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

Material and methods Study sites Study sites were in areas with little to no easy access to the public on state owned land and therefore no permits were needed to establish and follow up the study. The study took place between 2009 and 2019 in five E. nigrum dominated tundra heathlands in northern Fennos- candia between 69.17–70.02˚N and 18.75–20.9˚ E (Fig 1). The study sites were above in the tree line ecotone along a 200-km climatic gradient, ranging from oceanic to continental condi- tions. Snow cover at the study sites remains until mid-June and bedrock is mainly gabbro (Table 1). The study sites were chosen according to the following criteria: they were situated just above the tree line ecotone, had a minimum of 90% cover of E. nigrum, and had a maxi- mum slope of 5 degrees. The sites varied between exposed ridges to slightly sheltered depres- sions, but all had similar vegetation composition. Besides E. nigrum, other less abundant species were Betula nana, Vaccinium uliginosum, Vaccinium myrtillus and Vaccinium vitis- idaea [12]. The presence of herbivores inside the experimental plots was monitored through pellet counts in August each year. The pellet counts were minimal, averaging 4.5±1.05 plots Fig 1. Location of the study sites in Northern Norway. 1) Rebbenes island; 2) Skogsfjord; 3) Snarby; 4) Skibotn and 5) Guhkesja´vri. Each study site contained 10 blocks with three treatment plots each: (a) Control (Co): plot left untouched; (b) Extreme event (EE): all

plant biomass was cut at ground level and discarded; (c) Extreme event + litter (EELi): all plant biomass was cut at ground level and left as a litter layer. The numbers on the plot correspond to the species in the seed addition experiment. See text for further details. For site characteristics see Table 1. https://doi.org/10.1371/journal.pone.0245929.g001 PLOS ONE Niche construction effects on tundra after extreme event PLOS ONE | https://doi.org/10.1371/journal.pone.0245929 February 4, 2021 3 / 14