标题：Permafrost response to increasing Arctic shrub abundance depends on the relative influence
of shrubs on local soil cooling versus large-scale climate warming
摘要：Deciduous shrub abundance is increasing across the Arctic in response to climatic warming.
In a recent field manipulation experiment in which shrubs were removed from a plot and
compared to a control plot with shrubs, Blok et al (2010 Glob. Change Biol.16
1296–305) found that shrubs protect the ground through shading, resulting in a ~ 9% shallower active layer thickness (ALT) under shrubs compared to grassy-tundra, which led
them to argue that continued Arctic shrub expansion could mitigate future permafrost
thaw. We utilize the Community Land Model (CLM4) coupled to the Community
Atmosphere Model (CAM4) to evaluate this hypothesis. CLM4 simulates shallower ALT (~− 11 cm)
under shrubs, consistent with the field manipulation study. However, in an idealized pan-Arctic + 20% shrub area experiment, atmospheric heating, driven mainly by surface albedo changes related to
protrusion of shrub stems above the spring snowpack, leads to soil warming and deeper ALT (~+ 10 cm). Therefore, if climate feedbacks are considered, shrub expansion may actually increase
rather than decrease permafrost vulnerability. When we account for blowing-snow
redistribution from grassy-tundra to shrubs, shifts in snowpack distribution in low versus
high shrub area simulations counter the climate warming impact, resulting in a grid cell
mean ALT that is unchanged. These results reinforce the need to consider vegetation
dynamics and blowing-snow processes in the permafrost thaw model projections.
