Permafrost thaw in boreal peatlands is rapidly altering forest community composition

Boreal peatlands are frequently underlain by permafrost, which is thawing
rapidly. A common ecological response to thaw is the conversion of raised
forested plateaus to treeless wetlands, but unexplained spatial variation
in responses, combined with a lack of stand-level data, make it difficult
to predict future trajectories of boreal forest composition and
structure. We sought to characterize patterns and identify
drivers of forest structure, composition, mortality, and recruitment in a
boreal peatland experiencing permafrost thaw. To do this, we established a
large (10 ha) permanent forest plot (completed in 2014), located in the
Northwest Territories, Canada, that includes 40,584 mapped and measured
trees. In 2018, we conducted a comprehensive mortality and recruitment
recensus. We also measured frost table depth, soil moisture, soil
humification, and organic layer thickness within the plot between 2012 and
2018, and used habitat association tests to link these variables to forest
characteristics and dynamics. Forest composition and structure
varied markedly throughout the plot and were strongly governed by patterns
in permafrost presence and organic layer thickness. Overall, there was a
net loss of trees from the plot at a rate of 0.7% yr-1. Mortality of black
spruce, the dominant tree species, was more than double that of
recruitment and was strongly associated with permafrost thaw. In contrast,
recruitment of larch was over four times greater than mortality, and
occurred primarily in low-lying, permafrost-free wetlands with mineral
soil near the surface. The trends in tree demography and
underlying drivers suggest that spruce-dominated permafrost plateaus will
be converted into larch-dominated wetlands as permafrost thaw progresses
in boreal peatlands, particularly in areas where mineral soil is near the
surface. In the longer term, thaw could increase the hydrologic
connectivity of the landscape, resulting in widespread drainage and
re-vegetation by spruce, but we did not find evidence that this is
occurring yet. Given the increasing rates of permafrost thaw, and positive
feedbacks between thaw and forest change, we predict that larch abundance
will continue to increase in boreal peatlands over the coming decades,
leading to shifts in ecosystem function, wildlife habitat, albedo, and
snow dynamics.