As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release will be offset by increased production of Arctic and boreal biomass; however, the lack of robust estimates of net carbon balance increases the risk of further overshooting international emissions targets. Precise empirical or model-based assessments of the critical factors driving carbon balance are unlikely in the near future, so to address this gap, we present estimates from 98 permafrost-region experts of the response of biomass, wildfire, and hydrologic carbon flux to climate change. Results suggest that contrary to model projections, total permafrost-region biomass could decrease due to water stress and disturbance, factors that are not adequately incorporated in current models. Assessments indicate that end-of-the-century organic carbon release from Arctic rivers and collapsing coastlines could increase by 75% while carbon loss via burning could increase four-fold. Experts identified water balance, shifts in vegetation community, and permafrost degradation as the key sources of uncertainty in predicting future system response. In combination with previous findings, results suggest the permafrost region will become a carbon source to the atmosphere by 2100 regardless of warming scenario but that 65%–85% of permafrost carbon release can still be avoided if human emissions are actively reduced.

Variation in natural disturbance regime within a landscape is important for species population dynamics, because it controls spatial arrangement of sites providing regeneration and survival opportunities. In this study, we examine the differences in fire regime and evaluate possible sources of its variation between the surrounding mainland and the islands of Lake Duparquet (44.5 km²), a typical boreal lake in north-western Quebec, Canada. Dendrochronological reconstructions suggest that fires were frequent and of variable intensity on the islands, whereas fires were less frequent on the adjacent mainland, but were usually large and intense. Islands were significantly drier and warmer than the mainland, and maximum values of Fire Weather Index were significantly higher on the islands during both the early part of the fire season (May–June) and the whole fire season (May–September). The lightning density within the lake perimeter was significantly higher than in the surrounding mainland (0.63 v. 0.48 year^–1 km^–2 respectively). This pattern was a result of the differences in lightning density during the first half of the lightning season. The study suggests that more fire-prone local weather and higher frequency of lightning strikes could cause a higher frequency of low-intensity fires on the islands, compared with the mainland.

Abstract: The main objective of this paper is to evaluate whether future climate change would trigger an increase in the fire activity of the Waswanipi area, central Quebec. First, we used regression analyses to model the historical (1973–2002) link between weather conditions and fire activity. Then, we calculated Fire Weather Index system components using 1961–2100 daily weather variables from the Canadian Regional Climate Model for the A2 climate change scenario. We tested linear trends in 1961–2100 fire activity and calculated rates of change in fire activity between 1975–2005, 2030–2060, and 2070–2100. Our results suggest that the August fire risk would double (+110%) for 2100, while the May fire risk would slightly decrease (–20%), moving the fire season peak later in the season. Future climate change would trigger weather conditions more favourable to forest fires and a slight increase in regional fire activity (+7%). While considering this long-term increase, interannual variations of fire activity remain a major challenge for the development of sustainable forest management.

Résumé : Le principal objectif de cet article est d'évaluer si les changements climatiques futurs vont conduire à une augmentation de l'activité des feux dans la région de Waswanipi, dans le centre du Québec. Tout d'abord, nous avons utilisé des régressions linéaires pour modéliser la relation historique (1973–2002) entre les conditions météorologiques et l'activité des feux. Ensuite, nous avons calculé les composantes du système de l'Indice Forê-Météo à partir des simulations quotidiennes des conditions météorologiques du Modèle Régional Canadien du Climat pour le scénario A2 de changements climatiques (1961–2100). Nous avons testé les tendances linéaires de l'activité des feux sur la période 1961–2100, et calculé les taux de changement entre 1975–2005, 2030–2060, et 2070–2100. Nos résultats suggèrent que le risque de feu du mois d'août pourrait doubler (+110 %) d'ici 2100 alors que celui du mois de mai pourrait diminuer (–20 %). Ainsi, le pic saisonnier de l'activité des feux pourrait survenir plus tard dans la saison. Les changements climatiques futurs pourraient également créer des conditions plus favorables aux incendies forestiers, et donc à une légère augmentation de l'activité régionale des feux (+7 %). Bien que nos résultats suggèrent une augmentation à long terme de l'activité des feux, la variabilité interannuelle des feux demeure un défi important pour le développement d'un aménagement forestier durable.

Abstract: The main objective of this paper is to evaluate whether future climate change would trigger an increase in the fire activity of the Waswanipi area, central Quebec. First, we used regression analyses to model the historical (1973–2002) link between weather conditions and fire activity. Then, we calculated Fire Weather Index system components using 1961–2100 daily weather variables from the Canadian Regional Climate Model for the A2 climate change scenario. We tested linear trends in 1961–2100 fire activity and calculated rates of change in fire activity between 1975–2005, 2030–2060, and 2070–2100. Our results suggest that the August fire risk would double (+110%) for 2100, while the May fire risk would slightly decrease (–20%), moving the fire season peak later in the season. Future climate change would trigger weather conditions more favourable to forest fires and a slight increase in regional fire activity (+7%). While considering this long-term increase, interannual variations of fire activity remain a major challenge for the development of sustainable forest management.

Résumé : Le principal objectif de cet article est d’évaluer si les changements climatiques futurs vont conduire à une augmentation de l’activité des feux dans la région de Waswanipi, dans le centre du Québec. Tout d’abord, nous avons utilisé des régressions linéaires pour modéliser la relation historique (1973–2002) entre les conditions météorologiques et l’activité des feux. Ensuite, nous avons calculé les composantes du système de l’Indice Forê-Météo à partir des simulations quotidiennes des conditions météorologiques du Modèle Régional Canadien du Climat pour le scénario A2 de changements climatiques (1961–2100). Nous avons testé les tendances linéaires de l’activité des feux sur la période 1961–2100, et calculé les taux de changement entre 1975–2005, 2030–2060, et 2070–2100. Nos résultats suggèrent que le risque de feu du mois d’août pourrait doubler (+110 %) d’ici 2100 alors que celui du mois de mai pourrait diminuer (–20 %). Ainsi, le pic saisonnier de l’activité des feux pourrait survenir plus tard dans la saison. Les changements climatiques futurs pourraient également créer des conditions plus favorables aux incendies forestiers, et donc à une légère augmentation de l’activité régionale des feux (+7 %). Bien que nos résultats suggèrent une augmentation à long terme de l’activité des feux, la variabilité interannuelle des feux demeure un défi important pour le développement d’un aménagement forestier durable.

Questions: What climate variables best explain fire occurrence and area burned in the Great Lakes-St Lawrence forest of Canada? How will climate change influence these climate variables and thereby affect the occurrence of fire and area burned in a deciduous forest landscape in Témiscamingue, Québec, Canada?

Location: West central Québec and the Great Lakes-St Lawrence forest of Canada.

Methods: We first used an information-theoretic framework to evaluate the relative role of different weather variables in explaining occurrence and area burned of large fires (>200 ha, 1959-1999) across the Great Lakes-St Lawrence forest region. Second, we examined how these weather variables varied historically in Témiscamingue and, third, how they may change between the present and 2100 according to different scenarios of climate change based on two Global Circulation Models.

Results: Mean monthly temperature maxima during the fire season (Apr-Oct) and weighted sequences of dry spells best explained fire occurrence and area burned. Between 1910 and 2004, mean monthly temperature maxima in Témiscamingue showed no apparent temporal trend, while dry spell sequences decreased in frequency and length. All future scenarios show an increase in mean monthly temperature maxima, and one model scenario forecasts an increase in dry spell sequences, resulting in a slight increase in forecasted annual area burned.

Conclusion: Despite the forecasted increase in fire activity, effects of climate change on fire will not likely affect forest structure and composition as much as natural succession or harvesting and other disturbances, principally because of the large relative difference in area affected by these processes.

We examined the fire–climate relationship at the northern limit of commercial forest in western Quebec, a region where forest management is currently competing with fires for mature stands. The main objective was to determine if a particular climate signal would control the fire activity in this region when compared with other parts of the Quebec boreal forest.We used 500-hPa spatial correlation maps to compare the atmospheric patterns associated with the annual area burned (AAB) in the study area, the entire province of Quebec, the intensive (southern Quebec), and the restricted (northern Quebec) fire management zones. Next, dendroclimatic analyses were used to obtain tree-ring estimates of the AAB back to 1904 and to investigate the temporal stability of the fire–climate relationship. The climate controls associated with the AAB of the study area are intermediate between those associated with the AAB of the intensive and restricted fire management zones. The 500-hPa correlation patterns for the 1948–71 and 1972–2001 periods were relatively stable through time for the study area and for the restricted fire management zone. Our results provide a plausible mechanism for explaining the link between sea surface temperature and regional fire activity established in previous studies. They also provide information complementary to the Canadian fire danger rating system that uses daily weather data. © 2008 IJWF. All rights reserved.

The synchrony of regional fire regime shifts across the Quebec boreal forest, eastern Canada, suggests that regional fire regimes are influenced by large-scale climate variability. The present study investigated the relationship of the forest-age distribution, reflecting the regional fire activity, to large-scale climate variations. The interdecadal variation in forest fire activity in the Waswanipi area, north-eastern Canada, was reconstructed over 1720–2000. Next, the 1880–2000 reconstructed fire activity was analysed using different proxies of the Pacific Decadal Oscillation (PDO) and the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO). We estimated the global fire cycle around 132–153 years, with a major lengthening of the fire cycle from 99 years before 1940, to 282 years after 1940. Correlations between decadal fire activity and climate indices indicated a positive influence of the PDO. The positive influence of PDO on regional fire activity was also validated using t-tests between fire years and non-fire years between 1899 and 1996. Our results confirmed recent findings on the positive influence of the PDO on the fire activity over northern Quebec and the reinforcing role of the NAO in this relationship.

Abstract: The past decade has seen an increasing interest in forest management based on historical or natural disturbance dynamics. The rationale is that management that favours landscape compositions and stand structures similar to those found historically should also maintain biodiversity and essential ecological functions. In fire-dominated landscapes, this approach is feasible only if current and future fire frequencies are sufficiently low compared with the preindustrial fire frequency, so a substitution of fire by forest management can occur without elevating the overall frequency of disturbance. We address this question by comparing current and simulated future fire frequency based on 2 × CO2 and 3 × CO2 scenarios to historical reconstructions of fire frequency in the commercial forests of Quebec. For most regions, current and simulated future fire frequencies are lower than the historical fire frequency, suggesting that forest management could potentially be used to maintain or recreate the age-class distribution of fire-dominated preindustrial landscapes. Current even-aged management, however, tends to reduce forest variability by, for example, truncating the natural age-class distribution and eliminating mature and old-growth forests from the landscape. Therefore, in the context of sustainable forest management, silvicultural techniques that retain a spectrum of forest compositions and structures at different scales are necessary to maintain this variability and thereby allow a substitution of fire by harvesting.

Résumé :Au cours de la dernière décennie, un intérêt grandissant pour le développement d'approches d'aménagement basées sur notre compréhension de la dynamique historique des perturbations naturelles s'est manifesté. Ces approches reposent sur l'idée qu'un aménagement favorisant une composition des paysages et une structure des peuplements similaires à celles créées dans les forêts passées devrait aussi maintenir la diversité biologique et les fonctions écologiques essentielles de ces mêmes paysages et peuplements. Dans les paysages contrôlés par les feux, cette approche est possible seulement si les fréquences de feux actuelles et futures sont suffisamment faibles lorsque comparées aux fréquences pré-industrielles, cela afin de permettre de substituer le feu par la coupe forestière. Nous évaluons cette possibilité en comparant les fréquences de feux actuelles et futures aux fréquences historiques à partir d'études réalisées dans la forêt commerciale québécoise. Les fréquences actuelles et futures des feux, simulées en utilisant deux scénarios de concentration de CO2 (2× et 3× la concentration actuelle), sont plus faibles que les fréquences passées pour la majorité du territoire, suggérant que l'aménagement forestier pourrait potentiellement être utilisé afin de recréer la structure d'âge de la forêt soumise à un régime de feux sévères. Les aménagements équiennes actuels tendent toutefois à réduire la variabilité naturelle du système: par exemple, un aménagement équienne amputera, à terme, la structure d'âge de la forêt naturelle éliminant ainsi les forêts surannées et anciennes du paysage. Le développement de techniques de sylviculture permettant le maintien d'un spectre de compositions et structures forestières à différentes échelles de paysage est une des avenues proposées afin de maintenir cette variabilité. ©2006 NRC Canada

Five independent multicentury reconstructions of the July Canadian Drought Code and one reconstruction of the mean July–August temperature were developed using a network of 120 well-replicated tree-ring chronologies covering the area of the eastern Boreal Plains to the eastern Boreal Shield of Canada. The reconstructions were performed using 54 time-varying reconstruction submodels that explained up to 50% of the regional drought variance during the period of 1919–84. Spatial correlation fields on the six reconstructions revealed that the meridional component of the climate system from central to eastern Canada increased since the mid–nineteenth century. The most obvious change was observed in the decadal scale of variability. Using 500-hPa geopotential height and wind composites, this zonal to meridional transition was interpreted as a response to an amplification of long waves flowing over the eastern North Pacific into boreal Canada, from approximately 1851 to 1940. Composites with NOAA Extended Reconstructed SSTs indicated a coupling between the meridional component and tropical and North Pacific SST for a period covering at least the past 150 yr, supporting previous findings of a summertime global ocean–atmosphere– land surface coupling. This change in the global atmospheric circulation could be a key element toward understanding the observed temporal changes in the Canadian boreal forest fire regimes over the past 150 yr.

Area burned variability in the province of Ontario, Canada, was inferred from 25 treering width chronologies covering AD 1781-1982 and distributed largely across the Boreal Shield. The area burned estimates account for 39.5% of the variance in the actual area burned recorded from 1917 to 1981 and were verified using a split sample calibrationverification scheme. The reconstruction showed that a positive trend in area burned from ca. 1970-1981 was preceded by three decades during which area burned was amongst the lowest during the past 200 years. The area burned exhibited a trend toward increasing variance during the past century, recently reaching magnitudes similar to those seen prior to 1850. Signal analyses further identified the presence of two prominent periodic components in area burned that related to decade-to-decade variations. This will help to place the recent increase in area burned in a context relative to the long-term history of the province.

Forest fires in Canada are directly influenced by the state of the climate system. The strong connection between climate and fire, along with the dynamic nature of the climate system, causes the extent, severity and frequency of fires to change over time. For instance, many reconstructions of the history of forest fires across boreal Canada report a general decrease in fire activity since ~1850 which could, in part, result from changes in climate. Here we describe progress in characterizing the variability in fire-conducive droughts in the central and eastern Canadian boreal forests during the past three centuries. An extensive network of drought-sensitive tree-ring records from Manitoba, Ontario and Quebec was used to develop five multi-century reconstructions of the mean July Canadian Drought Code and one reconstruction of mean July and August temperatures. Correlation analyses with regional fire statistics (common period 1959-1998) showed that drought estimates have some skill to approximate fire activity and, hence, the estimates are relevant for the study of climate change impacts on Canadian forests. Spatial correlation analysis over the period 1768-1998 revealed that variability between the west and east has increased after the mid-19th century, specifically the decade-to-decade variability and the frequency of extreme events. Based on the synoptic characteristics of recent droughts, we interpret this change in variability as a response to an increasing frequency of upper level ridging and troughing over western and eastern Canada, respectively. The increasing horizontal movement of humid air masses over eastern Canada since ~1850 could have contributed to the creation of moister conditions that are less suitable for fire.

Résumé

Les feux de forêt sont fortement influencés par l'état du système climatique. Le lien étroit qui existe entre le climat et les feux ainsi que la nature dynamique de l'état du système climatique conduisent à des variations temporelles dans l'étendu, la sévérité et la fréquence des feux. Par exemple, plusieurs reconstitutions historiques des feux de forêt à travers le Canada boréal rapportent une diminution de l'activité des feux depuis ~1850 qui pourrait en partie être due à des changements du climat. Dans la présente étude, nous décrivons les progrès réalisés dans la caractérisation de la variabilité des sécheresses propices aux feux de forêt au cours des trois derniers siècles du centre à l'est de la forêt boréale canadienne. Un réseau étendu de séries d'accroissement radial d'espèces arborescentes sensibles à la sécheresse en provenance du Manitoba, du Québec, et de l'Ontario, a été utilisé pour développer cinq reconstitutions multi-centenaires de l'indice de sécheresse canadien (CDC) de juillet et une reconstitution des températures moyennes de juillet et août. Des analyses de corrélation effectuées sur des données régionales de l'activité des feux (période commune 1959-1998) ont démontré que les estimés de sécheresses étaient suffisamment fiables pour inférer la variabilité temporelle de l'activité des feux. Ces estimés sont donc pertinents pour l'étude de l'impact des changements climatiques sur la forêt canadienne. Des analyses de corrélation spatiale sur la période 1768-1998 ont démontrés que la variabilité entre l'ouest et l'est s'est accentuée depuis le milieu du 19e siècle, en particulier pour la variabilité inter-décennale et la fréquence d'événements extrêmes. À partir des caractéristiques synoptiques des sécheresses récentes, nous interprétons ce changement dans la variabilité comme une réponse à une augmentation de la fréquence des crêtes et creux barométriques au-dessus de l'ouest et de l'est du Canada, respectivement. L'accroissement du mouvement horizontal d'air humide sur l'est du Canada depuis ~1850 pourrait avoir contribué à la création de conditions plus humides qui sont moins propices aux feux.

Climate influences natural processes at multiple spatial and temporal scales. Consequently, climate change raises many challenges for sustainable forest management; among them, the integration of fire and forest management is increasingly discussed. We propose here an evaluation of the adaptive capacity of forest management under changing forest fire regimes under climate change in the boreal forest of Quebec. Adaptation begins by reinterpreting current practices dealing with climatically driven variability.Among them, fire suppression, and regeneration enhancement can contribute to coping with some impacts of climate change. However, there is an increasing need to develop more integrative and spatially explicit management strategies to decrease the vulnerability of forest management to changing fire risk. Some developing management strategies, such as fuel management or the triad approach (zoning system for conservation, intensive, and extensive forest management), present an interesting potential for integrating the fire risk in management plans.While fuel management and fire suppression are indicated for particularly severe fire regimes, protection against insects, and maintaining a shorter disturbance cycle using forest management represent the preferred adaptation options where the fire cycle is lengthening under climate change.

Le changement climatique pose de nombreux défis pour l’aménagement forestier durable car le climat influence les processus naturels à de multiples échelles spatiales et temporelles. Parmi ces enjeux, l’intégration du risque de feu à l’aménagement forestier est de plus en plus discutée.Nous proposons ici une évaluation de la capacité adaptative de l’aménagement forestier aux changements de régimes de feu sous l’influence du changement climatique en forêt boréale au Québec. L’adaptation commence par une réinterprétation de certaines pratiques forestières courantes qui traitent déjà de la variabilité climatique. Parmi elles, la suppression des feux et l’amélioration de la régénération peuvent contribuer à atténuer certains impacts du changement climatique. Cependant, il y a une demande grandissante pour des approches plus intégratives et spatialement explicites afin de diminuer la vulnérabilité de l’aménagement forestier face aux changements du risque de feu. Certaines stratégies en développement, comme l’aménagement du combustible et l’approche et de la triade (système de zones de conservation, d’aménagement intensif et extensif) présentent un potentiel intéressant pour intégrer le risque de feu à la planification forestière. Alors que l’aménagement du combustible et la suppression des feux sont indiqués pour les régimes de feu particulièrement sévères, la protection des forêts contre les insectes et le maintient d’un cycle de perturbation plus court par l’aménagement forestier sont des options d’adaptation privilégiées lorsque le cycle de feu s’allonge sous l’influence du changement climatique.

Over the past decades, there has been an increasing interest in the development of forest management approaches that are based on an understanding of historical natural disturbance dynamics. The rationale for such an approach is that management to favor landscape compositions and stand structures similar to those of natural ecosystems should also maintain biological diversity and essential ecological functions. In fire-dominated landscapes, this approach is possible only if current and future fire frequencies are sufficiently low, comparing to pre-industrial fire frequency, that we can substitute fire by forest management. We address this question by comparing current and future fire frequency to historical reconstruction of fire frequency from studies realized in the Canadian boreal forest. Current and simulated future fire frequencies using 2 and 3 x CO2 scenarios are lower than the historical fire frequency for many sites, suggesting that forest management could potentially be used to recreate the forest age structure of fire-controlled pre-industrial landscapes. There are however, important limitations to the current even-age management.

Fire history was reconstructed for an area of 15 000 km(2) 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.

Inter-annual and -decadal scale variability in drought over the Abitibi Plains ecoregion (eastern Canada) was investigated using a 380-year dendroclimatic reconstruction of the Canadian Drought Code (CDC; July monthly average) i.e., a daily numerical rating of the average moisture content of deep organic layers. Spectral analyses conducted on the reconstructed CDC indicated a shift in spectral power after 1850 leading toward a reduction in interdecadal variability and an increase in interannual variability. Investigation on the causes for this shift suggested a decrease in North Pacific forcing after the mid-nineteenth century. Cross-continuous wavelet transformation analyses indicated coherency in the 8-16 and 17-32-year per cycle oscillation bands between the CDC reconstruction and the Pacific Decadal Oscillation (PDO) prior to 1850. Following 1850, the coherency shifted toward the North Atlantic Oscillation (NAO). Principal component analysis conducted over varying time windows reaffirmed that the Pacific forcing was restricted to the period about 1750-1850. Prior to and after this period, the CDC was correlated with the NAO. The shift around 1850 could reflect a northward displacement of the polar jet stream induced by a warming of the sea surface temperature along the North Pacific coast. A northward displacement of the jet stream, which inhibits the outflow of cold and dry Arctic air, could have allowed the incursion of air masses from the Atlantic subtropical regions.

Trends and periodicities in summer drought severity are investigated on a network of Canadian Drought Code (CDC) monthly average indices extending from central Quebec to western Manitoba and covering the instrumental period 1913–1998. The relationship and coherency between CDC indices and ocean–atmosphere circulation patterns are also examined. Trend analyses indicate that drought severity is unchanged in eastern and central Canada. Composite analyses indicate that for most of the corridor, severe drought seasons occur with a combination of positive 500-hPa geopotential height anomalies centered over the Gulf of Alaska and over the Baffin Bay. Additional severe drought seasons develop across the corridor in the presence of positive height anomalies located over or upstream of the affected regions. According to spectral analyses, the North Atlantic and the North Pacific circulation patterns modulate the drought variability at the decadal scale. Our results lead us to conclude that climate warming and the increases in the amount and frequency of precipitation in eastern Canada during the last century had no significant impact on summer drought severity. It is unlikely that linear climate change contributed to the change in the boreal forest dynamics observed over the past 150 years.

Les tendances et périodicités dans la sévérité des sécheresses estivales sont analysées sur un réseau d'indices de sécheresse (CDC) moyens mensuels couvrant le corridor Québec–Manitoba et la période déterminante 1913–1998. Les relations et les cohérences entre les indices CDC et les principaux patrons de circulation atmosphérique sont également examinées. Les résultats obtenus n'indiquent aucun changement linéaire des conditions de sécheresse estivale, ni dans l'Est ni au centre du Canada. Des analyses composites indiquent qu'à travers le corridor, des saisons de sécheresse sévère ont lieu en combinaison avec des anomalies positives de la hauteur géopotentielle à 500 hPa centrées au-dessus du Golfe de l'Alaska et au-dessus de la Baie de Baffin. Des saisons de sécheresse additionnelles ont lieu à travers le corridor avec le développement d'anomalies positives au-dessus des régions affectées. D'après nos analyses spectrales, les patrons de circulations de l'Atlantique Nord et du Pacifique Nord agissent sur la variabilité des sécheresses dans l'échelle décennale. Nos résultats nous amènent à conclure que le réchauffement climatique et les augmentations de la quantité et de la fréquence des précipitations dans l'est du Canada au cours du dernier siècle n'ont eu aucun effet important sur la sévérité des sécheresses estivales. Il est donc peu probable que le changement climatique linéaire ait contribué au changement dans la dynamique de la forêt boréale enregistré au cours des 150 dernières années.©2004 NRC Canada

We modeled tree mortality three months after a wildfire in the mixed-wood boreal forest (Quebec, Canada) using data from 1963 trees in 36 stands burned under a wide range of fire behavior conditions during the 1997 Val Paradis fire. Stand composition influenced the char height: height of burn was lower in deciduous stands than mixed or coniferous stands. Analysis of species mortality rates revealed that Populus tremuloides Michx. Was the least fire-resistant species, whereas Picea mariana Moench and Pinus banksiana Lamb. Were the most resistant species. Efficient interactions for conifers exist between crown and cambial resistance to injury and fire behavior, as diameter at breast height (DBH), total tree height (TOTH), and mean bark thickness are characteristic variables throughout the fire behavior range. The best logistic regressions, relating probability of wildfire-induced mortality to morphology and fire variables, always entered char height (CH) at the first step of the stepwise procedure, followed by a morphological variable (DBH or TOTH). The Kappa coefficient used for model validations revealed that logistic regressions using morphologic and fire variables were very efficient as compared to logistic regressions based only on morphologic variables. Potential applications of these results by land managers are discussed.

In the `Des Vieux Arbres' ecological reserve situated within northwestern Québec, 40 band dendrometers were installed on 7 of the major boreal tree species. The late Spring–early Summer daily radial activity registered in 1997 was related to daily weather variables. For each tree species, the daily mean i) cumulative radial increment and ii) radial activity indexed series obtained by first-difference standardization were analyzed. The results indicate the existence of strong similarities among the 7 species. All showed strong synchronous fluctuations in radius during late winter and early spring. This period ended with a short but sharp increase in radial increments that marked the passage of water into the stem. This initial swelling, less obvious in Pinus species was followed by a prolonged period of little change in radial activity. Meteorological data indicated that air temperature was positively related to stem swelling during the late winter–early spring period. Both air and soil temperatures became negatively related to radial expansion once the passage of water has occurred in the stem. Starting in early June, all species registered a sustained increase in radial increments possibly associated with active cell division. After this, radial expansion was negatively related to air temperature and positively to rainfall.

General circulation model simulations suggest the Earth's climate will be 1-3.5 degreesC warmer by AD 2100. This will influence disturbances such as forest fires, which are important to circumpolar boreal forest dynamics and, hence, the global carbon cycle. Many suggest climate warming will cause increased fire activity and area burned. Here, we use the Canadian Forest Fire Weather Index to simulate future forest fire danger, showing the expected increase in most of Canada but with significant regional variability including a decrease in much of eastern Canada. These results are in general agreement with paleoecological data and general circulation model results from the 6000 calendar years BP interval, which was a time of a warmer climate that may be an analogue for a future climate. ©2001 NRC Canada

Stand age structures of Eastern White Pine (Pinus strobus L.) were analysed on three sites with different fire histories (latest fire 1760,1825 and 1941, respectively) in Québec to assess whether Eastern White Pine is regenerating or not at its northern limit. Previous paleo-botanical findings suggest that this species once extended ca 100 km further north than the present distribution limit in Québec. All three sites (dry habitats) represented vital populations where the individuals were established after the most recent fire. The results suggest that the relative lack of dry and/or disturbed sites in combination with a more severe fire regime that characterizes the northern lowlands, may explain why white pine expansion is restricted northwards.

Quantities and structural characteristics of coarse woody debris (CWD) (logs and snags) were examined in relation to stand age and composition in the Canadian mixedwood boreal forest. Forty-eight stands originating after fire (from 32 to 236 years) were sampled on mesic clay deposits. The point-centered quadrant method was used to record canopy composition and structure (living trees and snags). The line-intersect method was used to sample logs of all diameters. Total log load, mean snag density, and volume per stand were similar to other boreal stands. Linear and nonlinear regressions showed that time since fire and canopy composition were significant descriptors for log load changes, whereas time since fire was the only significant factor for snag changes. Coarse woody debris accumulation models through time since fire were different from the U-shaped model because the first initial decrease from residual pre-disturbance debris was missing, the involved species had rapid decay rates with no long-term accumulation, and the succession occurred from species replacement through time. ©2000 NRC Canada

Because some consequences of fire resemble the effects of industrial forest harvesting, forest management is often considered as a disturbance having effects similar to those of natural disturbances. Although the analogy between forest management and fire disturbance in boreal ecosystems has some merit, it is important to recognize that it has limitations. First, normal forest rotations truncate the natural forest stand age distribution and eliminate over-mature forests from the landscape. Second, in the boreal mixedwoods, natural forest dynamics following fire may involve a gradual replacement of stands of intolerant broadleaf species by mixedwood and then softwood stands, whereas current silvicultural practices promote successive rotations of similarly composed stands. Third, the large fluctuations observed in fire frequency during the Holocene limit the use of a single fire cycle to characterize natural fire regimes. Short fire cycles generally described for boreal ecosystems do not appear to be universal; rather, shifts between short and long fire cycles have been observed. These shifts imply important changes in forest composition at the landscape and regional levels. All of these factors create a natural variability in forest composition that should be maintained by forest managers concerned with the conservation of biodiversity. One avenue is to develop silvicultural techniques that maintain a spectrum of forest compositions over the landscape.

Despite increasing temperatures since the end of the Little Ice Age (ca. 1850), wildfire frequency has decreased as shown in many field studies from North America and Europe. We believe that global warming since 1850 may have triggered decreases in fire frequency in some regions and future warming may even lead to further decreases in fire frequency. Simulations of present and future fire regimes, using daily outputs from the General Circulation Model (GCM), were in good agreement with recent trends observed in fire history studies. Daily data, rather than monthly data, were used because the weather and, consequently, fire behavior can change dramatically over time periods much shorter than a month. The simulation and fire history results suggest that the impact of global warming on northern forests through forest fires may not be disastrous and that, contrary to the expectation of an overall increase in forest fires, there may be large regions of the Northern Hemisphere with a reduced fire frequency.

The objective of this study is to determine the factors responsible for the distribution of Pinus resinosa (red pine) at its northern limit in northwestern Quebec. Pinus resinosa is found only on islands and protected lake shores at its northern distribution boundary. The influence of climate on the germination of P. resinosa seed and on the phenology of P. resinosa was investigated in the Lake Duparquet region of northwestern Quebec. The results indicate that P, resinosa seed readily germinates at island- and inland sites, which indicates that germination is not responsible for the distribution of P. resinosa in the Lake Duparquet region. Also, cones and seeds developed normally at island sites and an inland plantation, which suggests that seed production is not hindering the expansion of P. resinosa. These results, together with other studies in the literature, suggest that no climatic factor could explain the present distribution of P. resinosa nor its northern limit. The fire regime appears to be responsible for the restriction of P. resinosa to lake environments and hinders the northward expansion of P. resinosa. The typical crown fire regime of the boreal forest is not conducive for P. resinosa regeneration and restricts the species to fire-sheltered locations such as islands and protected lake shores.