Carbon cycling after boreal forest fire: Wildfire activity has been increasing in the boreal forests of the Northern Hemisphere, releasing carbon into the atmosphere from biomass and soil, with potential feedback to climate warming. In a long-term study, Mack et al. analyzed wildfire impacts on the carbon balance of boreal forest in Alaska, with particular focus on forest-regeneration patterns. After fire, the species composition in most of the study sites changed from black spruce to a mixture of conifers and deciduous broadleaf tree species. The stands that had shifted to deciduous dominance stored fivefold more soil carbon than stands that returned to black spruce dominance. Therefore, the functional traits of deciduous trees compensated for the combustion loss of soil carbon, pointing to a potential mitigation of the feedback effect of boreal forest fire to climate warming. Science , this issue p. 280

Wildfire activity is increasing in boreal forests as climate warms and dries, furthering risks to rural and urban communities. In black spruce forests of Interior Alaska, fuel reduction treatments are increasingly used to reduce characteristics of fire behavior and create a defensible space for fire suppression. These treatments, however, introduce novel disturbance characteristics, making longer-term outcomes on ecosystem structure uncertain. By understanding how ecosystems respond to such treatments over time, we can improve management procedures and maintenance protocols so that they effectively reduce wildfire risk over decadal timescales. We remeasured a network of hand thinned and mechanically treated (‘shearbladed’) sites in Interior Alaska to assess how successional trajectories of tree dominance, understory composition, and permafrost change over ~20 years after treatment. We also assessed if these treatments reduce modeled surface rate of fire spread (ROS), flame length, and fireline intensity relative to an untreated black spruce stand, and if surface fire behavior changes over time. In thinned areas, soil organic layer (SOL) disturbance promoted tree seedling recruitment, with conifer seedlings dominating but deciduous broadleaf seedlings prevalent on the thinnest SOLs. In shearbladed areas conifer recruitment density increased over the sampling period at a slow rate (~0.2 stems m -2 yr -1 ) yet deciduous tree seedlings were nearly 20 times more abundant. Understory composition was not altered by thinning but in shearbladed treatments shifted from an initial high abundance of forbs and horsetail to tall deciduous shrubs and grasses over time. Thaw depth was greater in treated than unmanaged areas, and in shearbladed than thinned sites, but increased over time in both treatments. Although understory vegetation changed over time in shearbladed treatments, modeled surface fire behavior was constant, highlighting potential limitations of fuel models and the need for additional fuels-specific data to capture the changing vegetation structure within this treatment. Lastly, treatment effectiveness at reducing modeled surface ROS, flame length, and fireline intensity depended on the fuel model used for an untreated black spruce stand, pointing to uncertainties about the efficacy of these treatments at mitigating surface fire behavior. Overall, we show that fuel reduction treatments can promote low flammability, deciduous-dominated successional trajectories and that shearblading in particular has strong effects on understory composition and permafrost degradation that persist for nearly two decades after disturbance. Such factors need to be considered to enhance the design, management, and predictions of fire behavior in these treatments.