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Decreased cryogenic disturbance: one of the potential mechanisms behind the shrubification of non-sorted circles in subarctic Sweden
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

During the last few decades, the arctic has experienced large-scale vegetation changes often evident as widespread greening (increased NDVI) in satellite images. Understanding the mechanisms behind this greening is crucial for our ability to predict future vegetation changes. The objective of this study was to test the hypothesis that decreased cryogenic disturbances have contributed to the greening of patterned ground fields (non-sorted circles) in the study site (Abisko, Northern Sweden) during the last few decades. The hypothesis was tested by surveying the composition of plant communities across a gradient in cryogenic disturbance and by reinvestigating plant communities previously surveyed in the 1980s to scrutinise how these communities changed in response to reduced cryogenic disturbance. Whereas the historical changes in species occurrence associated with decreased cryogenic disturbances were fairly consistent with the changes along the contemporary gradient of cryogenic disturbances, the species abundance revealed important transient changes highly dependent on the initial plant community composition. Our results imply that the decreased environmental stress from differential heave during winter in recent decades is one of the mechanisms that has contributed to the greening of the patterned ground in the study area, although more than a few decades are likely required before this decreased stress contributes in a substantial way to the on-going shrubification. 

Keyword [en]
Patterned ground, Plant abundance, Non-sorted circles, Freeze/thaw-index, Cryogenic disturbance, Differential heave
National Category
Other Earth and Related Environmental Sciences
Research subject
Physical Geography
Identifiers
URN: urn:nbn:se:umu:diva-112507OAI: oai:DiVA.org:umu-112507DiVA: diva2:882510
Available from: 2015-12-15 Created: 2015-12-09 Last updated: 2015-12-17Bibliographically approved
In thesis
1. Cryogenic soil processes in a changing climate
Open this publication in new window or tab >>Cryogenic soil processes in a changing climate
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Kryogena mark processer i ett föränderligt klimat
Abstract [en]

A considerable part of the global pool of terrestrial carbon is stored in high latitude soils. In these soils, repeated cycles of freezing and thawing creates soil motion (cryoturbation) that in combination with other cryogenic disturbance processes may play a profound role in controlling the carbon balance of the arctic soil. Conditions for cryogenic soil processes are predicted to dramatically change in response to the ongoing climate warming, but little is known how these changes may affect the ability of arctic soils to accumulate carbon. In this thesis, I utilize a patterned ground system, referred to as non-sorted circles, as experimental units and quantify how cryogenic soil processes affect plant communities and carbon fluxes in arctic soils. I show that the cryoturbation has been an important mechanism for transporting carbon downwards in the studied soil over the last millennia. Interestingly, burial of organic material by cryoturbation appears to have mainly occurred during bioclimatic events occurring around A.D. 900-1250 and A.D. 1650-1950 as indicated by inferred 14C ages. Using a novel photogrammetric approach, I estimate that about 0.2-0.8 % of the carbon pool is annually subjected to a net downward transport induced by the physical motion of soil. Even though this flux seems small, it suggests that cryoturbation is an important transporter of carbon over centennial and millennial timescales and contributes to translocate organic matter to deeper soil layers where respiration proceeds at slow rates. Cryogenic processes not only affect the trajectories of the soil carbon, but also generate plant community changes in both species composition and abundance, as indicated by a conducted plant survey on non-sorted circles subjected to variable differential frost heave during the winter. Here, disturbance-tolerant plant species, such as Carex capillaris and Tofieldia pusilla, seem to be favoured by disturbance generated by the differential heave. Comparison with findings from a previous plant survey on the site conducted in the 1980s suggest that the warmer temperatures during the last decades have resulted in decreased differential heave in the studied non-sorted circles. I argue that this change in cryogenic activity has increased abundance of plants present in the 1980s. The fact that the activity and function of the non-sorted circles in Abisko are undergoing changes is further supported by their contemporary carbon dioxide (CO2) fluxes. Here, my measurements of CO2 fluxes suggest that all studied non-sorted circles act as net CO2 sources and thus that the carbon balance of the soils are in a transition state. My results highlight the complex but important relationship between cryogenic soil processes and the carbon balance of arctic soils.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2016. 18 p.
Keyword
Non-sorted circle, Carbon storage, Soil motion, Terrestrial photogrammetry, Cryoturbation, Carbon fluxes
National Category
Other Earth and Related Environmental Sciences
Research subject
Physical Geography
Identifiers
urn:nbn:se:umu:diva-112509 (URN)978-91-7601-361-8 (ISBN)
Public defence
2016-01-22, Stora Hörsalen, KBC-huset, Umeå universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2015-12-18 Created: 2015-12-09 Last updated: 2015-12-17Bibliographically approved

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Becher, MarinaOlofsson, JohanKlaminder, Jonatan
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