Origin and ecology of the spruce lichen woodlands in the Parc des Grands-Jardins, Quebec: An alternative stable state to the surrounding spruce moss forest

The spruce-lichen woodlands of the Parc des Grands-Jardins (Park), Quebec, Canada, are enigmatic, as they are situated 500 km south of their usual range in the lichen woodland zone found between the forest tundra and the closed boreal forest. Their co-existence within a spruce-moss forest matrix suggests the possible existence of alternative stable states. Two hypotheses have been proposed to explain their origin. The "remnant hypothesis" considers the spruce-lichen woodlands as remnants of the taiga that first occupied the region some 8000 years ago and which since has been slowly reduced in size due to a densification of the forest. The more recent catastrophic "insect-fire" hypothesis considers the lichen woodlands to be regressive spruce-moss forests that developed due to the combined disturbances of a spruce budworm epidemic followed shortly thereafter by a fire. The long-term vegetation and disturbance history of both lichen woodlands and spruce-moss forests along an east-west transect, corresponding to a precipitation and fire frequency gradient, were studied in order to investigate the factors controlling the creation and spatial distribution of the lichen woodlands and determine if they are successional stages or alternative stable states. Paleoecological tools, including the analysis of plant macrofossils, charcoal, head capsules of defoliating insects, and pollen, were used along with modern vegetation surveys to reconstruct the past and present disturbance dynamics of the Park.; No evidence was found supporting the remnant hypothesis, as moss forests previously occupied all lichen woodland sites, with the oldest record starting approximately 8300--9400 cal. BP. Shifts from moss forest to lichen woodland were identified by decreases in spruce macrofossils and the presence of dwarf birch fossils; dwarf birch is found only in open environments. At each site, spruce budworm head capsules were found preceding the charcoal layer delineating the shift to spruce lichen woodland, thus supporting the catastrophic, insect-fire origin of the lichen woodlands. Hiatuses in peat accumulation shortly after or at the time of the catastrophic disturbance indicate that the transformation affected site hydrology in addition to the forest.; A change to a higher fire frequency around 2500 cal BP was evident in the central and eastern sections of the Park, in which we find the lichen woodlands. A lesser fire frequency west of the lichen woodland zone is likely the result of a precipitation gradient caused by an elevated plateau west of the Park. Thus, while the spruce budworm affects the entire region, lichen woodlands are found exclusively within the area delimited by an increased fire frequency indicating that it is the superimposition of these two disturbance regimes that is responsible both for their creation and spatial distribution. Changes in atmospheric circulation patterns were likely responsible for the increase in late Holocene fires, the result being an increased possibility that lichen woodlands would be created as a consequence of combined insect-fire disturbances.; The relatively old dates for the inception of the studied lichen woodlands, which range between 580 and 1440 years cal BP, indicate that these lichen woodlands have not transformed into closed moss forests and have been able to maintain their open structure through time. Their persistence, along with their previous moss forest histories and their current existence adjacent to closed moss forests, indicate that they are alternative stable states to the spruce-moss forests and not a successional stage. In contrast to other examples of reported alternative stables, this one is a result of natural disturbances inherent to the system and not anthropogenic impacts.