COS 78-8
Boreal climate feedbacks from changes in carbon stocks after land-use change

Wednesday, August 12, 2015: 4:00 PM
303, Baltimore Convention Center
Jose M. Gruenzweig, Faculty of Agriculture, Food and Environment, Hebrew University of Jerusalem, Rehovot, Israel
David W. Valentine, School of Natural Resources and Extension, University of Alaska Fairbanks, Fairbanks, AK
F. Stuart Chapin III, Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK

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.