Harden, J.W., S.E. Trumbore, B.J. Stocks, A. Hirsh, S.T. Gower, K.P. ...

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F I R E I N T H E B O R E A L C A R B O N B U D G E T 181Fig. 3 The sensitivity of burn emissions to decomposition rateof charred remains (k char ) required to balance the C budget fordeep soil. Using a decomposition rate for deep soil ofk d = 0.0005 (turnover time of 2000 years; shown as lower linewith circles) to 0.0009 (1111 years; upper line with boxes) forblack spruce/feathermoss site, model results require a combinationof faster turnover time for charred material or higherburn emission to balance the C stored in deep soil. Shown ashorizontal lines, decomposition rates for char may lie somewherebetween ranges measured for k d and k s (Table 1).carbon as a result of increased heterotrophic respiration(O'Neill et al. 1997) and absence of vegetative cover. Asthe site recovers with new vegetation, dead and charredmaterial is protected by burial into deeper, colder layers(Goulden et al. 1998), and decomposition is offset by netprimary production. Based on the work of Rapalee et al.(1998), ®re scars are net C sources for about 30 years afterburning, after which time the systems become net sinksfor C. The loss of the insulating organic layers probablyalso has an effect on winter ¯uxes in which soils wouldbe colder to greater depths and may remain frozen for alonger period. However, the thickening of the activelayer after ®re suggests an overall warming. Althoughour model does not explicitly include a post-®rewarming or winter cooling, nor a slow transition duringregrowth, a sensitivity test suggested that a two-foldincrease in decomposition sustained for 20 years resultedin an underestimate of decomposition comparable toabout 3%NPP. This estimate, however, may change asdifferent model scenarios include changes in ®re returninterval and shifts in the C h and F terms.For a regional and global perspective on carbonexchange, carbon storage and decomposition may beextrapolated best by soil drainage class in associationwith ecosystem structure. For each drainage and ecosystemclass, we assigned relationships of C h , F, and NPP toareas of the globe based on the distribution of soildrainage and ecosystem class. Because the discontinuouspermafrost that underlies our study may disproportionatelyaffect ratios of F/(C h + F) relative to regionsFig. 4 A model of deep soil carbon storage (C d ) for wetland fensystems that burn very lightly, poorly drained spruce systemsthat burn severely, and well drained pine systems that decomposeand burn severely. The zig-zag pattern represents inputof dead and burned material from shallow to deep layers andsubsequent decline by decomposition between ®res. For a sitematuring after a burn, local Net Ecosystem Production (NEP)is represented by C accumulation in deep soil layers.Extrapolating NEP from local site measurements to a regionalscale or long-term average must include ®re emission lossesthat account for the major differences in carbon storage in fen,spruce and pine systems.lacking permafrost, the F/(C h + F) relationship may nothold true for the boreal forest in the absence ofpermafrost. Based on our areal extrapolations in thepresence of permafrost, however (Table 2), global boreal®re emissions could have been about 0.5±1 Pg C y ±1 overthe past century or millennia. This overlaps at least thelow end of some recent boreal ®re studies (Kasischkeet al. 1995a; Conard & Ivanova 1998) but is at least threetimes larger than extrapolations from some directemission estimates (Cahoon et al. 1994) and 3±10 timeslarger than estimates used in general circulation models(Houghton 1991; Randerson et al. 1997; Wittenburg et al.1998; see discussion below). While our estimates arehighly uncertain, they do emphasize the importance of®re emissions in closing the C budget over century-tomillenialtimescales, and they emphasize the linksbetween NPP, decomposition and ®re.DiscussionIf the distribution of stand age were to change through®re suppression or through changes in climate, then therelationships of NPP, k, C, and F would change and# 2000 Blackwell Science Ltd, Global Change Biology, 6 (Suppl. 1), 174±184
182 J . W . H A R D E N et al.Fig. 5 Carbon losses to ®re and decompositionwere calculated from model resultsdescribed in text. Long-term sums forNPP, C h for shallow (k s C s ), C h for deep(k d C d ) and C h for dead (k s C c ), and ®reemission F were divided by years of simulationfor annual uptake and lossrates of C exchange. Each drainage classwas modelled separately for runs where®re emissions were minimized (all C hvalues generally maximized within dataconstraints of Table 1) and maximized(C h minimized) to show range of possible®re emissions. Best estimates for F andC h are not shown here but are summarizedin Tables 1 and 2.result in a shift in F/(C h + F) and in NEP. Looking backthrough the past century, some investigators have notedvariations in ®re occurrence that coincided with variationsin ®re-season climate, but regional results vary.Compared to ad1850, burn areas and ®re occurrencewere notably low in tree-ring records in Quebec from1870 to 1988 (Payette et al. 1989; Bergeron & Archambault1993) and in charcoal deposition of Minnesota in the 19thCentury (Clark 1988). By contrast, ®res in the rest ofCanada after the 1980s have been more extensive than inprevious decades (Murphy et al. 1999). These variationsin climate and in ®re disturbance likely resulted in shiftsof F/(C h + F) and therefore in shifts of how fast and inwhat forms C was lost from the systems. More discreetstand-age information is needed before we can understandhow such shifts may relate to NEP or how shifts inclimate will affect F/(C h + F) and NEP.The reasons for the discrepancy between our estimatesof the long-term average carbon emissions and directestimates of C emissions are unclear but potentiallyimportant. First, estimates derived from observationsbefore the 1980s (Murphy et al. 1999) may be biasedtoward a lower ®re activity of the last century, assuggested by Bergeron & Archambault (1993). In thiscase, our long-term estimates suggest that we areentering a period of ®re activity that is atypical of thepast century but more typical for the boreal forest.Secondly, the technology for estimating emissions hasadvanced to increase the area burned by two-fold overthe past 5 years (Cahoon et al. 1994; Conard & Ivanova1998). Fire severity estimates have increased steadilyalso, as studies include a wider variety of stands (such asblack spruce with more moss and ground fuels) and amore careful accounting of ground fuels in general(Stocks 1989; Kasischke et al. 2000a). Whether ®re activityis actually on the rise or is being more carefullyaccounted, CO 2 emissions from boreal ®res are likely tobe much greater than have been assumed (e.g. compareSeiler & Crutzen 1980 with Cahoon et al. 1994 and Frenchet al. 2000). In addition, ®re emissions may increase in thefuture, particularly if summer droughts are sustained inthese regions leading to more episodic ®re events.Interactions between ®re disturbance and the activelayer above permafrost have demonstrated the importanceof insulating moss and soil layers in maintaining afrozen soil near the ground surface (Viereck 1983). Inthese regions species composition is highly dependent onsoil drainage (Rapalee et al. 1998), and NEP and decompositionare sensitive to soil drainage class (Trumbore &Harden 1997); thus it would follow that changes in ®rereturn intervals could potentially invoke changes inforest structure and carbon exchange. As ®re is episodicand sudden in the boreal forest, relative to gradualchanges in temperature and decomposition, the mechanismby which carbon may respond to regional climatechange is inherently different from other ecosystemswhere ®re is managed or of smaller scale. Furthermore,changes in permafrost regions can be irreversible whenshallow and deep freeze layers become separated(formation of talik and massive soil drainage).Depending on the balance between colder, deeper winterfreezes and warmer, deeper summer thaws in areaswhere organic mats were burned, a change to morefrequent ®res may prevent insulating layers from# 2000 Blackwell Science Ltd, Global Change Biology, 6 (Suppl. 1), 174±184
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Harden, J.W., S.E. Trumbore, B.J. Stocks, A. Hirsh, S.T. Gower, K.P. ...

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