Arctic tundra ecosystems are warming almost twice as fast as
the global average1. Permafrost thaw and the resulting release
of greenhouse gases from decomposing soil organic carbon
have the potential to accelerate climate warming2,3. In recent
decades, Arctic tundra ecosystems have changed rapidly4,
including expansion of woody vegetation5,6, in response to
changing climate conditions. How such vegetation changes
contribute to stabilization or destabilization of the permafrost
is unknown. Here we present six years of field observations
in a shrub removal experiment at a Siberian tundra site.
Removing the shrub part of the vegetation initiated thawing
of ice-rich permafrost, resulting in collapse of the originally
elevated shrub patches into waterlogged depressions within
five years. This thaw pond development shifted the plots from
a methane sink into a methane source. The results of our
field experiment demonstrate the importance of the vegetation
cover for protection of the massive carbon reservoirs stored
in the permafrost and illustrate the strong vulnerability of
these tundra ecosystems to perturbations. If permafrost
thawing can more frequently trigger such local permafrost
collapse, methane-emitting wet depressions could become
more abundant in the lowland tundra landscape, at the cost of
permafrost-stabilizing low shrub vegetation.