Conifer seedling distribution and survival in an alpine-treeline ecotone

The importance of seedling establishment to the position of alpine-treeline is recognized, yet little is known about factors affecting the survival of seedlings of treeline conifers during their initial years of growth and establishment. This establishment period may have the greatest mortality of all life stages until death of mature trees by disease or fire. Spatial and temporal patterns in the distribution and survival of seedlings of Picea engelmannii and Abies lasiocarpa were evaluated over four years in an alpine-treeline ecotone of the Snowy Range, Wyoming, USA. Seedlings and saplings of both species occurred most frequently near islands of adult trees. For P. englemannii, this appeared partly due to decreased survivorship of young seedlings (< 5 cm height) with greater distance away from tree islands. Survival of emergents of P. engelmannii was 28% greater on the north compared to south sides of tree islands, 48% lower on south-facing slopes compared to other aspects, and 70% greater with overhead cover such as tree branches. Survival of emergents was greater in microsites with grass cover (90% survival) compared to without ground cover (44% survival), but lowest in microsites surrounded, but not covered, by grass (19%). From 1994-1999, natural seedling emergence and survival was highest in 1995 (80% survival of 221 P. engelmannii, and 100% of seven A. lasiocarpa, in a sample area of 432 m 2), when the smallest mean difference in daily maximum and minimum temperatures occurred, and lowest in 1994 (30% of seven P. engelmannii), when above-average temperatures were accompanied by low rainfall and clear skies. The growth seasons of 1994 and 1995 had among the lowest and highest precipitation of the previous 30-year period, respectively. In an artificial seeding experiment, less than 20% of seedlings of both species survived their first complete year of growth. In the autumn of the second year, almost 25% additional mortality was observed in the remaining experimental seedlings when they were exposed to clear, cold skies without the normal protection of snowcover. Both spatial and temporal patterns of seedling survival suggest that exposure to high sunlight may exacerbate low-temperature and water stress in young conifer seedlings, inhibiting their establishment in this alpine treeline.