Boreal climate feedbacks from changes in carbon stocks after land-use change
The large boreal carbon (C) stocks in Alaska are vulnerable to losses from land-use change, such as clearcut logging of a pristine forest and deforestation for agricultural development. However, the consequences for C stocks are poorly known for the boreal region due to the paucity of field studies that address long-term changes in these stocks after anthropogenic disturbances. Above all, information is lacking on the fate of the large C stocks in permafrost soils that underlay 80% of the land in Alaska. Here we investigated impacts of logging in uplands and agricultural deforestation in lowlands on C and nitrogen (N) stocks in Interior Alaska, using chronosequences of up to 100 years (the entire EuroAmerican land-use history of the region), and synthesized results from other studies in the boreal region.
Two years after logging, ecosystem C stocks in upland forests were reduced by 11 kg m-2 (46% of the original ecosystem C stock; permafrost-free soils), mainly as a consequence of stem removal. Soil C and N stocks increased over the first few years after logging, but returned to pre-harvest levels during the following decades. Across the boreal region, mean initial C loss was four times greater, but long-term C cycling was similar in logged as compared to burned forests. Agricultural development in Alaskan lowlands permanently reduced ecosystem C stocks, reaching losses of 11 kg m-2 (34% of the ecosystem C stock) on non-permafrost soils after several decades and 31 kg m-2 (69%) on permafrost soils over 6 years. These C losses are much more rapid than the 5-6 kg m-2 over 500 years that models project to be lost by warming or warming-plus-wildfire in lowland boreal forests. As a consequence of the large C losses, both logging and deforestation may strongly impact climate forcing, potentially magnifying warming-induced C loss. These impacts can be reduced in lowlands by conserving permafrost-dominated sites for C storage and focusing agriculture on permafrost-free sites.