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206 REVIEW Effect of increased fire activity on global warming in the boreal forest Environ. Rev. Downloaded from www.nrcresearchpress.com by Université du Québec à Montréal on 08/28/14 For personal use only. France Oris, Hugo Asselin, Adam A. Ali, Walter Finsinger, and Yves Bergeron Abstract: Forest fires are an important disturbance in the boreal forest. They are influenced by climate, weather, topography, vegetation, surface deposits, and human activities. In return, forest fires affect the climate through emission of gases and aerosols, and changes in surface albedo, soil processes, and vegetation dynamics. The net effect of these factors is not yet well established but seems to have caused a negative feedback on climate during the 20th century. However, an increase in boreal forest fires is predicted by the end of the 21st century, possibly changing the effect of fires on climate change to a positive feedback that would exacerbate global warming. This review presents (1) an overview of fire regimes and vegetation succession in boreal forests; (2) the effects on climate of combustion emissions and post-fire changes in ecosystem functioning; (3) the effects of fire regime variations on climate, especially on carbon stock and surface albedo; (4) an integrative approach of fire effects on climate dynamics; and (5) the implications of increased fire activity on global warming by calculating the radiative forcing of several factors by 2100 in the boreal region, before discussing the results and exposing the limits of the data at hand. Generally, losses of carbon from forest fires in the boreal region will increase in the future and their effect on the carbon stock (0.37 W/m2/decade) will be greater than the effect of fire on surface albedo (−0.09 W/m2/decade). The net effect of aerosol emissions from boreal fires will likely cause a positive feedback on global warming. This review emphasizes the importance of feedbacks between fires and climate in the boreal forest. It presents limitations and uncertainties to be addressed in future studies, particularly with regards to the effect of CO2 fertilization on forest productivity, which could offset or mitigate the effect of fire. Key words: fire, radiative forcing, carbon cycle, surface albedo, carbonaceous aerosols. Résumé : Les feux de forêt constituent une importante perturbation en forêt boréale. Le climat, la température, la topographie, la végétation, les dépôts de surface et les activités humaines représentent autant de facteurs pouvant les influencer. À l'inverse, les feux de forêt affectent le climat par l'émission de gaz et d'aérosols et modifient l'albédo de surface, les processus édaphiques et la dynamique de la végétation. On ne connaît pas encore très bien l'effet net de ces facteurs, mais ils semblent avoir exercé une rétroaction négative sur le climat au cours du XXe siècle. Cependant, on prédit une augmentation des feux de forêt vers la fin du XXIe siècle susceptible de modifier leurs effets sur le climat vers une rétroaction positive capable d'exacerber réchauffement planétaire. Cette synthèse présente (1) une revue d'ensemble des régimes des feux et de la succession de la végétation en forêts boréales, (2) les effets sur le climat des émissions de combustion et des changements du fonctionnement des écosystèmes après feu, (3) les effets des variations du régime de feu sur le climat, particulièrement sur les changements de stock de carbone et d'albédo de surface, (4) une approche intégrée des effets des feux sur la dynamique du climat et (5) l'implication de l'augmentation de l'activité des feux sur le réchauffement planétaire en calculant le forçage radiatif de plusieurs facteurs vers 2100 en région boréale, avant de discuter les résultats et d'exposer les limites des données. Généralement, les pertes en carbone occasionnées par les feux de forêt en région boréale augmenteront dans le futur et leur effet sur les stocks de carbone (0,37 W/m2/décennie) sera plus grand que l'effet du feu sur l'albédo de surface (−0,09 W/m2/décennie). L'effet net des émissions d'aérosols venant des feux en forêt boréale causera vraisemblablement une rétroaction positive sur le réchauffement planétaire. Cette synthèse met l'accent sur l'importance des rétroactions entre les feux et le climat en forêt boréale. Elle présente les limites et les incertitudes à aborder dans les prochaines études, surtout en relation avec les effets de la fertilisation par le CO2 sur la productivité forestière, laquelle pourrait contrebalancer ou atténuer les effets du feu. [Traduit par la Rédaction] Mots-clés : feu, forçage radiatif, cycle du carbone, albédo de surface, aérosols carbonés. Introduction Boreal ecosystems store large amounts of carbon in plant biomass, as well as in soils and peatlands (Apps et al. 1993; Pan et al. 2011). They play an important role in climate regulation by storing anthropogenic carbon emissions (Bonan 2008). Forest fires, however, could counterbalance this effect by releasing stored carbon into the atmosphere (as CO2) through combustion of plant biomass and soil organic matter (Seiler and Crutzen 1980; Carcaillet et al. 2002; van der Werf et al. 2010; Hayes et al. 2011). When combustion is incomplete, emissions can include other gases such as carbon monoxide (CO), methane (CH4), and aerosols, with various impacts on climate (Carslaw et al. 2010; Simpson et al. 2011). Carbon emissions by boreal forest fires represent 9% of global fire emissions, behind the contribution of savanna, grassland, and tropical forest fires (van der Werf et al. 2010). However, while tropical forests are poor in soil organic matter, 40%–90% of carbon Received 12 September 2013. Accepted 9 December 2013. F. Oris, H. Asselin, and Y. Bergeron. Institut de recherche sur les forêts, Université du Québec en Abitibi-Témiscamingue, 445 boul. de l'Université, Rouyn-Noranda, QC J9X 5E4, Canada. A.A. Ali and W. Finsinger. Centre for Bio-Archaeology and Ecology (UMR5059 CNRS/UM2/EPHE), Institut de Botanique, 163 rue Broussonet, F-34090 Montpellier, France. Corresponding author: France Oris (e-mail: [email protected]). Environ. Rev. 22: 206–219 (2014) dx.doi.org/10.1139/er-2013-0062 Published at www.nrcresearchpress.com/er on 13 December 2013. Environ. Rev. Downloaded from www.nrcresearchpress.com by Université du Québec à Montréal on 08/28/14 For personal use only. Oris et al. emissions from boreal forest fires result from the combustion of forest soil organic layers and peat (Kasischke et al. 1995; Amiro et al. 2001b; van der Werf et al. 2010). Carbon accumulation in soils and peatlands occurs over hundreds to thousands of years (Wirth et al. 2002; Thornley and Cannell 2004; Kashian et al. 2006; Lecomte et al. 2006) but can be released almost instantaneously during a forest fire. By altering vegetation cover, forest fires also change surface albedo (Liu et al. 2005; Amiro et al. 2006), thus causing negative or positive radiative forcing (cooling or heating) by decreasing or increasing the amount of solar radiation absorbed in the climate system (Randerson et al. 2006). Increases in area burned are predicted in response to climate change in Canada (Flannigan et al. 2005) and Alaska (Bachelet et al. 2005; Balshi et al. 2009). In addition, areas with high fire risk will increase by 100% in Russia (Stocks et al. 1998). These predictions raise the question “Will increased forest fire activity in the boreal forest exacerbate global warming?”. This question is relevant, as the boreal forest carbon sink has decreased since 1997 and is believed to store 70% less carbon than in the previous decades, mainly because of climate change and more frequent forest fires (Hayes et al. 2011). Moreover, compound disturbances may be more common in response to global warming. Such disturbances (e.g., an insect outbreak closely followed by a fire) have caused regeneration failures and induced a change from spruce–moss forests to lichen woodlands (Payette et al. 2000; Jasinski and Payette 2005). Radiative forcing (RF) is often defined as a perturbation to the radiative energy budget relative to the pre-industrial state (Ramaswamy et al. 2001). In the case of fire disturbance, RF is used to quantify the per-unit area impact of forest fire relative to prefire state. Here we present a literature review to quantify the RF associated with boreal forest fires through changes in carbon stock, surface albedo, and aerosol emissions. Previous reviews have focused on one or the other of these three variables (Kasischke et al. 1995, 2000a, 2000b; Flannigan et al. 2009; Euskirchen et al. 2010); but to our knowledge, this is the first review to simultaneously consider the radiative forcing due to fire-induced changes in carbon stock, surface albedo, and aerosol emissions in the boreal forest. The review is organized as follows: (1) We provide an overview of fire regimes and vegetation succession in boreal forests; (2) we expose the effects on climate of combustion emissions and post-fire changes in ecosystem functioning; (3) we describe the effects of fire regime variations on climate, especially on carbon stock and surface albedo; (4) we examine integrated fire effects on climate dynamics; and (5) we estimate the implications of increased fire activity on global warming in the boreal region, before discussing the results and exposing the limits of the data at hand. Fire regimes and vegetation succession in boreal forests Databases on forest fires are available for Alaska and Canada since at least 1950 (Murphy et al. 2000; Stocks et al. 2002). In Russia, fire maps are only produced for regions where fires are suppressed, accounting for two thirds of Russian forests (Conard and Ivanova 1997). Nevertheless, accurate satellite data are available in Russia since 1996 (Sukhinin et al. 2004) and have been used in studies on the boreal forest (Balshi et al. 2007; Hayes et al. 2011). Fires in North American boreal coniferous forests are mostly crown fires that remove most of the trees and are thus called stand-replacing fires (Johnson 1992). Unburned, residual stands can account for 1%–25% of area burned (Delong and Tanner 1996; Madoui et al. 2010; Dragotescu and Kneeshaw 2012). Large fires (>200 ha) represent only 3% of total fires but contribute 97% of the area burned in Canada (Stocks et al. 2002). In Russia, about 80% of the burned areas are from surface fires, except during severe fire seasons where crown fires may represent more than 50% of all 207 fires (Conard et al. 2002; Soja et al. 2004). In the North American closed-crown boreal forest dominated by black spruce, the fire return interval (FRI) can vary from 100 to 500 years, whereas lichen woodland experiences lower FRIs (70–110 years) (Payette 1992). Most of the variation in area burned in the Canadian boreal forest is explained by the blockage of dry air masses in the troposphere which promotes dry conditions and thus large forest fires (Skinner et al. 1999). At the regional scale, dry soils and coarser surface deposits have been associated with shorter fire cycles (Harden et al. 2003; Mansuy et al. 2010). In Russia, the FRI was estimated at 90–130 years in larch (Larix sibirica), fir (Abies sibirica), and spruce (Picea obavata) forests, and 25–50 years in pine (Pinus sylvestris) forests (Conard and Ivanova 1997). Just after snowmelt, the soil is still moist, promoting low-severity fires; whereas the soil progressively dries until summer when severe fires occur (Turetsky et al. 2011). Peat fires can also occur (van Bellen 2011) and are mostly limited to the end of the fire season when peat is drier. Peat fires in western Canada account for 10%–15% of all fires (Turetsky et al. 2004). In boreal forests, post-fire tree regeneration is mainly controlled by species composition prior to disturbance (seed availability), competition (shade tolerance), surface deposit, as well as fire severity and frequency (Johnstone and Kasischke 2005; Johnstone and Chapin 2006a; Greene et al. 2007). An increase in fire frequency creates a younger landscape and promotes the installation of early successional, shade-intolerant broadleaved species (e.g., Populus tremuloides and Betula papyrifera) and fire-adapted pioneer species (e.g., Picea mariana and Pinus contorta var. latifolia) (Johnstone and Chapin 2006a). In the spruce–moss forests of eastern Canada, increased fire frequency over the past 50 years caused a shift from dense forests to open woodlands (Girard et al. 2008). Fire severity affects vegetation dynamics by altering the thickness of the soil organic layer (Johnstone and Chapin 2006b). High seedling establishment has been reported on organic soils less than 2.5 cm thick (Johnstone and Chapin 2006b). However, a negative correlation between fire severity and post-fire recruitment was found in boreal larch forests of northeastern China (Cai et al. 2013) and a black spruce forest of Alaska (Johnstone and Kasischke 2005). After high-severity fires, pioneer broadleaved species can rapidly recolonize the sites (Greene and Johnson 1999; Johnstone and Kasischke 2005). Response to fire severity also depends on habitat type, with neutral and negative responses in lichen woodlands and moist conifer forests, respectively (Johnstone and Chapin 2006b). Emissions from wildfires Combustion proceeds through a sequence of stages (ignition; flaming, glowing, and pyrolysis; glowing and pyrolysis (smoldering); glowing; and extinction) (Lobert and Warnatz 1993). Combustion through flaming mostly produces CO2, NO, and N2O, while smoldering mostly produces CO and CH4 (Lobert and Warnatz 1993). Carbon emissions Estimates of total carbon released from fire (Ct) can be obtained from a widely used equation requiring quantification of area burned (A), fuel loading (biomass per unit area), and proportion of biomass consumed (Seiler and Crutzen 1980; French et al. 2000): (1) Ct ⫽ A[(Cg × g) ⫹ (Ba × fc × a)] where Ba is aboveground biomass; fc is the fraction of carbon in aboveground biomass (percent); Cg is density of the carbon in ground-layer components; and a and g are, respectively, the aboveground and ground-layer carbon fraction consumed during a fire. Published by NRC Research Press 208 Environ. Rev. Vol. 22, 2014 Table 1. Estimated annual carbon emissions by fires in different boreal regions. Region Period Emission (Tg C/year) Burned area (106 ha/year) Northern Hemisphere Boreal area 1977−1990 101 9.4 1998 1995−2003 1980−1994 1997−2004 1997−2006 1997−2009 1997−2006 1997−2004 1997−2010 Based on FRI 1998 1998−2002 1998−2010 1996−2002 1996−2002 1959−1999 1990−2008 1950−1999 1996−2002 290−383 106−209 53 44 51 54 255 133 128 194 135−190 116−520 92 208.8−215.7 32.2−32.9 27 23 4.5 13.7−13.9 17.9 3−23.6 2.6 2.3 2.57 Environ. Rev. Downloaded from www.nrcresearchpress.com by Université du Québec à Montréal on 08/28/14 For personal use only. BONA BOAS Russia Siberia Eurasia Canada Alaska 7.15 9.7 12 13.3 6.88−11.17 5.35 5.383 3.9 2 0.66 0.22 0.289 Note Reference Auclair and Carter 1993 Fire severity range, including peat fires Fire severity range Weighting by fire severity CASA model TEM model CASA model TEM model CASA model CASA model Adjusted from Dixon and Krankina (1993) Fire severity range Fire severity range ILIS TEM; with or without CO2 fertilization TEM; with or without CO2 fertilization Different fuel types Inventory in managed forest GIS-based model TEM; with or without CO2 fertilization Kasischke and Bruhwiler 2003 Kasischke et al. 2005 French et al. 2000 van der Werf et al. 2006 Hayes et al. 2011 van der Werf et al. 2010 Hayes et al. 2011 van der Werf et al. 2006 van der Werf et al. 2010 Conard and Ivanova 1997 Conard et al. 2002 Soja et al. 2004 Shvidenko et al. 2011 Balshi et al. 2007 Balshi et al. 2007 Amiro et al. 2001b Stinson et al. 2011 French et al. 2002 Balshi et al. 2007 Note: BONA, boreal North America; BOAS, boreal Asia; FRI, fire return interval; CASA, Carnegie–Ames–Stanford Approach; TEM, terrestrial ecosystem model; ILIS, integrated land information system. Variations in estimates of total carbon released among different boreal regions (Table 1) are probably due to (1) the availability of reliable data on area burned, especially in Russia where data are scarce; (2) spatial variability of fire severity; and (3) inclusion of fire severity in the models (% combustion of plant biomass and soil organic matter). Most studies, however, reported similar values, except Balshi et al. (2007) who reported greater carbon emissions in Alaska. This can be explained by their use of the carbon consumption of soil organic matter estimates from French et al. (2000) in their study, which reported higher consumption for the Alaska sites. Table 2. Emission factors (g/kg) for different fire types in Canada. Gaseous emissions To precisely characterize fire emissions, the emission factor (EF) is frequently used. It is reported as the amount (in grams) of gas emitted by kilogram dry matter burned (Andreae and Merlet 2001). Forest fires in boreal regions represent 18% of greenhouse gas emissions in North America and Northern Asia (Amiro et al. 2001a), with 2.5 times as much emissions from Asia than from North America (van der Werf et al. 2010). About 99% of the carbon released from fires is composed of CO2, CO, and CH4 (Cofer et al. 1998; Simpson et al. 2011). The CO2 EF from boreal forest fires varies between 1210 and 1616 g/kg in Canada (Cofer et al. 1998; Simpson et al. 2011), considerably more important than the EFs of CO and CH4 (Table 2). CO, CH4, and NOx (NO and NO2) can influence atmosphere chemistry, including a contribution to the formation of tropospheric ozone (O3), an important greenhouse gas (Crutzen and Carmichael 1993; Sudo and Akimoto 2007). Emissions from forest fires contributed 13.8% and 12.4% of global fire emissions of CO and CH4 in 1998, respectively (Kasischke and Bruhwiler 2003). Variations in gas emissions by forest fires explained 85% of the variation in atmospheric CO concentration in the northern hemisphere (excluding tropical forests) between 1995 and 2003 (Kasischke et al. 2005). effects on climate depend on many parameters such as optical depth, particle size, and single scattering albedo (SSA) (Hansen et al. 1980; Reid et al. 2005; Myhre et al. 2013). Particles emitted by forest fires contain more than 50% carbon, including both organic carbon (OC) and elemental carbon (EC) (Lobert and Warnatz 1993). Elemental carbon (also known as black carbon) is the main lightabsorbing constituent in aerosols and has the second largest radiative power at the global scale, with 0.2– 1.1 W/m2 (Jacobson 2001; Bond et al. 2013). Its elimination could reduce global average temperature by 0.5–1.0 °C (Ramanathan and Carmichael 2008). However, many factors can contribute to uncertainties in the estimates of the direct forcing by aerosols, including a lack in EC emission estimates (Bond et al. 2013). Indeed, EC emissions varied from 0.016 Tg EC/year in 1960–1969 to 0.041 Tg EC/year in 1990– 1997 (Lavoué et al. 2000) and to 0.12 Tg EC/year in 1997–2000 for Canadian fires (Park et al. 2003). Organic carbon scatters solar radiation and cools the climate, except for some OC components which are sunlight absorbers (Forster et al. 2007; Myhre et al. 2013). Emission factor values of OC range between 8.6 and 9.7 g/kg, greater than for EC with 0.56 ± 0.19 g/kg. Organic carbon is the largest component of extratropical forest biomass burning aerosols (Andreae and Merlet 2001). Aerosols also have indirect or semi-direct effects on climate. Large clouds of smoke emitted by fires reduce solar radiation that reaches the surface (Randerson et al. 2006; Ramanathan and Carmichael 2008). In addition, deposition of light-absorbing particles on snow and ice reduces the albedo and may accelerate warming. Flanner et al. (2007) estimated the radiative forcing of EC from boreal fires and pollution deposited on snow in 1998 Aerosol emissions During a forest fire, aerosols (also called particulate emissions) play an important role in climate regulation at the regional scale due to their short half-life (hours to weeks). Direct radiative forcing by fire-emitted aerosols was equivalent to 17 ± 30 W/m2 during the first year after a boreal fire (Randerson et al. 2006). Aerosol CO2 CO CH4 NOx Mixed combustiona Flaming combustionb Smoldering combustionb 1616±180 113±72 4.7±2.9 0.97±0.12 1530±80 100±10 2.4±0.6 n/a 1210±100 184±15 7.8±2 n/a Note: n/a, not available. aSmoke and flame (Simpson et al. 2011). bCofer et al. 1998. Published by NRC Research Press Oris et al. 209 Region Respiration increase? Alaska Alaska Alaska Alaska Alaska Finland Siberia Yes Yes Yes Yes Yes No Yes Note Reference Severe fires in black spruce forest; net loss of 20 t C/ha 15–20 years after a fire Rh measured only in the first year, release of 202 g C/m2/year in black spruce forest Release of 1.8−11.0 Mg C/ha, 7–15 years after fire in black spruce forest Release of 100 g C/m2/year in a young black spruce population Release of 14.7 t C/ha in black spruce stands over a decade after fire Seven-fold decrease during 3 years in a Scots pine forest Increase in Rh after fire in a larch forest correlated with increase in temperature and organic carbon content Kasischke et al. 2000b Randerson et al. 2006 O'Neill et al. 2003 Bond-Lamberty et al. 2004 Kim and Tanaka 2003 Fritze et al. 1994 Sawamoto et al. 2000 and 2001. Isolating the contribution of boreal fires (12% in 2001 and 20% in 1998), the radiative forcing ranged from −0.0055 W/m2 (in 2001) to −0.01001 W/m2 (in 1998). This forcing is low, but its effectiveness is greater than that of direct absorption by EC because of snow melting. Aerosols can also act as cloud condensation nuclei (CCN), form small cloud drops, and thus cool the climate by increasing cloud albedo (Chuang et al. 2002; Lohmann and Feichter 2005). This indirect effect is called the cloud albedo effect. A cooling of 0.34 W/m2 at the global scale has been attributed to EC emissions from fires and pollution causing the formation of CCN (Spracklen et al. 2011). Moreover, smaller drops need longer to reach sizes at which they can fall as rain, thus increasing cloud cover and resulting in additional cooling. This second indirect effect is termed cloud lifetime effect (Albrecht 1989). Elemental carbon can also decrease the relative humidity of the cloud layer by a semi-indirect effect, leading to evaporation of cloud drops, decreased cloud cover and albedo and thus climate warming (Hansen et al. 1997; Ramanathan and Carmichael 2008). However, this effect depends on several factors, including cloud type and the altitude of the EC relative to the cloud (Koch and Del Genio 2010). The single scattering albedo (SSA) is defined as the ratio of radiation scattering to the sum of scattering and absorption coefficients. If SSA is less than 0.85, aerosols are responsible for a positive (heating) radiative forcing. If it is greater than 0.95, the forcing is negative (cooling). Between 0.85 and 0.95, the effect can be positive or negative depending on cloud cover and surface albedo (Ramanathan et al. 2001). For boreal and temperate forest fires, a synthesis of in situ measurements of optical properties of fresh smoke particles placed this ratio between 0.6 and 0.97 (Reid et al. 2005). It varied according to fuel type, fire intensity, flame height, and combustion phase (Radke et al. 1991; Reid et al. 2005). Post-fire ecosystem functioning Carbon fluxes Carbon dioxide fluxes from vegetation regeneration (photosynthesis and autotrophic respiration (Ra)) and soil (heterotrophic respiration (Rh)) are key elements in the post-fire restoration of the boreal forest carbon pool (Chapin et al. 2006). Tree mortality causes a decrease in autotrophic respiration (Ra) (Kasischke et al. 2000b; Sawamoto et al. 2000; Zhuang et al. 2002; Kim and Tanaka 2003; Bergner et al. 2004). At the microbial scale, however, the pattern is unclear (Amiro et al. 2003). Higher soil temperatures following fire increase microbial soil respiration (Rh), while the removal of micro-organisms in severe fires could cause a decrease in respiration. However, most studies find an increase in Rh after fire (Table 3; Fig. 1), which is included in process-based climate models (Auclair and Carter 1993; Dixon and Krankina 1993). These indirect biogenic carbon losses could be greater than the direct losses from combustion during fire (Auclair and Carter 1993; Dixon and Krankina 1993; Kasischke et al. 1995). Greenhouse gases (CO2 and CH4) emitted in the first year after a fire were responsible for a 6 and a 8.3 ± 3 W/m2 positive radiative forcing in Manitoba and Alaska, respectively (Randerson et al. 2006). Hence, boreal Fig. 1. Conceptual net ecosystem production (NEP), net primary production (NPP), and heterotrophic respiration (Rh) dynamics after a fire in the boreal forest. Carbon fluxes (g C/m2 /year ) Environ. Rev. Downloaded from www.nrcresearchpress.com by Université du Québec à Montréal on 08/28/14 For personal use only. Table 3. Heterotrophic respiration after fire in boreal forests. Sink NPP NEP Rh Source Time since last fire Years forest ecosystems remain carbon sources for several years after fire because of increased soil respiration, decomposition of dead wood, and absence of vegetative cover (Fig. 1). Usually, net primary productivity (NPP = photosynthesis – Ra) first increases and culminates to high values at an early age, before later declining (Gower et al. 1996; Chen et al. 2002; Goulden et al. 2011; Alexander et al. 2012). However, old-growth forests are not necessarily a carbon source and have great sequestration potential (Luyssaert et al. 2008; Wei et al. 2013). Net ecosystem productivity (NEP = NPP – Rh) returns to its pre-fire level after 10–40 years (Amiro 2001; Chen et al. 2002; Amiro et al. 2003; Bond-Lamberty et al. 2004; Goulden et al. 2011). However, even after NEP has reached its pre-fire level, recovery of the initial carbon stock can take up to 235 years (Kashian et al. 2006). Long-term carbon accumulation varies between forest types and soil drainage. A modeling exercise by Harden et al. (2000) showed that drier forests generally had greater carbon losses to fire than wetter forests. In boreal regions characterized by permafrost, increased soil temperature following fire may lead to permafrost degradation (Brown et al. 1983; Burn 1998; Burn et al. 2009). After the 1968 Inuvik fire in the Northwest Territories, a 50 cm increase of the thickness of the active layer (surface layer of permafrost that thaws in the summer) occurred over an 8-year period (Brown et al. 1983; Mackay 1995). This effect likely depends on the residual depth of the organic layer. Residual depths >7–12 cm should protect permafrost from melting (Yoshikawa et al. 2003). Permafrost melting can restore the soil respiration process, with major implications for the carbon cycle. A recent modeling study estimated that future fires will lead to a 1.1 m increase of the thickness of the active layer by 2100 in Alaska (Genet et al. 2013). With warming Published by NRC Research Press Environ. Rev. Downloaded from www.nrcresearchpress.com by Université du Québec à Montréal on 08/28/14 For personal use only. 210 added to changes in fire regime, they projected a 9.8 kg C/m2 decrease of the soil carbon stock by 2100 (Genet et al. 2013). Solid elemental carbon, more commonly known as charcoal, is formed during the combustion of plant material. It represents a potential carbon sink because it is resistant to decomposition (de Lafontaine and Asselin 2011). Fire temperature, burn time, and type and size of fuels influence charcoal production (Czimczik et al. 2005; Fréjaville et al. 2013). Between 0.7% and 2.0% of the initial organic matter can be converted to charcoal during boreal forest fires (Hart and Luckai 2013). An estimated 8%–10% of soil carbon is estimated to be under the form of charcoal (Hart and Luckai 2013), up to 20%–24% in some Siberian soils (Schulze et al. 1999). Albedo Albedo is the fraction of solar radiation reflected back into space. It plays an important role in climate regulation: the higher the albedo, the cooler the climate (Eugster et al. 2000). Boreal forests have a dark canopy (with a low albedo) that obscures the snow-covered ground in winter. The albedo is higher for deciduous forests compared with coniferous forests (Betts and Ball 1997; Lukeš et al. 2013). By removing vegetation cover, fire acts on albedo in two different ways. Albedo increases in winter and decreases in summer, respectively cooling and warming the climate. As the difference in albedo before and after fire is greater in winter than in summer, the net effect is that albedo increases in the early years after a fire (5–55 years; Amiro et al. 2006; Randerson et al. 2006; O’Halloran et al. 2012), and it decreases later on due to the afforestation process (Liu et al. 2005). However, if post-fire succession is dominated by broad-leaved species, an increase in albedo will be observed in contrast to the pre-fire coniferous stand (Liu and Randerson 2008). Evapotranspiration Evapotranspitation is the heat flux from the Earth's surface to the atmosphere that is associated with evaporation or transpiration of water at the surface and subsequent condensation of water vapor in the troposphere. This latent heat flux has a subsequent impact on climate by affecting cloud cover and precipitation, and thus the radiative energy balance. Higher evapotranspiration causes a cooling of the regional climate (Eugster et al. 2000; Liu et al. 2005). In Alaska, the loss of moss that helds soil moisture and an increase in soil temperature have explained a 33% decrease in annual evapotranspiration 3 years after fire (Liu et al. 2005). Thereafter, if post-fire succession is dominated by broad-leaved species, an increase in evapotranspiration will be observed (Liu and Randerson 2008). Environ. Rev. Vol. 22, 2014 However, vegetation recovery may take longer when very severe fires occur on thin organic soils (Asselin et al. 2006). Even if the annual rate of storage was greater after a high intensity fire, a theoretical study estimated that the carbon stock returns to its pre-fire value more rapidly if fire severity is low (Thornley and Cannell 2004). North American boreal forests are characterized by infrequent severe fires and FRI long enough to allow the forest to return to a mature state before the next fire (Payette 1992). In Russia, fires are less severe but more frequent (Conard and Ivanova 1997; Wooster and Zhang 2004). For a comparable area, Russian fires may burn less intensely and produce fewer emissions than those in North America (Wooster and Zhang 2004). Satellite images (MODIS) from Alaska and central western Canada showed that albedo was higher in severely burned areas due to a higher proportion of deciduous regeneration (Beck et al. 2011; Jin et al. 2012). However, in areas where jack pine colonizes burned stands, the post-fire albedo is lower than that in the prefire stand (Goetz et al. 2007). Increased fire severity could offset the negative albedo feedback by releasing more carbon (Turetsky et al. 2011). The shift from spruce–moss forest to lichen woodlands due to compound disturbances in the eastern Canadian boreal forest (Jasinski and Payette 2005; Girard et al. 2008) causes an increase in albedo that offsets the carbon loss due to reduced tree density and thus causes a net radiative forcing of −0.094 mW/m2 (Bernier et al. 2011). Impact of fire regime on climate at broader scales As the number and size of fires vary from year to year, carbon losses (or gains) also change each year (van der Werf et al. 2006). For example, emissions from forest fires in Siberia in 1998 accounted for 14%–20% of global emissions by fire (Conard et al. 2002), whereas in “normal” years they represent only 6.4% of total emissions (van der Werf et al. 2010). Other studies suggest that the strength of the carbon sink in the North American boreal forest has declined in recent decades (Kurz and Apps 1999; Balshi et al. 2007; Hayes et al. 2011) due to an increase in forest fires. A decrease in the time interval between fires across the boreal forest could lead to a decrease in carbon stock and even a shift from the boreal forest being a carbon sink to it being a source. The fire regime threshold beyond which the boreal forest becomes a carbon source depends on forest productivity and is thus difficult to estimate. Significant increase in NEP would be required for a 100% increase in area burned predicted in Canada (Flannigan et al. 2005; Kurz et al. 2008; Metsaranta et al. 2010). Impact of fire regime variations on climate Integrative approach of fire effects on climate dynamics Impact of fire regime on climate at the stand scale A decrease in the time interval between fires results in a decrease of the carbon stock (Kasischke et al. 1995; Wirth et al. 2002; O'Neill et al. 2003; Thornley and Cannell 2004). In addition, younger landscapes have been associated with lower carbon stocks as biomass is lower in young populations than in older ones (Harmon et al. 1990; Pregitzer and Euskirchen 2004; Wang et al. 2011). However, as the fire return interval (FRI) decreases, less carbon is released per fire event because the ecosystem does not have the time to accumulate carbon (Wirth et al. 2002; Zhuang et al. 2006). Thus, if fire frequency increases, forests might continue to store carbon but will inevitably be a weaker carbon sink. This hypothesis was studied by Brown and Johnstone (2011) who measured the consumption of organic soil under different FRIs (long: 94 years; short: 15 years) in Yukon. They found that the carbon stock of the forest under short FRI represented only 60% of the post-fire carbon found under long FRI. Vegetation recovery is usually denser and quicker after severe fires (Lecomte et al. 2006; Simard et al. 2007; Shenoy et al. 2011). Only a few studies have estimated the combined boreal forest fire effects on climate (Table 4). Most studies focused on the stand scale, and data are scarcer at the boreal forest biome scale (Table 4). The negative radiative forcing of post-fire changes in albedo (−4.2 W/m2; Randerson et al. 2006) was estimated to be greater than the positive radiative forcing of changes in NEP (+1.6 W/m2; Randerson et al. 2006) in Alaska over 80 years. In Manitoba, the average net radiative forcing over 100 years was estimated at −0.9 ± 2.4 W/m2 (O’Halloran et al. 2012). However, integrated post-fire changes in albedo and NEP have caused a short-term positive radiative forcing (Randerson et al. 2006; O’Halloran et al. 2012), but the trend rapidly (<5 years) reversed in Alaska in contrast to Manitoba ca. after 50 years (Table 4). In addition to post-fire changes in albedo and NEP, Randerson et al. (2006) estimated that the effect of aerosols released from fires and the cumulative fire effect caused a positive radiative forcing during more than 10 years. Hence, combined boreal fire effects seem responsible for climate warming shortly after fire, followed by climate cooling 80–100 years after fire (Table 4). Published by NRC Research Press Region Studied effects Carbon stock changes − boreal forest scale Canada Net ecosystem carbon fluxes Period Results Feedback Temporal scale Reference 1920−1989 Increase in carbon release and decrease in carbon stock since 1980 Decrease in carbon stock with an increase in fire frequency and release of an average of about 0.1 Gt C/year Loss of 10%−30% in NPP after fire Positive 9 years Kurz and Apps 1999 Positive 500 years Thornley and Cannell 2004 Positive 6500 years Harden et al. 2000 Saskatchewan Fire frequency on NPP and carbon storage Fire return interval of 100 years Boreal forest of North America Boreal forest Fire-free interval Last 6500 years NEP 1997−2006 73% decrease insink strength compared to 20th century Positive 9 years Hayes et al. 2011 Radiative budget − stand scale Alaska Post-fire change in albedo 1950−2004 Negative 50 years following fire Lyons et al. 2008 Alaska, Saskatchewan and Manitoba 150-year forest chronosequence Surface radiative forcing of −6.2 W/m2 compared with mature coniferous forest Decrease of 12−24 W/m2 of net surface radiation in summer Negative 1−3 years following fire Amiro et al. 2006 Negative 15 years following fire Liu et al. 2005 Increase in albedo due to shift Negative from black spruce–moss forest to spruce–lichen woodland responsible for a radiative forcing of −0.094 mW/m2 Greater deciduous biomass in Negative more severely burned area; winter and summer albedo were higher in high-severity burns Increases in spring albedo above Negative pre-fire levels in all burn severity classes, and higher for high severities After successive fires Bernier et al. 2011 10−50 years following fires Beck et al. 2011 5−7 years following fire Jin et al. 2012 80-year fire cycle Randerson et al. 2006 100-year fire cycle O’Halloran et al. 2012 Alaska Radiative budget, post-fire change in albedo and evapotranspiration Radiative budget 1-year forest chronosequence in three sites 80, 15, and 3 years following a fire 2000−2008 Boreal forest of eastern North America Radiative budget/post-fire change in albedo Alaska Post-fire change in albedo along gradients of burn severity 1950−1991, 60-year forest chronosequence Central western Canada Post-fire change in albedo along gradients of burn severity 2000−2009 Combined effects − stand scale Boreal forest -Alaska GHG, aerosols, albedo, and black carbon deposition on snow Boreal forest CO2 and post-fire change in Manitoba albedo Decrease of 17 W/m2 in annual net surface radiation Calibrated on a 1999 fire in Alaska During the first year: 34 ± 31 W/m2 Over an 80-year fire cycle: −2.3 W/m2 150-year forest chronosequence First 50 years: 0.59 W/m2 Last 50 years: −2.4 W/m2 Over 100 years: −0.9 W/m2 Positive the first year/ negative over 80 years Positive the first 50 years/negative over 100 years Note: A positive feedback refers to a warming, whereas a negative feedback refers to a cooling. NPP, net primary production; NEP, net ecosystem productivity. 211 Published by NRC Research Press Environ. Rev. Downloaded from www.nrcresearchpress.com by Université du Québec à Montréal on 08/28/14 For personal use only. Oris et al. Table 4. Estimations of fire effects on climate at different scales in boreal forests. 212 Environ. Rev. Vol. 22, 2014 Environ. Rev. Downloaded from www.nrcresearchpress.com by Université du Québec à Montréal on 08/28/14 For personal use only. Table 5. Summary of future fire activity in North America and Russia. A1B and A2 are IPCC scenarios with the most intense climate forcing, whereas B1 and B2 are scenarios with intermediate climate forcing (IPCC 2007). Region Period or scenario Result Reference Canada 1959−97 vs. 2080−99 (3×CO2) Flannigan et al. 2005 Canada 2×CO2; 3×CO2 Alaska Western North America 1922−96 vs. 2025−99 1991−2000 vs. 2091−2100 (range of two IPCC scenarios: A2 and B2) 2001−2007 vs. 2091−2100 (range of scenarios A2, A1B, and B1) 1980−1989 vs. 2×CO2 74%–118% increase in area burned (initial value of 1779 Mha) 34% increase in area burned and 3% increase in soil consumption (kg/m2) by fire; 93% increase in area burned and 6% increase in soil consumption (kg/m2) by fire 14%–34% increase in area burned 3.5- to 5-fold increase in area burned Western Canada Russia Russia Central Russia Quebec Eastern Canada Quebec 1981−2000 vs. 2100 2001−2007 vs. 2091−2100 (range of scenarios A2, A1B, and B1) 2×CO2; 3×CO2 1959−99 vs. 2046−65 and 2081−2100 (range of three IPCC scenarios: A2, A1B, and B1) 1971−2000 vs. 2071−2000 (IPCC A2 without or with unlimited tree dispersion) 54%–69% of all fires as crown fires (initial value of 57%) 2- to 3-fold increase in high fire risk area in June and July 2.5-fold increase in high fire risk area 6%–22% of all fires as crown fires (initial value of 6%) 0.2389% increase in annual burn rate; 0.4441% increase in annual burn rate (initial value of 0.1725%/year) 0.29−0.40 increase in annual burn rate; 0.35−0.45 increase in annual burn rate (initial value of 0.22%) 10%−25% increase in fire occurrence; under unlimited dispersal scenarios, increases in fire occurrence are predicted to be less important in the boreal forest, and trends could even be reversed In both studies, the fire was severe and induced a more productive 40-year deciduous succession phase with an albedo higher than the pre-fire black spruce forest (Randerson et al. 2006; O’Halloran et al. 2012). In addition, calculations of radiative forcing emissions were based on the release of about 1.5–1.7 kg C/m2 during fire. This is close to Canada's 1959–1999 average of 1.3 kg C/m2 (Amiro et al. 2001a), but far from the average release estimates of 2.95 ± 0.12 and 6.15 ± 0.41 kg C/m2 between 1983 and 2005 in early-season and late-season Alaskan fires, respectively (Turetsky et al. 2011). Although albedo is higher after more severe fires (Beck et al. 2011; Jin et al. 2012), the question is to know if there is a fire severity threshold beyond which there would be no change in albedo but still an increase in carbon emissions. Future feedbacks between fire and climate Climate change effects on future boreal forest fire dynamics Global circulation models (GCMs) predict increased temperatures in boreal ecosystems. In Canada, the greatest increases in temperature are predicted for the north and central regions during the period 2041–2070 (Price et al. 2001). Winter minima will be the most impacted with increases of 6 °C. A 5% increase in mean annual precipitation is predicted across the country, with considerable regional variation and greater increases in summer (IPCC 2007). In Quebec, winter temperatures are expected to increase between 5 and 9 °C and summer temperatures between 2 and 3.5 °C by the end of the 21st century (Logan et al. 2011). In Alaska, a 4.5–10 °C warming is expected for 2100, along with a 21% increase in precipitation (Christensen et al. 2007). In Russia, temperature and precipitations are projected to increase by 4.3 °C and 15%, respectively (Christensen et al. 2007). Many studies have evaluated the effect of climate change on fires, especially in North America (Bachelet et al. 2005; Flannigan et al. 2005; Bergeron et al. 2006, 2010; Balshi et al. 2008; Amiro Amiro et al. 2009 Bachelet et al. 2005 Balshi et al. 2009 de Groot et al. 2013b Stocks et al. 1998 Malevsky-Malevich et al. 2008 de Groot et al. 2013b Bergeron et al. 2006 Bergeron et al. 2010 Terrier et al. 2013 et al. 2009; de Groot et al. 2013b), and to a lesser extent in Russia (Stocks et al. 1998; Malevsky-Malevich et al. 2008; de Groot et al. 2013b) (Table 5). These studies suggest that the increase in precipitation will not be sufficient to counteract the increase in temperature, thus resulting in increased fire incidence in the boreal forest. This increase is often estimated in terms of area burned, with a range of increase across the boreal forest from 14% to 450% (Table 5). Effect of increased fire frequency on climate In Quebec, carbon release estimates 3000 years before present were 80% higher than current emission levels (Bremond et al. 2010). It is thus likely that past boreal wildfires made higher contributions to the global carbon stock than current ones. With an increased fire activity due to climate change, boreal wildfires could again make major contributions to the global carbon stock. Studies in Canada (Kasischke et al. 1995; Zhuang et al. 2006; Amiro et al. 2009; Balshi et al. 2009), Alaska (Kasischke et al. 1995; Bachelet et al. 2005; Zhuang et al. 2006; Balshi et al. 2009), and Russia (Kasischke et al. 1995; Zhuang et al. 2006) have so far mostly focused on future carbon emissions compared with other effects of fire on climate (Table 6). An increase of 24%–450% in carbon emissions was predicted depending on regions and studies. A compilation of data available from the literature allowed us to calculate the regional radiative forcing of albedo and carbon stock changes due to fire for the present and future (end of 21st century) in North America and in the whole boreal region (Fig. 2). Currently, the radiative forcing due to fires is an average of 0.04 ± 0.02 (mean ± standard deviation) W/m2/decade in the boreal region. At the end of the 21st century, this effect will be almost ten times greater, with an average of 0.37 ± 0.03 W/m2/decade. Other factors influence carbon stocks, such as climate change and the CO2 fertilization effect on photosynthesis. Currently, adding the effects of climate and fire, carbon stock changes have a cooling effect of Published by NRC Research Press Oris et al. 213 Table 6. Summary of the impacts of future increase in boreal forest fires on the carbon stock, albedo, and aerosols. Region Environ. Rev. Downloaded from www.nrcresearchpress.com by Université du Québec à Montréal on 08/28/14 For personal use only. Carbon stock changes North America Studied effect Result Reference Theoretical productivity and respiration with different growth rates Boreal forest could be a carbon sink in the future if decomposition rate does not change 313 Tg of CO2 equivalent released per year in the horizon 2100 compared to the current release of 162 Tg of CO2 equivalent per year 473 Tg C released per year in 2100 compared to the current release of 276 Tg C/year Loss of 27.1– 51.9 Pg carbon if there is a 50% increase in area burned Loss of 17−19 Tg C/year in 2025−2099 Kurz et al. 2008 Canada Carbon release by fires Northern high latitude ecosystems Carbon emissions considering effects of permafrost, CO2 fertilization, and fire Boreal forest Carbon dynamics depending on area burned Carbon dynamics with climate, fire, and ecosystem interactions Carbon stock changes depending on two scenarios of area burned Alaska Managed forest in Canada North America Radiative budget Alaska Carbon emissions and carbon stock changes Canadian forest will be a carbon sink with a probability of 70% and 35% if there is no change in area burned and if there is an increase by a factor of 2–4 by 2100, respectively 2.5- to 4.4-fold increase in carbon emissions by 2091−2100; loss of 18−25 Tg C/year in 2025−2099 in Alaska North American boreal forest Changes in albedo under future fire regimes (100%−200% increase in area burned) Radiative budget changes due to forest fires Changes in forest age distributions increase summer albedo responsible for a cooling of −0.9 W/m2/decade A doubling of area burned would cool the surface by 0.23 ± 0.09 °C during winter and spring months (0.43 ± 0.12 °C for a quadrupling of area burned) Aerosols Canada Black carbon emissions Emissions of 45 and 65 Gg/year for 2×CO2 and 3×CO2 scenarios respectively, compared to the current value of 34 Gg/ year Minor role in the reduction of continental snow cover area; except in Quebec and eastern Siberia when all the aerosol forcings and atmospheric feedbacks were included by 2050 Radiative forcing of −0.10 to 0.4 W/m2 by 2100 because of changes in future aerosol abundance 90% increase of aerosol emissions by 2050 for a 54% increase in area burned North of 30°N Changes in snow cover due to black carbon emissions Global scale Aerosol emissions Western United States Aerosol emissions 0.055 ± 0.096 W/m2/decade in the boreal region and 0.065 ± 0.062 W/m2/decade in North America. In the future, the effect of climate change on carbon stock might be positive (Balshi et al. 2009) or negative (Kasischke et al. 1995; Zhuang et al. 2006). The difference between these studies is the inclusion (or not) of carbon release from permafrost thawing. The CO2 fertilization effect on photosynthesis always has a positive effect (carbon storage). As a result, only scenarios including the CO2 fertilization effect have the potential to offset carbon losses from fires (and permafrost thawing), with an average cooling of 0.27 ± 0.14 W/m2/ decade in the future. The average of the other scenarios (without CO2 fertilization) corresponds to a warming of 0.21 ± 0.04 W/m2/decade. A higher proportion of deciduous forests in the landscape after fire will change the surface albedo and cause climate cooling (Table 6). In Alaska, this radiative forcing was estimated to −0.09 W/m2/decade in 2003–2100 (Euskirchen et al. 2009). In North America, changes in albedo and evapotranspiration due to a dou- Amiro et al. 2009 Zhuang et al. 2006 Kasischke et al. 1995 Bachelet et al. 2005 Metsaranta et al. 2010 Balshi et al. 2009 Euskirchen et al. 2009 Rogers et al. 2013 Amiro et al. 2009 Ménégoz et al. 2013 Carslaw et al. 2010 Spracklen et al. 2009 bling of the area burned would cool the surface by 0.23 ± 0.09 °C during winter and spring months (Rogers et al. 2013). The effect of forest fires on carbon stock will be higher than that of albedo (Fig. 2). Their sum will be a warming between 0.19 and 0.41 W/m2/ decade. Adding the climate and CO2 fertilization effects on carbon stock change scenarios, future radiative forcing will range between −0.72 and 0.21 W/m2/decade (Fig. 2). Uncertainty around the future increase in area burned leads to important variations in estimates of radiative forcing by aerosols (Fig. 3). On average, the direct effect of fire-produced aerosols will exert a positive radiative forcing of 0.18 W/m2 by 2100 in the boreal region. The effect will be smaller in Canada and Russia compared with that in Alaska (Fig. 3). Discussion The fire-induced CO2 radiative forcing is estimated between 0.34 and 0.50 W/m2/decade at the end of the 21st century (Fig. 2). Published by NRC Research Press 214 Environ. Rev. Vol. 22, 2014 Fig. 2. Summary of radiative forcing (W/m2/decade) due to changes in carbon uptake and (or) emission and land surface albedo in boreal forests. Solid symbols represent positive radiative forcing and open symbols represent negative radiative forcing. Carbon releases were converted in W/m2 as in Zhuang et al. (2006). Where appropriate, mean and standard errors were calculated. HAE, high anthropogenic CO2 emissions scenario; MAE, medium anthropogenic CO2 emissions scenario; *, with CO2 fertilization effect. 2 References Radiative forcing (W/m /decade) Environ. Rev. Downloaded from www.nrcresearchpress.com by Université du Québec à Montréal on 08/28/14 For personal use only. -1 -0.5 0 0.5 Balshi et al. (2007) Boreal region* Hayes et al. (2011) Boreal region* Metsanrata et al. (2010) Canada Balshi et al. (2009) Scenario A2* Balshi et al. (2009) Scenario B2* Balshi et al. (2009) Scenario A2 Balshi et al. (2009) Scenario B2 Balshi et al. (2007) Boreal region* Hayes et al. (2011) Boreal region* Balshi et al. (2007) Boreal region Zhuang et al. (2006) Boreal region Zhuang et al. (2006) Scenario HAE* Zhuang et al. (2006) Scenario MAE* Zhuang et al. (2006) Scenario HAE Zhuang et al. (2006) Scenario MAE Kasischke et al. (1995) Boreal region Hayes et al. (2011) North America* Balshi et al. (2007) North America* Balshi et al. (2007) North America Balshi et al. (2009) Scenario B2* Balshi et al. (2009) Scenario A2* Balshi et al. (2009) Scenario A2 Balshi et al. (2009) Scenario B2 Euskirschen et al. (2009) Albedo Present Present Future Future Canada Boreal region average 0.05 0.10 0.15 0.20 Fire effect Combined effect of fire and climate, with (*) or without CO2 fertilization (boreal region) Future Russia 0.00 2 Future Alaska -0.05 1.5 Present Fig. 3. Summary of predicted radiative forcing (W/m2) due to aerosols emitted by fires between today and 2100 in Alaska, Canada, Russia, and the boreal region as a whole. Values do not represent the radiative effect of the total burden but refer to the change in the abundance of aerosols due to fire between today and 2100. To calculate the radiative forcing, the hypothesis that changes in aerosol emissions depend on changes in area burned was used. The direct effect of aerosols was estimated using the method of Carslaw et al. (2010). Future changes in aerosol concentration were calculated using the ratio of the future change in emissions (90%) to the future change in surface organic carbon concentration due to fires (30% according to Spracklen et al. 2009). Changes in future aerosol concentration were then multiplied by the current direct radiative forcing of aerosols estimated by Forster et al. (2007) (from –0.05 to 0.20 W/m2). -0.10 1 0.25 0.30 0.35 0.40 Radiative forcing by aerosols (W/m 2 ) The impact on climate of carbon emissions from fires is primarily measured by changes in carbon stock, which equate to the difference between carbon emissions and carbon storage by forests. As CO2 is needed for photosynthesis, a CO2 fertilization effect could increase the stock of terrestrial carbon captured by plants (Amthor 1995; Long et al. 2004). Accounting for such a CO2 fertilization effect on photosynthesis, carbon storage could offset warming by carbon emissions due to fire (Fig. 2; Zhuang et al. 2006). This negative feedback is uncertain, however, because the response of vegetation to increased atmospheric CO2 concentration varies according to other factors (water and nutrient availability and species-specific responses) and a saturation point can be reached (Cao and Woodward 1998; Canadell et al. 2007). Some studies also suggested that CO2 fertilization might not be enough to offset carbon losses due to disturbances (Kurz et al. 2008; Combined effect of fire and climate, with (*) or without CO2 fertilization (North America) Albedo (Alaska) Metsaranta et al. 2010). Moreover, permafrost thawing in response to climate change will release into the atmosphere carbon that has been trapped in soils for millennia (Schuur et al. 2009). When permafrost thawing is included in the models, CO2 fertilization does not offset carbon losses under low and medium anthropogenic CO2 emission scenarios for the 21st century (Zhuang et al. 2006). Balshi et al. (2009) did not include this effect in their model and, therefore, estimated positive carbon stock changes. Fire severity is an important parameter to consider when calculating carbon emissions (Turetsky et al. 2011). Studies (Kasischke et al. 1995; Bachelet et al. 2005; Zhuang et al. 2006; Kurz et al. 2008; Balshi et al. 2009; Metsaranta et al. 2010) on the carbon stock changes do not take into account changes in fire severity within the fire season, nor the average increase in fire severity due to global warming, estimated at 6% by 2100 in Canada (Amiro et al. 2009). The radiative forcing due to fire-induced emissions of GHGs is, therefore, underestimated. However, increased fire severity could also induce an increase in albedo (Beck et al. 2011; Jin et al. 2012) and, therefore, climate cooling. The direct effect of aerosols is higher than the variation in carbon stock by fire emissions of GHGs and changes in albedo. However, the effect of aerosols is temporary while the others are long-lasting. Aerosols interact with the atmosphere in complex ways, and thus important variations exist in the estimation of radiative forcing, including negative and positive values (between −0.075 and 0.33 W/m2 in 2100; Fig. 3). The effect of elemental carbon deposition on snow and ice remains the only effect of aerosols producing a positive radiative forcing, which would not be offset in the future. This effect is, however, believed to be small compared with the direct and indirect effects, but have greater efficiency by accelerating snowmelt (Flanner et al. 2007). According to a recent simulation study, a future increase in biomass burning will play a minor role in the reduction of continental snow cover through snow darkening if atmospheric feedbacks are not included (Ménégoz et al. 2013). However, in a scenario considering all the aerosol forcings and atmospheric feedbacks, a significant decrease of mean number of days with snow at the surface was simulated in Quebec and eastern Siberia (Ménégoz et al. 2013). If boreal forests do not support the predicted increase in fire frequency and (or) area burned, they might shift to a new, non-forest stable state (Girard et al. 2008). Hence, the predicted increase in forest fires might not be sustained. The Alaskan boreal forest was Published by NRC Research Press Environ. Rev. Downloaded from www.nrcresearchpress.com by Université du Québec à Montréal on 08/28/14 For personal use only. Oris et al. likely not resilient to a climate change that occurred 5500 years ago and has grown from a canopy dominated by white spruce to a dominance by black spruce (Hu et al. 2006). In eastern Canada, the boreal forests gradually changed during the Holocene (e.g., 8000 years) and have generally been resilient to changes in fire regime (Carcaillet et al. 2010). However, a gradual opening of the northern Quebec boreal landscape has resulted from higher fire frequencies during the last 3000 years (Asselin and Payette 2005). In eastern Canada, the increase in burn rate predicted by the end of the 21st century remains within the natural range of variability recorded in the past (Bergeron et al. 2010). The effect of compound disturbances on forest resilience, which may be more common in response to global warming, is also an important issue (Paine et al. 1998; Payette and Delwaide 2003). Compound disturbances (e.g., an insect outbreak closely followed by a fire) have caused regeneration failure which caused a shift from a closed forest to an open woodland ecosystem (Payette et al. 2000; Jasinski and Payette 2005). Such an ecosystem change would likely have an impact on climate through changes in albedo. Uncertainties and limitations The interaction between fire and vegetation is a major limitation in assessing the influence of boreal forest on climate. With increasing temperature, species typical of temperate environments could migrate northward (Burton and Cumming 1995; Terrier et al. 2013). These temperate species would act negatively on fire spread due to their high moisture content and low flammability (Hély et al. 2000). A high proportion of broad-leaved species could have offset the effect of a warmer and drier climate on fire activity during the mid-Holocene in eastern Canadian boreal forests (Girardin et al. 2013). Models used to estimate the area burned or future carbon emissions do not take into account changes in vegetation (e.g., conifer versus broad-leaved). Yet, fire occurrence and the resulting carbon emissions vary depending on fuel type. To better understand the effect of increased fire on climate, it is necessary to develop better prediction models of area burned, taking into account vegetation dynamics. A second challenge is to determine what amount of carbon will be released as a result of soil decomposition and permafrost thawing (Goulden et al. 1998; Gruber et al. 2004; Allison and Treseder 2011; Koven et al. 2011). An increase in the decomposition rate of soil organic matter, which is directly proportional to ground temperature (Lloyd and Taylor 1994), is predicted (Davidson and Janssens 2006; Euskirchen et al. 2006; Allison and Treseder 2011), implying a potential decrease in net ecosystem productivity. The amplitude of this positive feedback is uncertain (Dufresne et al. 2002), but could be higher than the negative feedback due to CO2 fertilization (Cox et al. 2000). The 1%–26% loss of permafrost expected during the 21st century (Koven et al. 2012) would also release significant amounts of carbon (Davidson and Janssens 2006; Allison and Treseder 2011; Koven et al. 2011). O'Donnell et al. (2011) estimated that NPP would have to increase by 7%–14% in the next two to three centuries to offset carbon losses due to the interaction between fire and permafrost. The third challenge is to take into account spatial variability in fire regimes. Variations between and within countries in terms of temperature, precipitation, area burned, fire severity, carbon emissions (with different severity and type of fuel), and carbon storage (forest productivity in mixed, deciduous, coniferous forests and dense and open forests) need further investigations. For example, Russia differs from North America in various ways (Wooster and Zhang 2004; de Groot et al. 2013a). Future research efforts should focus on the Russian boreal forest, which received considerably less attention so far. Temporal variability is also important to take into account, as fire regimes can differ markedly from year to year or over longer time periods (Larsen 1997; Duncan et al. 2003; Bergeron et al. 2004; Balzter et al. 2005; Kasischke and Turetsky 2006). Finally, to determine the effect of aerosols on climate is a big challenge. Difficulty in separating the origins of aerosols (anthro- 215 pogenic versus natural) complicates model calibration. Aerosol production depends on numerous variables (e.g., temperature, wind speed, precipitation, changes in antioxidants, trace gases, and radiation) and is difficult to predict regionally. In addition, the effect of aerosols depends on the relationship between their abundance and concentration at the surface, which is not linear (Spracklen et al. 2009). Coupled climate–chemistry models should thus be developed. Data on long-term aerosol emissions by fire are needed to calibrate the models. Conclusion It is not yet possible to simultaneously take into account all fire effects on climate, but some conclusions can nevertheless be made. In general, research findings point towards a future increase in carbon emissions from boreal forest fires, with a likely greater effect on climate than that of post-fire albedo change. However, possible limits to the CO2 fertilization effect on photosynthesis make uncertain the future of the carbon stock in boreal forests. The carbon stock also varies according to other disturbances, such as insect infestations and diffuse mortality (Kurz and Apps 1999; De Grandpré et al. 2008). For example, the Canadian boreal forest is currently a source of carbon because of the combined effects of fire and mountain pine beetle outbreaks (Natural Resources Canada 2012). Only the direct aerosol effect was calculated in this review with a large range of radiative forcings on climate. The indirect aerosol effect (CCN) could be equivalent or greater, but with negative values (Forster et al. 2007). Elemental carbon deposited on snow and ice would thus be the only uncompensated positive radiative forcings due to aerosols. The partial results presented here suggest that boreal forest fires are more likely to warm the climate than to cool it in the future, especially in North America. Acknowledgements Funding was provided by the Natural Sciences and Engineering Research Council of Canada (NSERC Strategic Project Grant #41344411) and the Fonds québécois de recherche – Nature et Technologies (FQRNT). References Albrecht, B.A. 1989. Aerosols, cloud microphysics, and fractional cloudiness. Science, 245(4923): 1227–1230. doi:10.1126/science.245.4923.1227. PMID: 17747885. 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