The Effects of Recent Floods and Geomorphic Processes on Red Ash Populations, Upper St Lawrence Estuary, Québec

Effects of recent floods on red ash (Fraxinus pennsylvanica Marsh.) forest margins were studied along the upper St Lawrence Estuary in eastern Canada. Major floods amplified by tides left many injuries on riparian red ash trees, which allows dating of past disturbances based on stand structure and dendrochronological analysis. The formation of ice scars on stems, the development of basal sprouts, and the inhibition of population regeneration on shore, provide evidence of a recent increase in shore disturbance. Since the 1950s and especially the 1970s, a landward displacement of the tree line occurred as a result of increasing shore erosion. Usually the ice foot on the shore disintegrates in situ in April, but since the 1950s, early snow-melts in mid-winter have been causing sudden floods that raise the ice foot to the edge of the backshore forest, leaving many signs of damage. Increasing winter climatic variability since the 1950s seems responsible for such variations in flood regime.