A POSTFIRE SHIFT FROM LICHEN SPRUCE TO LICHEN TUNDRA VEGETATION AT TREE LINE

The environmental changes associated with a fire-induced shift from old-growth lichen-spruce krummholz to lichen-tundra vegetation have been evaluated at a tree line site in northern Quebec. Tree ring and growth form patterns of black spruce (Picea mariana) remains lying on the ground in a lichen-tundra community were used to reconstruct, within a 4600-m2 quadrat, the structure of a conifer stand at the time of the bum (approximately AD 1750). The prefire spruces were the last members of a long regenerative sequence leading to maintenance of a lichen-spruce stand formed after a bum approximately 1700 yr BP. At this time postfire recruitment was most likely facilitated by favorable climatic conditions. Before the 1750 fire event the krummholz was predominantly maintained by layering, due to severe climatic conditions at least since the beginning of the Little Ice Age (approximately AD 1580). The site was deforested by the 1750 fire, because of the limited regenerative potential of stunted spruce. Postfire spruce exclusion has been responsible for major environmental changes associated with a thinner snow cover. Along the border of the deforested site snow cover depth was controlled by living spruces, whereas postfire shrubs (mostly dwarf birch, Betula glandulosa) of the lichen-tundra stand were unable to trap drifting snow. A significant decrease in thickness of the snow cover following deforestation was deduced from a comparative analysis of stem morphology of prefire spruces and present snow conditions. Inception of gelifluction lobes and mudboils was also caused by a change in the soil thermal regime associated with a reduced snow cover. Our results confirm the hypothesis that several lichen stands of the forest-tundra are postfire communities succeeding from degraded conifer stands during cold periods of the late Holocene.