Fire history was reconstructed for an area of 15 000 km2 located in the transition zone between the mixed and coniferous forests in Quebec's southern boreal forest. We used aerial photographs, archives, and dendroecological data (315 sites) to reconstruct a stand initiation map for the area. The cumulative distribution of burnt area in relation to time since fire suggests that the fire frequency has decreased drastically since the end of the Little Ice Age (about 1850) in the entire region. However, a large part of the area was burned between 1910 and 1920 during intensive colonization and when the climate was very conducive to fire. For the period 1920–1945, large fires have mainly been concentrated in the more populated southern area, while few fires have been observed in the virgin coniferous forest in the north. Despite slight differences between the south and the north, fire cycles or the average number of years since fire are not significantly different. Since 1945, there have been far more fires in the south, but the mean fire size was smaller than in the north. These results suggest that the transition between the mixed and coniferous forests observed in the southern boreal forest cannot be explained by a difference in fire frequency, at least during the last 300 years. As climatic factors and species potential distribution did not vary significantly from south to north, we suggest that the transition from mixedwood to coniferous forests is mainly controlled by fire size and severity. Smaller and less severe fires would favor species associated with the mixedwood forests as many need survivors to reinvade burnt areas. The abundance of deciduous species in mixedwood forests, together with the presence of more lakes that can act as firebreaks, may contribute to decreases in fire size and severity. The transition between the two vegetation zones could be related to the initial setting following the vegetation invasion of the area during the Holocene. In this context, the limit of vegetation zones in systems controlled by disturbance regimes such as fires may not have reached a balance with current climatic conditions. Historical legacies and strong positive feedback between disturbance regimes and composition may filter and delay the responses to changes in climate.

S. 2002. Natural fi re regime: a guide for sustainable management of the Canadian boreal forest. Silva Fennica 36(1): 81-95.The combination of certain features of fi re disturbance, notably fi re frequency, size and severity, may be used to characterize the disturbance regime in any region of the boreal forest. As some consequences of fi re resemble the effects of industrial forest harvesting, conventional forest management is often considered as a disturbance that has effects similar to those of natural disturbances. Although the analogy between forest management and fi re disturbance in boreal ecosystems has some merit, it is important to recognise that it also has its limitations. Short fi re cycles generally described for boreal ecosystems do not appear to be universal; rather, important spatial and temporal variations have been observed in Canada. These variations in the fi re cycle have an important infl uence on forest composition and structure at the landscape and regional levels. Size and severity of fi res also show a large range of variability. In regions where the natural matrix of the boreal forest remains relatively intact, maintenance of this natural variability should be targeted by forest managers concerned with biodiversity conservation. Current forest management tends to reduce this variability: for example, fully regulated, even-aged management will tend to truncate the natural forest age distribution and eliminate over-mature and old-growth forests from the landscape. We suggest that the development of strategic-level forest management planning approaches and silvicultural techniques designed to maintain a spectrum of forest compositions and structures at different scales in the landscape is one avenue to maintain this variability. Although we use the boreal forest of Quebec for our examples, it is possible to apply the approach to those portions of the boreal forest where the fi re regime favours the development of even-aged stands in burns.

Although the concept of forest ecosystem management based on Le concept d'arnknagement &yst6mique, soit une approche #am& natural disturbance has generated a great deal of interest, few connagement forestier s'inspirant des perturbations naturelles, suscrete examples exist of FEM principles being put into application.cite de plus en plus d'intkret auprks des forestiers mais son Silvicultural practices that emulate natural disturbances are proapplication concrkte en est encore B ses dkbuts. Des pratiques sylviposed with examples from the principal vegetation zones of coles s'apparentant aux perturbations naturelles sont proposkes Quebec. With the exception of the large-scale use of careful en donnant des exemples pour les principales zones de vCg6ta-logging to protect advanced regeneration in ecosystems generally tion du Qukbec. A l'exception de l'utilisation 2i grande Cchelle de controlled by fire, stand-level silvicultural practices currently used la coupe avec protection de la rkg6nkration et des sols dans des are reasonably similar to natural disturbances, although impor6cosystkmes gknkrdement cont18lCs par les incendies, les pratiques tant differences exist. In contrast, at the forest-level, even-aged ~y l v i c~l e s rkalistes couramment B l'kchelle des ~e u~l e m e n t s management, as is currently rarely adequate sont assez similaires aux perturbations naturelles, bien qu'il duction of the variety of age ,-lasses, stand types, and structural existe encore des diffkrences importantes. Par contre, l'amkcomponents d y found in the boreal forest. A model that allows nagement normal des for& kquiennes, tel que pratiquk actuellean even-aged management approach inspired by natural dynamment, Permet rarement de reproduire adkquatement la varikt6 des ics is proposed.classes d'$ges, des types de peuplements et des composantes structurales habituellement rencontrks en for& boreale. Un modkle permettant d'appliquer une approche d'amknagement Cquienne s'inspirant de la nature est proposk.

Summary1 Studies on the variability of natural fire regimes are needed to understand plant responses in a changing environment. Since vegetation changes might follow or trigger changes in fire frequency, climate models suggest that changes in water balance will accompany current global warming, and the response of fire regimes to Holocene hydro-climate changes and vegetation switches may thus serve as a useful analogue for current change. 2 We present high-resolution charcoal records from laminated cores from three small kettle lakes located in mixed-boreal and coniferous-boreal forest. Comparison with some pollen diagrams from the lakes is used to evaluate the role of the local vegetation in the fire history. Fire frequency was reconstructed by measuring the separation of peaks after detrending the charcoal accumulation rate from any background. 3 Several distinct periods of fire regime were detected with fire intervals. Between c. 7000-3000 cal. year  , fire intervals were double those in the last 2000 years. Fire frequency changed 1000 years earlier in the coniferous-boreal forest than in the mixedboreal forest to the south. The absence of changes in combustibility species in the pollen data that could explain the fire frequency transition suggests that the vegetation does not control the long-term fire regime in the boreal forest. 4 Climate appears to be the main process triggering fire. The increased frequency may be the result of more frequent drought due to the increasing influence of cool dry westerly Pacific air-masses from mid to late Holocene, and thus of conditions conducive to ignition and fire spread. In east Canada, this change matches other long-term climate proxies and suggests that a switch in atmospheric circulation 2-3000 years ago triggered a less stable climate with more dry summers. Future warming is moreover likely to reduce fire frequency.