How do reindeer grazing-induced ecosystem state transitions determine the effects of climate warming on carbon cycling in sub-arctic tundra?

Theme 1. Environmental protection
Session Name 1.11 Reindeer & caribou in the Arctic system: Interactions between environmental, social and biophysical processes
Presentation Type Oral
Author(s) Maria Väisänen (University of Oulu, Finland), Henni Ylänne (University of Lund, Sweden), Elina Kaarlejärvi (University of Umeå, Sweden), Sofie Sjögersten (University of Nottingham, United Kingdom), Sari Stark (University of Lapland, Finland)
Abstract text

Reindeer has a circumpolar distribution and often induces ecosystem state transition, in which low-productive shrub tundra turns into productive grass tundra. Tundra plays a crucial role in atmospheric CO2 control by harboring vast soil carbon reserves. However, climate warming may enhance decomposition and thereby soil CO2 emissions that can positively feedback to warming. The reindeer-induced state transition may affect the balance between soil CO2 emissions and plant photosynthetic CO2 uptake and, consequently, ecosystem carbon stocks. These shifts could determine the impacts of warming on tundra carbon cycling but currently, these interactions remain poorly known.


We conducted laboratory and field warming experiments accompanied by measurements of carbon cycling in arctic-alpine tundra (Raisduoddar Fjell, northern Norway) that has experienced state transition due to a fence separating ungrazed shrub tundra from a heavily grazed tundra where grasses have displaced shrubs. Laboratory incubations showed that grazing dampened the warming-induced increase in soil CO2 release in comparison to no grazing. Field warming with open-top-chambers (summers 2010-2014) increased both ecosystem CO2 emission and photosynthetic uptake and thus, net CO2 balance (i.e. sink) remained stable in the grazed tundra. In the ungrazed tundra, CO2 emissions increased resulting in decreased net CO2 sink. Regardless of these different responses in net gas balance, warming induced a similar loss in soil carbon storage in both grazing treatments. We discuss how reindeer-induced ecosystem shifts may result in different process-pathways yet with similar net outcomes under warming. We discuss the pronounced effects of reindeer on ecosystem functions in comparison to climate changes.    

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