Author: isabel

Arctic grazers are known to prevent the establishment of deciduous shrubs and, under certain conditions, promote the dominance of evergreen shrubs. As these shrubs associate with contrasting soil fungal communities, a grazer-induced shift in shrub abundance could alter the processes sustained by soil fungi – such as soil carbon sequestration. This role of grazers is of particular interest today now that both deciduous and evergreen shrubs are expanding their ranges, and arctic grazer could determine their relative success.

We assessed soil fungal community composition and its links to soil carbon storage in two contrasting long-term reindeer grazing regimes in the subarctic treeline ecotone at the border between Finland and Norway. We found that root-associated basidiomycetes and free-living moulds and yeasts were characteristic to the regime with only wintertime grazing, whereas few taxa of root-associated ascomycota dominated fungal communities on the open heaths in the year-round grazing regime. These patterns in fungal functional guild abundance were linked to organic soil carbon storage: root-associated ascomycetes were more abundant in plots with high soil carbon storage, whereas saprotrophic guilds were typical to plots with low soil carbon storage.

Overall, these findings suggest that when grazers promote the dominance of evergreen dwarf shrubs, they may induce shifts in soil fungal communities that, in the long term, increase soil carbon sequestration. This implies an overlooked role of grazers and grazing regimes in controlling soil carbon storage at the treeline ecotone.

Reference: Ylänne, H., Madsen, R.L., Castaño, C., Metcalfe, D.B. and Clemmensen, K.E. (2021) Reindeer control over subarctic treeline alters soil fungal communities with potential consequences for soil carbon storage. Global Change Biology, pp. 1– 15. https://doi.org/10.1111/gcb.15722

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Text by Henni Ylänne, Lund University

Pictures from the two grazing regimes by Henni Ylänne

In the Arctic, soil fungal communities may be intrinsically shaped by heavy grazing, which may locally induce an ecosystem change that couples with increased soil temperature and nutrients, and where climate change induced shrub encroachment is less likely to occur than in lightly grazed conditions. Due to these reasons, differences in grazing intensity may control the effects of climate change on fungal communities and thereby carbon and nutrient cycling.

We tested how contrasting long-term grazing intensities affect the responses of soil fungal communities to short-term warming and increased nutrient availability. We used a study design along a reindeer migration route, where 50-years history of an annually occurring pulse of heavy grazing during reindeer migration has shifted the subarctic tundra ecosystem towards increased graminoid dominance, higher soil temperature and nutrient availability. We found that heavy grazing had led to distinct shifts in soil fungal communities when compared to light grazing. Furthermore, the long-term grazing difference largely overrode the effects of short-term warming and fertilization, and the changes in the soil fungal communities caused by our experimental treatments were not unidirectional under different grazing intensities.

Our results demonstrate the determinant role of long-term difference in grazing intensity in shaping fungal communities and their responses to abiotic changes. Further, it reveals that if grazing shifts the fungal communities in Arctic ecosystems to a different state, this may dictate ecosystem responses to further abiotic changes. These results incline that the intensity of grazing cannot be left out when predicting future changes in fungi-driven processes in the tundra.

You can read the full article here: https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.17623

Reference: Ahonen, S.H., Ylänne, H., Väisänen, M., Ruotsalainen, A.L., Männistö, M.K., Markkola, A. and Stark, S. (2021) Reindeer grazing history determines the responses of subarctic soil fungal communities to warming and fertilization. New Phytologist.

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Text by Saija Ahonen, PhD student University of Oulu

Picture: Reindeer in Ráisduoddar (photo credit Sari Stark)

Decades of biogeographic theory suggests that a species geographic range is limited by abiotic conditions towards its high elevation limit and antagonistic biotic interactions towards its low elevation range limit. While many studies have supported the high-elevation range limit prediction, rigorous tests of the low-elevation and antagonistic interaction prediction have been limited. With this work, we tested if three alpine grass species are restricted to the top of the West Elk Mountains, Colorado, USA, in part by higher mammal herbivory rates towards and beyond their low-elevation range limit.

We transplanted individuals of our focal species (Elymus scribneri, Festuca brachyphylla, and Poa alpina) in the core of their range, at their range limit, and in a novel-beyond range limit site that reflected ~2 °C warming. At each site, we factorially excluded above and belowground mammal herbivore access to plants using fencing. Additionally, we collected demographic data from natural populations of the focal species and applied experimental treatment effect sizes to vital rates models (i.e., growth, survival, and reproduction) to project experimental effects on population growth. We found, generally, that herbivory increases from the core of the species’ range towards the limit and novel sites and that restricting above and belowground mammal access to plants increased their growth and reproduction more in limit and novel habitats than in their core range. Additionally, mammal exclusion increased population growth over controls most in limit and novel sites.

Our study suggested that higher herbivory below a species’ elevation range limit contributes to the plant’s exclusion from lower elevations. Importantly, the abiotic environment in novel habitats was hospitable for the individuals, but mammalian herbivory in range limit and novel habitats may drive focal species population growth rates below replacement. Results suggest that as mammals shift their foraging range upslope with further climate change, increased herbivory may drive the local extinction of these alpine restricted grass species.  

You can read the full article here: https://onlinelibrary.wiley.com/doi/epdf/10.1111/ele.13829

Reference: Lynn, J.S., Miller, T.E.X. and Rudgers, J.A. (2021) Mammalian herbivores restrict the altitudinal range limits of alpine plants. Ecology Letters.

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Text by Joshua S. Lynn, University of New Mexico.

Picture: View of an elevation transect used for the experiment in the West Elk Mountains, Colorado, USA. (Photo credit: Joshua S. Lynn).

Oligotrophic boreal forests with lichen dominated field layer are important winter pastures for reindeer (Rangifer tarandus). In addition to the lichen dominated, drier sunlit habitats, these forests may also have moister shaded habitats with varying moss abundance. Mosses, in turn, control vascular plant-soil interactions, yet they all can also be altered by reindeer grazing.

We determined how two decades of reindeer exclusion affects feather moss (Pleurozium schreberi) depth, and the accompanying soil N dynamics, plant foliar N and stable isotopes of N and C in two contrasting habitats of an oligotrophic Scots pine forest. The study species were pine seedling (Pinus sylvestris), bilberry (Vaccinium myrtillus), lingonberry (V. vitis-idaea) and feather moss. We found that moss carpet was 80% deeper after excluding reindeer both in shaded and sunlit habitats. In the humus horizon, the proportion of heavy N isotope increased due to exclusion in the sunlit habitats and, in the mineral soil, exclusion also increased inorganic N in both habitats. These soil responses were correlated with moss depth. Foliar chemistry did not respond to reindeer exclusion and varied solely due to habitat but depending on species identity.

We conclude that despite strong reindeer grazing-induced shifts in mosses and subtler shifts in soil N, the N dynamics of vascular vegetation remain unchanged. These indicate that plant-soil interactions are resistant to shifts in grazing intensity, a pattern that appears to be common across boreal oligotrophic forests.

Reference: Väisänen M, Tuomi M, Bailey H, Welker JM. Plant and soil nitrogen in oligotrophic boreal forest habitats with varying moss depths: does exclusion of large grazers matter? Oecologia

https://doi.org/10.1007/s00442-021-04957-0

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Text by Maria Väisänen, University of Oulu and University of Lapland.

Picture: Reindeer in an oligotrophic Scots pine forest in NE Finland (photo credit: Noora Kantola)

Rapid warming is predicted to increase insect herbivory across the tundra biome, yet how this will impact the community and ecosystem dynamics remains poorly understood. More insect herbivory could reduce potential gains in Arctic plant growth, by serving as a top–down control on tundra vegetation. Additionally, many tundra ecosystems experience severe insect outbreaks, where large numbers of insects emerge and consume large amounts of leaves and other plant material, which can have leaf lasting damage to shrubs. To explore how tundra-insect herbivore systems respond to warming, we measured shrub traits and leaf herbivory damage at 16 sites along a landscape gradient in western Greenland. Here we show that shrub leaf insect herbivory damage on two dominant deciduous shrubs, grey willow (Salix glauca) and dwarf birch (Betula nana), was positively correlated with increasing temperatures throughout the first half of the 2017 growing season. We found that the majority of insect herbivory damage occurred in July, which was outside the period of rapid leaf expansion that occurred throughout most of June. Leaf-chewing insects caused the most leaf damage in both shrub species. Additionally, insect herbivores removed a larger proportion of dwarf birch leaf biomass in warmer sites, which is due to a combination of increased herbivory with a coinciding decline in leaf biomass. These results suggest that the effects of rising temperatures on both insect herbivores and plants are important to consider when predicting the trajectory of Arctic tundra shrub expansion.  

Reference: Finger-Higgens, R., DeSiervo, M., Ayres, M.P. and Virginia, R.A., 2021. Increasing shrub damage by invertebrate herbivores in the warming and drying tundra of West Greenland. Oecologia, pp.1-11.
https://doi.org/10.1007/s00442-021-04899-7

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Text by Rebecca Finger-Higgens, Darmouth College.

Picture: Fieldwork in Kangerlussuaq (photo credit: Rebecca Finger Higgens)