Tree regeneration responses to Chusquea montana bamboo die-off in a subalpine Nothofagus forest in the southern Andes

Question: The aim of the present study is to examine how a rare flowering and massive die-off event of a dominant understorey bamboo affected tree regeneration processes in a subalpine Nothofagus forest. In particular, we assess the effects of the die-off event on tree seedling demography (establishment and survival) and on the height growth of previously established juveniles, and we determine how these effects vary in relation to stand age and canopy conditions. Location: Carirrine Pass and Choshuenco Volcano at 1200 in a.s.l. in a mixed subalpine temperate forest in northern Patagonia (40 degrees S, 71 degrees W), spanning the Chilean and Argentinian border. Methods: Regeneration processes in Nothofagus pumilio and N. dombeyi were examined under different canopy conditions (closed or gap), in both young and old stands, and in patches of flowered and non-flowered Chusquea bamboo during the first three growing seasons following the simultaneous flowering and die-off of Chusquea montana f. montana in November-December 2001. We installed permanent plots under all forest conditions and measured seedling and sapling height growth and new seedling establishment following the bamboo die-off. To assess potential variation in tree seed sources, seed fall was collected and seed viability was measured. To assess for potential variation in the understorey light conditions and infer its influence on tree regeneration, hemispherical photographs of the canopy were taken at each plot. Results: Bamboo die-off triggered accelerated growth in height of Nothofagus seedlings and saplings in gaps in old stands. Bamboo die-off did not result in new tree seedling establishment; however, die-off did increase the survival of newly established seedlings. Conclusion: The predominant response of Nothofagus spp. to the bamboo die-off event was the accelerated height growth of previously established juveniles (i.e. a re-organization response). Successful recruitment into the canopy of these relatively shade-intolerant tree species appears to be dependent oil previously existing canopy gaps. It is likely that the release in height of previously established juveniles of Nothofagus after the bamboo die-off will eventually result in development of even-sized and relatively even-aged, patches of canopy trees.