The creation of alternative stable states in the southern boreal forest, Quebec, Canada

The southernmost spruce-lichen woodlands in the Parc des Grands-Jardins, Quebec, Canada, are situated 500 km south of their usual range in the northern lichen woodland zone. Their co-occurrence within a spruce-moss forest matrix suggests the existence of alternative stable states. We investigate the possibility of these spruce-lichen woodlands as an alternative stable state along with the factors contributing to their origin and spatiotemporal distribution. Analysis of plant rnacrofossils, charcoal, head capsules of defoliating insects, and pollen were used along with vegetation surveys to reconstruct the past and present disturbance dynamics along an east-west transect, corresponding to a precipitation and fire frequency gradient. At each site, spruce budworm head capsules were found preceding the charcoal layer delineating the shift to spruce-lichen woodland, demonstrating the compound disturbance (insect-fire) origin of the lichen woodlands. Moss forests previously occupied all lichen woodland sites, with the oldest record starting ca. 8300-9400 yr BP. A change to a higher fire frequency around 2500 yr BP was evident in the lichen woodland zone of the park. A lower fire frequency west of the lichen woodland zone likely is a result of orographic precipitation. While the spruce budworm affects the entire region, lichen woodlands are found exclusively within an increased fire frequency zone. Thus, it is the superimposition of these two disturbance factors that is responsible both for their creation and spatial distribution. Lichen woodland inception dates range between 580 and 1440 yr BP, demonstrating that these lichen woodlands have maintained their open structure with time and have not transformed into closed forests. Their persistence, along with their previous moss forest histories and current occurrence adjacent to closed moss forests, indicate that they are an alternative stable state to the spruce-moss forests and not a successional stage. In contrast to other examples of reported alternative stable states, this one is a result of natural disturbances inherent to the system and not anthropogenic impacts.