Abstract

1 Nothofagus pumilio forms an abrupt alpine timberline (AT) at 690 m a.s.l on Balseiro mountain, 54 degrees S, Tierra del Fuego, Chile. Fruit and seed rain (quantity and quality), emergence, density and survival of seedlings were studied across an altitudinal gradient (450-740 m) from within the forest to above the AT. 2 Fruit rain generally declined with altitude but increased between 630 and 690 m before falling sharply beyond the AT. The proportion of seed-bearing fruits, seed viability and seed mass also declined with increasing altitude, but showed no recovery in the vicinity of the AT. 3 Fruit rain decreased exponentially with distance into the alpine zone, so that effective dispersal rarely exceeded 20 m beyond the AT and few of these fruits contained seeds. 4 Seedling emergence and density decreased with altitude both within the forest and into the alpine zone so that seedlings were found no more than 10-20 m above the AT. Even in 1996, when fruit production was high, successful seedling recruitment was limited to lower altitudes. 5 There was little correlation between altitude and the percentage survival of naturally occurring seedlings within the forest. However, transplanted seedlings survived better at the AT itself than immediately inside the forest, and showed high mortality in the alpine zone. 6 The most severe bottlenecks for tree recruitment within the forest appeared to be seedling emergence and seed production. Above the AT, seed viability and emergence were the principal bottlenecks. 7 Although not all demographic variables declined altitudinally, the overall probability of adult establishment decreased with increasing altitude and became very low once the protection by the tree canopy became unavailable. A demographic model explaining the origin and abrupt character of the AT studied is presented.