Resumen

In a site in northern Peru, montane rain forest forms an upper timberline on steep slopes and another timberline lower down on valley bottoms. Both timberline forests had 7-m-tall canopies, but other features were very dissimilar. The upper timberline forest had few stems, relatively low basal area, and little regeneration, while the lower timberline forest was markedly different in species composition and had abundant populations of seedlings and saplings, and large basal area. Data from intervening plots along an elevational transect clarified that these contrasts originated in the differential presence and abundance of the tree and shrub species, some of which were limited to edge habitats on either or both of the timberlines, and others that regenerated in interior forest but only within narrow elevational ranges. These patterns may be typical of tropical timberlines in areas of glacially modified topography.