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

expected to increase in frequency and intensity in northern regions which can play an impor- tant role in shaping tundra ecosystems [4]. The impact of these extreme events can affect the recovery capacity of the dwarf shrub tun- dra which is usually dominated by a few shrub species [5]. Though most studies focus on effects and changes of deciduous tundra shrubs, there are still gaps in our understanding of how evergreen shrubs respond to climate change and disturbance [6]. For example some ever- green dwarf shrubs appear to be both sensitive [7, 8] and resistant [9] to climatic variation, hence it can be challenging to predict the consequences of extreme events in the tundra. Furthermore, the outcome of extreme events might also be mediated by species interac- tions. For example, niche constructor species that modify their own or other species environ- ments through their metabolism and legacy effects (sensu Mathews et al. 2014), may represent a structuring force after disturbance in tundra ecosystems [10], and may therefore modify the expected outcome of extreme events by favouring only those species that tolerate these struc- turing forces. Nevertheless, studies on the increase or decrease of evergreen shrubs are less common than those on deciduous shrubs and deeper knowledge of their effects on northern ecosystems is needed [6]. E. nigrum L. [11, 12] is an evergreen dwarf shrub which is common in tundra ecosystems [13]. E. nigrum has niche constructing capacities which are related to it being an allelopathic

species [14]. It produces and releases large amounts of phytochemicals which are known to inhibit growth and establishment of other vascular plant species [15–17]. Furthermore, it forms dense mats of clonal growth, allowing only a few species to establish [18] and it has heavy recalcitrant litter [19] which impairs soil microbial processes [14]. There is evidence that niche construction by E. nigrum can mediate the recovery of heavy disturbed E. nigrum heathlands in continental areas, by restricting the growth of faster growing subordinate species [20–22]. Furthermore, E. nigrum has recently been found to be resistant to climatic fluctuation [9] and to mediate the positive effects of increasing summer temperatures in tundra plant communities [23]. Specifically, the expected increase in diversity and biomass of faster growing species associated with higher summer temperatures was shown to be less apparent when E. nigrum was present in the plant community, especially at high abundance. In contrast, dwarf shrub abundance was correlated with E. nigrum presence in plant commu- nities [23–25]. In combination, this suggest that niche construction by E. nigrum might be the decisive factor determining the recovery of tundra heathlands after extreme events indepen- dent of climate. If so, no vegetation shifts are expected following extreme events even under favourable climates in E. nigrum dominated heathlands. To test this hypothesis, we aimed to imitate the natural disturbance caused by extreme

events at a sufficient level as to assess their effects on ecosystem recovery of E. nigrum heath- lands under different climates. We conducted a manipulation experiment in a total of 144 plots in five E. nigrum dominated tundra heathlands located along a 200-km gradient, from continental to oceanic climate in Northern Norway with sharp contrasts of temperature and precipitation (Fig 1). We simulated an extreme event by removing all aboveground vegetation and studied the recovery of the tundra heathlands over 10 years. Based on the lasting niche constructing abilities of E. nigrum (e.g. Aerts et al 2010) and the temperature mediating effect of E. nigrum presence in tundra plant communities [23], we hypothesize a slow recovery of all the disturbed plots along the entire climatic gradient. Fur- thermore, we expect the heathlands to have a low regenerative capacity through seed bank and seed rain [26], and that the addition of seeds from several faster growing species will not pro- mote a faster recovery of the disturbed plots due to the allelopathic effect of E. nigrum on seed germination and seedling growth [16]. PLOS ONE Niche construction effects on tundra after extreme event PLOS ONE | https://doi.org/10.1371/journal.pone.0245929 February 4, 2021 2 / 14 Victoria T. Gonzalez. The publication charges for this article have been funded by a grant from the publication fund of UiT- The Arctic University of Norway. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: No authors have competing interests.