Abstract
Tundra ecosystems are changing rapidly. The Arctic greening trend of the past decades is generally associated with climate warming-induced shrub expansion, but has recently halted for reasons yet unknown. This proposal addresses shrub decline due to abrupt permafrost thaw (thermokarst), which forms a potential explanation for this trend break. At our long-term research site in Siberian lowland tundra we observe both local shrub expansion and shrub drowning due to local permafrost collapse. In poorly drained lowland tundra such shrub decline due to thermokarst could be a likely alternative for the widely assumed shrub expansion. Moreover, we hypothesize abrupt permafrost thaw to be triggered by extreme wet summers. We propose a combination of landscape and local-scale monitoring with experimentation to assess the dynamics and drivers of shrub decline. We will use a time-series of high-resolution satellite images to assess the balance between shrub decline and shrub expansion at the landscape scale. In field studies we will 1) explore whether the recent thaw pond expansion can be traced back to climatic events using dendrochronology on drowned shrubs and 2) experimentally test if we can trigger permafrost collapse in a novel precipitation experiment. A better understanding of what determines shrub expansion and shrub decline is important as the vegetation shifts and permafrost degradation have important consequences for the climate and greenhouse gas balance. We expect that the proposed research on the drivers of shrub decline and expansion in an understudied Arctic region will generate a novel perspective on Arctic landscape changes.
Netherlands