In subalpine forests, extreme dry slab avalanches with fracture heights between 0.8 and 1.5m, corresponding to slope angles from 35 to 45° in the starting zone, and mean return periods of 30 to 300y, may start in openings with downslope lengths of as little as 30m and widths of 15m. Larch stands near timberline have an open structure with tree distances often exceeding 15m. Larch stands only slightly affect the formation of the snow cover. Additional support of the slab is limited to distortions of the stress field by the not very numerous stems. Therefore formation of extreme avalanches is not significantly hindered. Dense spruce stands, particularly if multilayered, effectively prevent formation of extreme slab avalanches. If the avalanche flow is not braked on a distance downslope from the fracture line of 30 to 60m, depending on slope angle and fracture height, standard trees with diameters ≤ 0.3m will be broken. The minimum extents of weak and super-weak layers for slab formation (25 and 5m, respectively), as well as the ranges of typical supports to the new snow slab scale with slab thickness. Dense spruce stands significantly affect snow distribution and radiation balance, and therefore the formation of weak layers.