Changing Alpine Treeline v12

The alpine treeline ecotone (ATE) is an area of transition high on mountains where closed canopy forests from lower elevations give way to the open alpine tundra and rocky expanses above. Alpine tundra is an island biome and its ecotone with forest is subject to change, and like oceanic islands, alpine tundra is subject to invasion - or the upward advance of treeline. The invasion of tundra by trees will have consequences for the tundra biome as invasion does for other island flora and fauna. To examine the invasibility of tundra we take a plant's-eye-view, wherein the local conditions become extremely important. Among these local conditions, we find geomorphology to be exceptionally important. We concentrate on aspects of microtopography (and microgeomorphology) and microclimate because these are the factors that matter: from the plant's-eye-view, but we pay attention to multiple scales. At coarse scales, snow avalanches and debris flows are widespread and create "disturbance treelines" whose elevation is well below those controlled by climate. At medium scales, turf-banked terraces create tread-and-riser topography that is a difficult landscape for a tree seedling to survive upon because of exposure to wind, dryness, and impenetrable surfaces. At fine scales, turf exfoliation of the fronts of turf-banked risers, and boulders, offer microsites where tree seedlings may find shelter and are able to gain a foothold in the alpine tundra; conversely, however, surfaces of needle-ice pans and frost heaving associated with miniature patterned ground production are associated with sites inimical to seedling establishment or survival. We explicitly consider how local scale processes propagate across scales into landscape patterns.