Gap characteristics, as well as forest structure, were investigated in Nothofagus pumilio (Poepp. et Endl.) Krasser stands on two slopes in Tierra del Fuego to test the following hypotheses: (i) there exists an altitudinal decline in tree height and stem diameter, (H) this should result in gaps of decreasing size and abundance towards the upper forest, and (iii) this, in turn, should result in a longer suppressed growth of seedlings near the treeline (that is, an older seedling bank). Mean tree height on both slopes decreased significantly with increasing altitude from 21 to 11 m and from 13 to 8 m. Mean stem diameter declined with increasing altitude from 50 to 28 cm and from 19 to 16 cm, but this trend was only significant on one slope analyzed. Gap frequency declined with increased elevation from 33 to only 3 in 2-km transects. Mean gap size also decreased significantly with increased elevation from 91 to 11 m(2). Mean seedling age varied with altitude, but the expected relationship was significant only when gap frequency was controlled. The best-fit model indicates a negative correlation between altitude and tree height and stem diameter, which are positively correlated and act indirectly on gap frequency via gap size. Although restrictions on seedling growth caused by small and infrequent gaps do not seem to be stronger at upper forest elevations, they may produce a bottleneck in the life cycle of the species in the long term. This phenomenon could be overcome either through faster seedling growth and higher survival at the treeline or through an episodic regeneration, as other research has documented previously.
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