Distribution and dynamics of balsam fir (Abies balsamea [L.] Mill.) at its northern limit in the James Bay area

A regional assessment of balsam fir distribution along a transect between 49 degrees 30' N and 53 degrees 30' N suggests that balsam fir exists as isolated populations scattered across large expanses of Quebec's boreal forest. At its northern limit, balsam fir is primarily associated with environments that have a low fire frequency, such as the coastal lands along James Bay and the shores of major hinterland water bodies. Detailed mapping of balsam fir populations, stratigraphical study of supporting organic soils, and age-structure data of forest stands around Duncan Lake (53 degrees 30' N, 78 degrees W) constituted the core of the local analysis of balsam fir ecology at its northern limit. The presence of macrofossils in a representative organic soil profile of the spruce-fir forests of the Duncan Lake area suggests that balsam fir is present in the region since 3830 years BP. Its scarcity in the peat profile indicates a low population density during much of the site history. Age-structure analysis indicates the four spruce-fir forests studied in the Duncan Lake area are old-growth, dating back to at least the 17th or 18th centuries. Balsam fir behaves like a late successional species, appearing 100-150 years after black spruce in each site. Layering started during the 19th century, and became the principal means of regeneration for both species during the 20th century. The correlation between age frequency distribution and negative exponential model estimates ranges from 0.57-0.93 For balsam fir, and 0.93-0.95 for black spruce. The distribution of residuals along the regression slope suggests a period of low black spruce recruitment between ca 1860-1930; this was followed by a sharp increase until ca 1970. Balsam fir followed a similar, though delayed, pattern. The results suggest that marginal spruce-fir communities in the Duncan Lake area have experienced accelerated layering during the 20th century. Because few dead fir trees were found in the field, this may represent a long-term increase in fir population densities. Increased snow fall since the turn of the century may have favoured the growth of Sphagnum and the rooting of basal branches; this has resulted in a continuous increase in population density.