Moderate drop in water table increases peatland vulnerability to post-fire regime shift

Northern and tropical peatlands represent a globally significant carbon reserve accumulated over thousands
of years of waterlogged conditions. It is unclear whether moderate drying predicted for northern peatlands
will stimulate burning and carbon losses as has occurred in their smaller tropical counterparts where the
carbon legacy has been destabilized due to severe drainage and deep peat fires. Capitalizing on a unique
long-term experiment, we quantify the post-wildfire recovery of a northern peatland subjected to decadal
drainage. We show that the moderate drop in water table position predicted for most northern regions
triggers a shift in vegetation composition previously observed within only severely disturbed tropical
peatlands. The combined impact of moderate drainage followed by wildfire converted the low productivity,
moss-dominated peatland to a non-carbon accumulating shrub-grass ecosystem. This new ecosystem is
likely to experience a low intensity, high frequency wildfire regime, which will further deplete the legacy of
stored peat carbon.