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 for
est 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 altitud
e, 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 the
se fruits contained seeds.
4 Seedling emergence and density decreased with altitude both within the fo
rest 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 fore
st, and showed high mortality in the alpine zone.
6 The most severe bottlenecks for tree recruitment within the forest appear
ed to be seedling emergence and seed production. Above the AT, seed viabili
ty and emergence were the principal bottlenecks.
7 Although not all demographic variables declined altitudinally, the overal
l 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 s
tudied is presented.