Slumping activity and forest vegetation along the northeastern shore of Waskesiu Lake, Prince Albert National Park, Saskatchewan

Abstract

Changes in forest cover along the shore of Waskesiu Lake, Saskatchewan, can be related to slumping. This study evaluates the impact of slumping along the lakeshore on the structure of the vegetation. The oldest trees in the undisturbed forest were approximately 100 years old, whereas the trees which had established on recent scars were typically less than 10 years old. Tree rings were used to determine the ages of the reaction wood in bent trees that had survived the disturbances; this indicated that recent slumping events occurred in 1973, 1979, and 1984. These years either received higher than normal precipitation or had above normal lake levels. Recent slope disturbances were characteristically colonized by a sparse cover of herbaceous plants and tree seedlings which contrasted with the adjacent slopes which supported a mixed forest cover dominated by White Spruce, Balsam Fir, and Paper Birch. Herbaceous ground cover on the slumps was composed of 35 herb and grass species compared to 14 species on the intervening slopes. Similarly, the shrub cover was more diverse on the slumps (9 species) than on the adjacent slopes (4 species). The greatest density of herbs and grasses occurred near the base of the slumps, where soil moisture conditions were more suitable for plant growth. Slumping is most likely caused by subsurface seepage, with annual fluctuations in lake level and seasonal wave activity possibly contributing to the process. As well as causing higher lake levels, wet years can also promote slope instability by increasing the water content of the surficial materials. However, there is no simple relationship between precipitation, temperature, lake level and the occurrence of slumping.