Fires are a key disturbance of boreal forests. In fact, they are the main source of renewal and evolution for forest stands. The variability of fire through space and time results in a diversified forest mosaic, altering their species composition, structure and productivity. A resilient forest is assumed to be in a state of dynamic equilibrium with the fire regime, so that the composition, age structure and succession stages of forests should be consistent with the fire regime. Dense spruce-moss stands tend, however, to diminish in favour of more open stands similar to spruce-lichen stands when subjected to more frequent and recurring disturbances. This study therefore focused on the effects of spatial and temporal variations in burn rates on the proportion of open stands over a large geographic area (175,000 km2) covered by black spruce (Picea mariana (Mill.) Britton, Sterns, Poggenb.). The study area was divided into 10 different zones according to burn rates, as measured using fire-related data collected between 1940 and 2006. To test if the abundance of open stands was unstable over time and not in equilibrium with the current fire regime, forest succession was simulated using a landscape dynamics model that showed that the abundance of open stands should increase progressively over time in zones where the average burn rate is high. The proportion of open stands generated during a specific historical period is correlated with the burn rate observed during the same period. Rising annual burn rates over the past two decades have thereby resulted in an immediate increase in the proportion of open stands. There is therefore a difference between the current proportion of open stands and the one expected if vegetation was in equilibrium with the disturbance regime, reflecting an instability that may significantly impact the way forest resources are managed. It is apparent from this study that forestry planning should consider the risks associated with the temporal variability of fire regimes on the forest ecosystem, as the resulting changes can have a significant impact on biodiversity and allowable cut estimates.

La capacité d’une forêt à produire du bois est en relation avec les interactions existant entre sa capacité de régénération, les caractéristiques physiques du site (climat, dépôt de surface, drainage) et les perturbations. De façon minimale, pour que son aménagement soit durable, une forêt doit être suffisamment productive et capable de se régénérer après une perturbation de façon telle à ce que sa capacité de production soit maintenue ou améliorée. À cet égard, nous avons évalué la productivité en bois d’une vaste région (175?000 km2) qui recouvre l’étendue latitudinale de la pessière noire fermée (Picea mariana (Mill.) B.S.P). L’indice de qualité de station et l’indice de densité relative ont été utilisés pour identifier les peuplements qui ne peuvent atteindre un volume minimal de bois et des dimensions minimales d’arbres en une révolution. Une méthode non paramétrique a été utilisée pour estimer leur valeurs pour tous les peuplements de l’aire d’étude. Cette imputation a été faite en utilisant soit uniquement les attributs physiques du site pour estimer une productivité potentielle indépendante de l’historique du peuplement, soit une combinaison d’attributs physiques et de végétation pour estimer une productivité actuelle. La proportion de peuplements productifs a ensuite été estimée à l’échelle de paysages de taille variant entre 39 et 2491 km2. Les facteurs physiques du site expliquent à eux seuls 84 % de la variabilité dans le pourcentage potentiel de peuplements productifs (78 % pour le pourcentage actuel de peuplement productifs) et leur combinaison cause une transition abrupte de la productivité dans l’aire d’étude. Toutefois, le taux de brûlage, lorsque considéré seul, explique 63 % de la variabilité de la proportion de peuplements actuellement productifs et 41 % de la différence relative entre les pourcentages actuels et potentiels de peuplements productifs. Ces résultats ont des implications pour la planification stratégique de l’aménagement au moment de la stratification du territoire, puisque il est supposé que la superficie productive reste constante dans le temps alors qu’elle semble être en lien avec le taux de perturbation.

When forest fires are taken into account during timber supply analyses, planned harvest rates are necessarily reduced to prevent potential timber shortages due to future forest fires. Because fire events are highly unpredictable, forest managers are reluctant to proactively reduce harvest targets, as it results in an immediate revenue loss. We explored a simple but proactive way of including the risks and uncertainties of fire in forest management planning through the identification of low productivity forest areas most vulnerable to fire in two different boreal forest zones. Site index and relative density index were used to estimate the time required to reach different harvesting thresholds based on stem size and tree density. We varied the production objective by using three different thresholds of minimum stem size (dm3/tree) and stand yield (m3/ha) (50 dm3/tree – 50 m3/ha, 70 dm3/tree – 70 m3/ha, 90 dm3/tree – 90 m3/ha). We estimated the time required to reach these thresholds and the proportion of forest zone that could exceed them. Fire cycle length was then used to assess the survival likelihood (probability of reaching the threshold at the stand scale when considering fire risk). An alternative rate of return was also used as an indicator of profit exposure to fire risk. When survival likelihood and alternative rate of return are considered jointly with time-declining interest rates, minimum survival likelihoods need to be higher for longer fire cycles. The proportion of stands vulnerable to fire served to decide whether or not to include fire risk into strategic planning. The identification of major break points in the vulnerability assessment also helped to decide which minimum harvesting threshold is appropriate as a function of the productivity characteristics and fire cycle of the forest under management.