Future Spruce Budworm Outbreak May Create a Carbon Source in Eastern Canadian Forests Caren C. Dymond, 1,2 * Eric T. Neilson, 1 Graham Stinson, 1 Kevin Porter, 3 David A. MacLean, 4 David R. Gray, 3 Michel Campagna, 5 and Werner A. Kurz 1 1 Natural Resources Canada, Canadian Forest Service, 506 West Burnside Road, Victoria, British Columbia V8Z 1M5, Canada; 2 Ministry of Forests and Range, Government of British Columbia, P.O. Box 9504, Stn Prov Govt, Victoria, British Columbia V8W 9C1, Canada; 3 Natural Resources Canada, Canadian Forest Service, P.O. Box 4000, 1350 Regent Street South, Fredericton, New Brunswick E3B 5P7, Canada; 4 Faculty of Forestry and Environmental Management, University of New Brunswick, P.O. Box 4400, Fredericton, New Bruns- wick E3B 5A3, Canada; 5 Ressources Naturelles et faune Que ´ bec, 880, chemin Sainte-Foy, 10e e ´ tage, Que ´ bec, Quebec G1S 4X4, Canada ABSTRACT Spruce budworm (Choristoneura fumiferana Clem.) is an important and recurrent disturbance throughout spruce (Picea sp.) and balsam fir (Abies balsamea L.) dominated forests of North America. Forest carbon (C) dynamics in these ecosystems are affected during insect outbreaks because millions of square kilome- ters of forest suffer growth loss and mortality. We tested the hypothesis that a spruce budworm out- break similar to those in the past could switch a forest from a C sink to a source in the near future. We used a model of ecosystem C to integrate past spruce bud- worm impact sequences with current forest man- agement data on 106,000 km 2 of forest in eastern Que ´ bec. Spruce budworm-caused mortality decreased stand-level merchantable C stocks by 11–90% and decreased ecosystem C stocks by 2–10% by the end of the simulation. For the first 13 years (2011–2024), adding spruce budworm significantly reduced ecosystem C stock change for the landscape from a sink (4.6 ± 2.7 g C m -2 y -1 in 2018) to a source (-16.8 ± 3.0 g C m -2 y -1 in 2018). This result was mostly due to reduced net primary production. The ecosystem stock change was reduced on average by 2 Tg C y -1 for the entire simulated area. This study provides the first estimate that spruce budworm can significantly affect the C sink or source status of a large landscape. These results indicate that reducing spruce budworm impacts on timber may also pro- vide an opportunity to mitigate a C source. Key words: carbon cycle; net primary produc- tion; net biome production; boreal forest; defolia- tion; ecosystem carbon; CBM-CFS3; ecosystem stock change. INTRODUCTION Spruce budworm (Choristoneura fumiferana (Clem.), Lepidoptera: Tortricidae) is a native insect in spruce (Picea sp.) and balsam fir (Abies balsamea L.) forests of eastern North America. Spruce budworm outbreaks have occurred periodically in these forests for cen- turies (Blais 1983; Royama 1984). These outbreaks play an important role in the natural history of these Received 30 November 2009; accepted 23 June 2010; published online 23 July 2010 Electronic supplementary material: The online version of this article (doi:10.1007/s10021-010-9364-z) contains supplementary material, which is available to authorized users. Author Contributions: CD led writing of the article and performed research. EN performed research and contributed to the article. GS per- formed research, contributed to the study design study, and contributed to the article. KP, DM, and DG contributed to the study design, models, and contributed to the article. MC analyzed data, and WK conceived of the study and contributed to the article. *Corresponding author; e-mail: Caren.Dymond@gov.bc.ca Ecosystems (2010) 13: 917–931 DOI: 10.1007/s10021-010-9364-z Ó 2010 UKCrown: Natural Resources Canada, Government of Canada 917