Processes that cause natural unforested areas in Canadian Rockies below temperature treeline

Abstract

Naturally treeless areas surrounded by closed canopied forests are common below the temperature treeline in mountains. These areas are a result of several possible conditions, e.g. exposed un-weathered bedrock, blockfields, talus slopes, landslides, lithology and others. Here we show a Foothills ridge that is treeless because of its structural geology, lithology, regolith depth, regolith temperature gradient, wind and precipitation, all of which facilitate frost heave that in turn uproots seedlings. The ridge has a SE-NW striking fault that is perpendicular to the direction of the winter wind that blows snow off the ridge. There are nearby seed sources (within 15m) and enough growing season regolith moisture for tree seed germination. However, in the fall and winter the seedlings are uprooted due to regolith movement. The mechanism for this regolith movement is frost heave caused by the ridge's deeply weathered regolith, wind-blown snowless surface that results in a steep temperature gradient near the regolith surface with a close subsurface of near freezing (frozen fringe) followed by an unfrozen zone all in the top 30cm. The snow free ridge is caused by snow transport processes modeled in Liston's SNOWMODEL as a result of the ridge shape and orientation. The Rempel's frost heave process model explains that the cause is the regolith texture with no insulating snow cover resulting in the formation of near surface pump of water from a frozen fringe into a freezing zone at the surface.