Aim Our two main goals are first to evaluate the resilience of the boreal forest according to latitude across the closed-crown forest zone using the post-disturbance distribution and cover of lichen woodlands and closed-crown forests as a metric, and second to identify the disturbance factors responsible for the regeneration and degradation of the closed-crown forest according to latitude since the 1950s.
Location The study area extends between 70°00′ and 72°00′ W and throughout the closed-crown forest zone, from its southern limit near 47°30′ N to its northern limit at the contact with the lichen woodland zone at around 52°40′ N.
Methods Recent (1972–2002) and old (1954–1956) aerial photos were used to map the distribution of lichen woodlands across the closed-crown forest zone. Forest disturbances such as fire, spruce budworm (Choristoneura fumiferana (Clemens)) outbreak, and logging were recorded on each set of aerial photos. Each lichen woodland and stand disturbance was validated by air-borne surveys and digitized using GIS software.
Results Over the last 50 years, the area occupied by lichen woodlands has increased according to latitude; that is, 9% of the area that was occupied by closed-crown forests has shifted to lichen woodlands. Although logging activities have been concentrated in the same areas during the last 50 years, the area covered by logging has increased significantly. Outbreaks by the spruce budworm occurred predominantly in the southern (47°30′ N to 48°30′ N) and central (48°53′ N to 50°42′ N) parts of the study area, where balsam fir stands are extensive. In the northern part of the study area (51°–52°40′ N), extensive fires affected the distribution and cover of closed-crown forests and lichen woodlands.
Main conclusions Over the last 50 years, the area occupied by closed-crown forests has decreased dramatically, and the ecological conditions that allow closed-crown forests to establish and develop are currently less prevalent. Fire is by far the main disturbance, reducing the ability of natural closed-crown forests to self-regenerate whatever the latitude. Given the current biogeographical shift from dense to open forests, the northern part of the closed-crown forest zone is in a process of dramatic change towards the dominance of northern woodlands.