Seedling occurrence in alpine treeline conifers: A case study from the central Rocky Mountains, USA

Information concerning the occurrence of very young (1- to 10-year-old) tre e seedlings in the alpine treeline ecotone is rare. Seedling occurrence of the dominant conifers Picea engelmannii and Abies lasiocarpa was measured i n the treeline ecotone of the Medicine Bow Mountains, Wyoming (central Rock y Mountains, USA), an area composed of elongated toe islands separated by o pen meadows (ribbon forest) that grade into the closed forest. No seedlings were found on the windward sides of tree islands, while a mean of 0.6 seed lings.m(-2) occurred on the leeward (downward) sides. These values compared to the 4.2 seedlings.m(-2) in the closed forest. In addition, a strong cor respondence was found between snowpack depth and seedling abundance, with d epths that were either too shallow (< 0.5 m) or too deep (> 1.5 m) associat ed with fewer or no seedlings. A. lasicarpa seedlings made up much less of the overall seedling population in the ribbon forest (6%) than in the close d forest (22%). Seedling establishment in this portion of the alpine treeli ne ecotone appears to be occurring at a low rate that differs between the t wo dominant species and may be strongly influenced by wind-driven snow accu mulation patterns. (C) Elsevier, Paris.