Dynamics of scree slopes in forested environments: processes, modelling, and Holocene evolution

Scree slopes offer an environment conducive to the occurrence of several gravity-driven geomorphic processes, the frequency and magnitude of which are highly variable in time and space. These processes locally prevent vegetation colonization, despite the climatic warming that favors a general progression and consolidation of forest fronts, and this from an altitudinal as well as a latitudinal point of view. The position of the forest front is therefore due, in addition to the climate, to the geomorphological dynamics of mass transfer processes on these steep slopes. Rarely observed in a cold temperate climate, at low altitude and enclosed in the forest environment, this makes the Northern Gaspé region (Province of Quebec, Eastern Canada) unique and complex for studying the interactions between geomorphic processes, talus slope geometry and morphology and forest dynamics during the Holocene.

The mapping inventory of 43 active talus slopes in the coastal valleys of the Northern Gaspé, and their geometric characteristics, allowed the identification of statistically significant variables that explain scree slope morphology in relation to two groups of dominant geomorphic processes, namely: 1) snow avalanches and debris flows, and 2) frost-coated clast flows and rockfalls. The first group is found on East and North facing slopes while the second is found on West and South facing slopes. The variables indicative of high geomorphic activity (active area, altitudinal treeline, Ho/Hi ratio) all suggest increased geomorphic activity on West and South facing slopes. Conversely, the East and North facing slopes are largely stabilized, but occasionally disturbed by debris-flow channels and snow-avalanche corridors. The location and intensity of these disturbances depend essentially on the morphology of the rockwall and the talus slope, in addition to climatic parameters.

The evolution of the West and South facing slopes is more complex as geomorphic processes have continued to disrupt the vegetation colonization, started 7250 years BP, on the slopes where the rockwalls are still active. However, the reasons for the presence or absence of frost-coated clast flows having a notable impact on the sediment budget and the altitudinal treeline remain unclear.