Development of a lichen-white spruce stand along the rapidly emerging coast of Hudson Bay in subarctic Quebec

Development of a lichen-white spruce stand along the rapidly emerging coast of Hudson Bay in subarctic Quebec. Population structure and tree growth of a white spruce stand (Picea glauca [Moench] Voss) were studied along the emerging coastline of Manitounuk Strait on the east coast of Hudson Bay. Isostatic rebound in this area is still one of the most rapid in the world (1.2 to 1.5 m/century). The study aimed to determine the relation between the development of a shoreline shrub fringe and the expansion of white spruce and to decipher the environmental changes that led to a scattered tree population over the last century. The colonization of the raised shore by white spruce is closely dependent on the earlier development of a dense shrub fringe. The shrubs form a narrow strip just above the tidal flat, where the substrate is maintained moist by the seepage of ground water from adjacent lands. In winter, shrubs collect the wind-blown snow and protect the abondant white spruce seedlings locally. With the rapid land emergence, the watertable is lowered and the hygrophilous shrub fringe displaces slowly seaward. The degradation of shrubs change the snowpack depth at the position of the formerly established spruces. Only the most vigourous saplings survive. A scattered population withstand this severely exposed environment. Its only possibility for further expansion is behind permafrost mounds that soon develop on surfaces exposed to the wind and behind raised beach ridges. The raised topography favours the accumulation of snow behind obstacles and white spruce takes advantage of such protected sites to expand. The steps of the colonization process are highlighted by the distribution of shrubs and trees according to their age, the white spruce shoot elongation rates and tree-ring widths, and the recent vegetation changes indicated by the plant macrofossil remains since the end of the 19th century.