Episodic regeneration at the Nothofagus pumilio alpine timberline in Tierra del Fuego, Chile

I The short- and long-term dynamics of Nothofagus pumilio were investigated across an altitudinal gradient in Tierra del Fuego. Declining probability of establishment of individuals with increasing altitude (spatial restriction) is expected to lead to increasingly sporadic recruitment towards the alpine timberline (AT) (temporal restriction). Longevity of adult trees and difficulties in establishment at altitude should result in a static AT. 2 The number of annual growth buds of seedlings suggests that the species regenerates episodically at all altitudes in cycles of a similar length (7-8 years) to those for seed production. Nevertheless, the seedling density in a year of high regeneration (1996) was considerably higher at lower altitudes than at the AT. 3 Counting of the growth rings of adult individuals revealed a massive episode of establishment between 1800 and 1850 at all altitudes. There was poor establishment between 1920 and 1980. The age range for the entire population fluctuated between 49 and 377 years in 1996. 4 Although there is no evidence for more frequent recruitment at lower altitudes, differential regeneration ensures that establishment there is more probable. 5 Individuals at the AT had an average age of 160 years in 1996, suggesting that the border has indeed remained static for at least a century and a half. 6 The spatial restriction determines that the AT cannot advance until removal of the temporal restriction allows turnover to occur.