Abstract
The climate-driven encroachment of shrubs into the Arctic is accompanied by shifts in soil fungal communities that could contribute to a net release of carbon from tundra soils. At the same time, arctic grazers are known to prevent the establishment of deciduous shrubs and, under certain conditions, promote the dominance of evergreen shrubs. As these different vegetation types associate with contrasting fungal communities, the belowground consequences of climate change could vary among grazing regimes. Yet, at present, the impact of grazing on soil fungal communities and their links to soil carbon have remained speculative. Here we tested how soil fungal community composition, diversity and function depend on tree vicinity and long-term reindeer grazing regime and assessed how the fungal communities relate to organic soil carbon stocks in an alpine treeline ecotone in Northern Scandinavia. We determined soil carbon stocks and characterized soil fungal communities directly underneath and >3m away from mountain birches (Betula pubescens ssp. czerepanovii) in two adjacent 55-year-old grazing regimes with or without summer grazing by reindeer (Rangifer tarandus). We show that the area exposed to year-round grazing dominated by evergreen dwarf shrubs had higher soil C:N ratio, higher fungal abundance and lower fungal diversity compared with the area with only winter grazing and higher abundance of mountain birch. Although soil carbon stocks did not differ between the grazing regimes, stocks were positively associated with root-associated ascomycetes, typical to the year-round grazing regime, and negatively associated with free-living saprotrophs, typical to the winter grazing regime. These findings suggest that when grazers promote dominance of evergreen dwarf shrubs, they induce shifts in soil fungal communities that increase soil carbon sequestration in the long term. Thus, to predict climate-driven changes in soil carbon, grazer-induced shifts in vegetation and soil fungal communities need to be accounted for.
Highlights
Arctic and subarctic regions are rapidly warming, which is leading to northward and altitudinal range expansion of shrubs and trees (Myers-Smith et al, 2019; Tape et al, 2012; Terskaia et al, 2020)
We test the hypotheses, that (H1) year-round grazing by reindeer and greater distance to mountain birch trees increase the relative abundance of ericoid mycorrhizal fungi (ErM), while decreasing ectomycorrhizal fungi (EcM) and saprotrophic fungi, and that (H2) ErM-dominated communities are associated with higher organic soil carbon stocks than EcM-dominated communities, if (H3) the EcM fungi belong to the cord-forming exploration type and/or if (H4) the mycorrhizal shift from EcM to ErM is accompanied with lower abundance of free-living saprotrophs
Based on the captured plant reads in the soil samples, the year-round grazing regime (YGR) was characterised by higher abundance of evergreen ericaceous dwarf shrubs, that is, E. hermaphroditum, P. caerulea and C. vulgaris, and bryophytes compared with the winter grazing regime (WGR) (Table 1)
Summary
Arctic and subarctic regions are rapidly warming, which is leading to northward and altitudinal range expansion of shrubs and trees (Myers-Smith et al, 2019; Tape et al, 2012; Terskaia et al, 2020). Given what is known about the role of herbivores in climate- driven vegetation changes and their potential to promote an evergreen shrub–dominated plant community, herbivores' potential to increase tundra soil carbon stocks via promoting the dominance of ErM has been proposed (Vowles & Björk, 2019), yet evidence for this mechanism is lacking We test this idea across two adjacent grazing regimes, which have experienced either only winter grazing or year-round grazing by reindeer due to long-standing differences in Finnish and Norwegian reindeer husbandry. We test the hypotheses, that (H1) year-round grazing by reindeer and greater distance to mountain birch trees increase the relative abundance of ErM, while decreasing EcM and saprotrophic fungi, and that (H2) ErM-dominated communities are associated with higher organic soil carbon stocks than EcM-dominated communities, if (H3) the EcM fungi belong to the cord-forming exploration type and/or if (H4) the mycorrhizal shift from EcM to ErM is accompanied with lower abundance of free-living saprotrophs
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