Ecosystems (2010) 13: 917–931 DOI: 10.1007/s10021-010-9364-z Ó 2010 UKCrown: Natural Resources Canada, Government of Canada 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. Kurz1 1Natural Resources Canada, Canadian Forest Service, 506 West Burnside Road, Victoria, British Columbia V8Z 1M5, Canada; 2Ministry of Forests and Range, Government of British Columbia, P.O. Box 9504, Stn Prov Govt, Victoria, British Columbia V8W 9C1, Canada; 3Natural Resources Canada, Canadian Forest Service, P.O. Box 4000, 1350 Regent Street South, Fredericton, New Brunswick E3B 5P7, Canada; 4Faculty of Forestry and Environmental Management, University of New Brunswick, P.O. Box 4400, Fredericton, New Bruns- wick E3B 5A3, Canada; 5Ressources 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 adding spruce budworm significantly reduced an important and recurrent disturbance throughout ecosystem C stock change for the landscape from a spruce (Picea sp.) and balsam fir (Abies balsamea L.) sink (4.6 ± 2.7 g C m-2 y-1 in 2018) to a source dominated forests of North America. Forest carbon (-16.8 ± 3.0 g C m-2 y-1 in 2018). This result was (C) dynamics in these ecosystems are affected during mostly due to reduced net primary production. The insect outbreaks because millions of square kilome- ecosystem stock change was reduced on average by ters of forest suffer growth loss and mortality. We 2TgCy-1 for the entire simulated area. This study tested the hypothesis that a spruce budworm out- provides the first estimate that spruce budworm can break similar to those in the past could switch a forest significantly affect the C sink or source status of a from a C sink to a source in the near future. We used a large landscape. These results indicate that reducing model of ecosystem C to integrate past spruce bud- spruce budworm impacts on timber may also pro- worm impact sequences with current forest man- vide an opportunity to mitigate a C source. agement data on 106,000 km2 of forest in eastern Que´bec. Spruce budworm-caused mortality decreased Key words: carbon cycle; net primary produc- stand-level merchantable C stocks by 11–90% and tion; net biome production; boreal forest; defolia- decreased ecosystem C stocks by 2–10% by the end tion; ecosystem carbon; CBM-CFS3; ecosystem of the simulation. For the first 13 years (2011–2024), stock change. Received 30 November 2009; accepted 23 June 2010; published online 23 July 2010 INTRODUCTION Electronic supplementary material: The online version of this article (doi:10.1007/s10021-010-9364-z) contains supplementary material, Spruce budworm (Choristoneura fumiferana (Clem.), which is available to authorized users. Lepidoptera: Tortricidae) is a native insect in spruce Author Contributions: CD led writing of the article and performed research. EN performed research and contributed to the article. GS per- (Picea sp.) and balsam fir (Abies balsamea L.) forests of formed research, contributed to the study design study, and contributed eastern North America. Spruce budworm outbreaks to the article. KP, DM, and DG contributed to the study design, models, have occurred periodically in these forests for cen- and contributed to the article. MC analyzed data, and WK conceived of the study and contributed to the article. turies (Blais 1983; Royama 1984). These outbreaks *Corresponding author; e-mail: [email protected] play an important role in the natural history of these 917 918 C. C. Dymond and others forest ecosystems, and have strong influence on severe because it kills host trees in a single year, has stand succession and landscape vegetation dynamics been forecast to reduce forest C stocks by 270 Tg C (Baskerville 1975; MacLean 1984). Repeated defo- over 20 years as a result of decreased uptake and liation of host trees during budworm outbreak increased decomposition from beetle-killed trees periods results in growth loss and mortality of (Kurz and others 2008a). We anticipate that a severely defoliated trees. Mortality of dominant or spruce budworm outbreak of similar magnitude co-dominant canopy trees creates openings for to those that have occurred in the past will also succession by the younger trees. Spruce budworm have significant impacts on the forest C budget in outbreaks have occurred across tens of millions of affected regions. square kilometers in eastern North America several Large insect outbreaks can have considerable times during the twentieth century (Williams and impact not just regionally, but also at the scale of Birdsey 2003), and are anticipated to do so again in Canada’s National Forest Carbon Budget (Kurz and the future (Candau and Fleming 2005; Gray 2008). Apps 1999; Kurz and others 2008b). These previous The spruce budworm primarily attacks balsam fir, analyses took into account the impact of spruce white spruce (Picea glauca Moench. Voss.), and red budworm outbreaks as one of several processes spruce (Picea rubens Sarg.). Budworm larvae feed ongoing simultaneously on the landscape. We predominantly on current-year foliage, so the loss in tested the hypothesis that a spruce budworm out- leaf area only becomes severe after several consec- break similar to those in the past could switch utive years of defoliation (MacLean and Ostaff affected forests from a C sink to a source in the near 1989). Outbreaks generally end while there is still future. Our objectives were to (1) use historically mature spruce and fir foliage available in the land- derived, spatially explicit defoliation sequences and scape (Royama 1984). the SBWDSS to generate estimates of budworm Growth loss and tree mortality during past out- impacts; (2) apply these in the Carbon Budget breaks have resulted in significant reductions in Model of the Canadian Forest Sector (CBM-CFS3) timber supply (Sterner and Davidson 1982). It was (Kurz and others 2009) to estimate C dynamics in estimated that Canada lost 44 Mm3 of timber in the boreal forest of eastern Canada from 2000 to just 5 years (1977–1981) at the peak of the last 2024; and (3) compare both stand- and landscape- budworm outbreak, which affected as much as level model outputs to independent data from the 200,000 to 500,000 km2 y-1 (Kettela 1983; Simp- literature. Our study was relatively short-term and son and Coy 1999; NFDP 2008). As a consequence, therefore did not explicitly take into account global considerable investment has since been made in change factors on forest growth, decomposition, or scientific research to develop a sound biological and disturbances. Given the uncertainty of post-out- ecological understanding of spruce budworm pop- break dynamics, we terminated the landscape-scale ulation dynamics and associated impacts on forest simulations in 2024, even though impacts were dynamics. Tools such as the Spruce Budworm projected to continue somewhat beyond this time. Decision Support System (SBWDSS) were devel- We also did not assess the probability of a spruce oped to synthesize and make scientific information budworm outbreak although the insect tends to accessible to forest managers, helping them inte- display cyclical population dynamics (for example, grate budworm considerations directly into forest Boulanger and Arsenault 2004) and budworm management planning processes (MacLean 1996; defoliation in Que´bec has increased from 3000 ha MacLean and others 2001). in 2003 to 338,000 ha in 2009 (QMRNF 2009). Insect disturbance agents that have eruptive outbreak dynamics and widespread impacts in terms of reduced growth and increased mortality MATERIALS AND METHODS may exert considerable influence on the forest Study Area carbon (C) and other nutrient cycles (see overview in Lovett and others 2006). One recent study The study focused on 106,000 km2 within a investigated the impact of a defoliator (gypsy moth, 146,000 km2 region of eastern Que´bec, Canada Lymantria dispar L.) on net ecosystem exchange. (Figure 1). The forest was composed of about Clark and others (2009) found stands that were net 60,000 km2 of spruce-fir dominated stand types, C sinks without the defoliator became net sources 35,000 km2 of hardwoods, mostly sugar maple when there was severe defoliation of the canopy (Acer saccharum Marsh.), with the balance com- trees. Across the 1600 km2 landscape, they esti- posed of jack pine (Pinus banksiana Lamb.), or a mated a reduction in C uptake of 41%. Mountain mixture of hardwood and softwood species. White pine beetle (Dendroctonus ponderosae Hopk.), more birch (Betula papyrifera Ehrh.), larch (Larix laricina Spruce Budworm Impact on Forest Carbon 919 Figure 1. The study area includes 106,000 km2 of forest within a 146,000 km2 landscape in Canada’s eastern boreal forest. Kock.), and poplar (Populus sp.) are also important that allow the model to represent key ecological species in the region. The area is characterized by processes and allow users to compare estimates of rolling hills with hardwoods on the more well- stocks with field measurements. CBM-CFS3 simu- drained sites. Black spruce (Picea mariana Mill. BSP) lates annual changes in C stocks of each pool due to and eastern white cedar (Thuja occidentalis L.) tend growth, litter fall, turnover, decomposition, natural to grow in areas of organic soils and poorly drained disturbances, and forest management (Figure 2). sites (Boulanger and Arsenault 2004), whereas Where a direct comparison has been done, the balsam fir is often found with yellow birch (Betula basic model outputs of net primary productivity alleghaniensis Britton) on the more mesic sites. The (NPP) and heterotrophic respiration (Rh) were anthropological influence on forest structure found to be generally similar to those derived from through wood harvesting began in the early nine- flux tower estimates (Trofymow and others 2008).