Role of shifting vegetation cover in Arctic silicon cycling

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1. Background

Arctic ecosystems are experiencing the effects of climate change to a much larger degree than most other places on Earth. Rapid Arctic warming is changing the types of plants that proliferate in tundra ecosystems, with shrubs often out-competing grass and sedge species, and permafrost thaw increasing the area of wetlands.


Such changes in plant species composition have direct impacts for plant silica (SiO2) cycling in the Arctic. Land plants consume vast quantities of silica, typically incorporated passively in their tissue as the plant consumes water (i.e. transpires). However, the amount of silica accumulated by plants varies dramatically by species type, with large ranges of plant silica concentrations observed across species. Rates of silica export from land to the sea have important consequences for carbon cycling in aquatic systems, since silica concentrations in coastal Arctic waters can often dictate phytoplankton growth rates, or rates of primary production.

2. Methods

We made the first measurements of how much silica is stored in above- and belowground portions of the three main types of tundra found in the Alaskan Arctic (moist acidic tundra, moist non-acidic tundra, and wet sedge tundra). Field sites spanned a 300 km latitudinal gradient in central and northern Alaska, USA. We also examined plant silica accumulation across three main tundra types found in the Arctic (MAT, moist non-acidic tundra (MNT), and wet sedge tundra (WST)). Plant samples were harvested in late June 2015, which was the period of peak biomass according to phenology data (days 180-200) in 2015. At each site, aboveground harvests were conducted at three haphazardly chosen 0.25 m2 plots. Plots were located at least 5 meters apart from one another. At each plot, replicate samples (at least 3 g) of all species present in both the tussock and inter-tussock portions of tundra within the plots were collected. Aboveground portions of bryophytes and graminoids were cut roughly 1 cm above the litter layer. We then estimated how shift in tundra species composition alters net plant silica storage. 

3. Results

BSi concentrations in live Eriophorum vaginatum, a tussock-forming sedge that is the foundation species of tussock tundra, were not significantly (p<0.05) different across the three main sites. Concentrations of BSi in live aboveground tissue were highest in the graminoid species (0.55 ± 0.07 % BSi in sedges from WST, and 0.27 ± 0.01% in E. vaginatum across the three MAT sites). Both inter-tussock tundra species and shrubs contained substantially lower BSi concentrations than E. vaginatum


We find that increasing shrub abundance will increase aboveground net silica storage in land plants, whereas increasing wetland area will have the opposite effect, by decreasing net silica storage in aboveground vegetation. These results are driven mostly by shifts in total biomass production, rather than plant silica concentrations, as shrub tundra ecosystems are more productive than tussock tundra ecosystems, while wetlands are less productive than the other tundra types.


4. Conclusions

Silica (SiO2) accumulation by terrestrial vegetation is an important component of the biological silica cycle because it improves overall plant fitness and influences export rates of silica from terrestrial to marine systems. However, most research on silica in plants has focused on agricultural and forested ecosystems, and knowledge of terrestrial silica cycling in the Arctic, as well as the potential impacts of climate change on the silica cycle is severely lacking. 


Changes in the amount of silica stored in land plants will likely alter the amount of silica exported from terrestrial systems into downstream waters. Our results highlight that Arctic warming will alter the amount of silica stored in land plants, with potential unrealized consequences for downstream ecosystems.


Our calculations suggest that shrub expansion via warming will increase BSi storage in Arctic land plants due to the higher biomass associated with shrub tundra, whereas conversion of tussock tundra to WST via permafrost thaw would produce the opposite effect in the terrestrial plant BSi pool. Such changes in the size of the terrestrial vegetation silica reservoir could have direct consequences for the rates and timing of silica delivery to receiving waters in the Arctic. 

5. Future ideas/collaborators needed to further research?

I'm looking to collaborate with other Arctic plant scientists to determine if the trends we observe here are observed elsewhere across the Arctic.

6. Please share a link to your paper

10.1111/1365-2435.12912

Comments

1
Jackie Walker
8 months ago

Good research

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Dr. Carey joined Babson in 2017 as an Assistant Professor in the Math & Science Division. She currently teaches Case Studies in Ecological Management (NST 2020), and Oceanography (NST 1060). Her re...

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