Tree recruitment at the Nothofagus pumilio alpine timberline in Tierra delFuego, Chile

Cuevas, JG

Jg. Cuevas, Tree recruitment at the Nothofagus pumilio alpine timberline in Tierra delFuego, Chile, J ECOLOGY, 88(5), 2000, pp. 840-855

74

INGLESE

Article

Environment/Ecology

JOURNAL OF ECOLOGY

0022-0477 → ACNP

88

5

2000

840 - 855

ISI

0022-0477(200010)88:5<840:TRATNP>2.0.ZU;2-U

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.