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) tree seedlings in the alpine treeline ecotone is rare. Seedling occurrence of the dominant conifers Picea engelmannii and Abies lasiocarpa was measured in the treeline ecotone of the Medicine Bow Mountains, Wyoming (central Rocky Mountains, USA), an area composed of elongated toe islands separated by open 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 seedlings.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 correspondence was found between snowpack depth and seedling abundance, with depths that were either too shallow (< 0.5 m) or too deep (> 1.5 m) associated 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 closed forest (22%). Seedling establishment in this portion of the alpine treeline ecotone appears to be occurring at a low rate that differs between the two dominant species and may be strongly influenced by wind-driven snow accumulation patterns. (C) Elsevier, Paris.