lack of seed availability but other constrains to vegetation growth. Indeed, previous studies in continental E. nigrum heathlands where E. nigrum and/or neighbouring species were removed, found no change in plant community composition after seven [20] or eight years [21], which were attributed to the legacy effects left by E. nigrum in the soil environment, mainly through the production and release of phyto-

chemicals [46]. The results presented here are in accordance with these studies, but further confirm that niche construction by E. nigrum is not only apparent in continental areas but also across an entire gradient of climatic conditions ranging from oceanic to continental climate. Thus, because E. nigrum is able to dominate heathlands under a range of habitats and climates [13, 23], a potential increase of extreme events not tolerated by E. nigrum could bring lasting changes to large areas of the subarctic tundra dominated by E. nigrum. Our results also suggest that under an increase in extreme events scenario, E. nigrum and potentially other slow grow- ing ecosystem retarding species will gradually regain dominance over the disturbed areas. This is substantiated by that E. nigrum has a positive growth response even after being exposed to significant cold spells in winter [9], suggesting it can tolerate a range of extreme events and still keep, and potentially extend, its dominance. We found higher lichen cover in extreme event plots than in control plots, suggesting that extreme events could contribute to an increased lichen biomass in tundra areas previously dominated by evergreen shrubs [47, 48]. Nevertheless, bare soil was much more common than cover of lichen, moss and vegetation biomass (Fig 4), indicating that large sections of the plots were bare, what could contribute to erosion and soil degradation over time. Thus an increase in extreme events on E. nigrum heathlands could potentially promote the browning trend

which is being registered in some parts of the tundra [49]. Overall, we show how a common niche constructor species can dramatically affect ecosys- tem recovery after extreme events independent of climate being continental or oceanic. Our study contributes to the growing literature on the important ecosystem-modifying role of E. nigrum in tundra plant communities. Supporting information S1 Fig. Mean seedling count from seed sowing experiment per site (± 95% confidence inter- vals) between 2012–2016 across all sites. Continentality index per site is presented in paren- thesis. Species included Avenella flexuosa, Solidago virgaurea, Calamagrostis phragmitoides, Chamerion angustifolium, Bistorta vivipara, Rhinantus minor and Melampyrun pratense. (DOCX) S2 Fig. Mean shoot length from bioassay experiment (± 95% confidence intervals). Species used were Avenella flexuosa, Rumex acetosa and Solidago virgaurea growing for three weeks in each of the three soil types (i.e. sampled from control plots (C), from extreme event plots (EE) PLOS ONE Niche construction effects on tundra after extreme event PLOS ONE | https://doi.org/10.1371/journal.pone.0245929 February 4, 2021 11 / 14