Abstract
Variations in the architecture and axis structure of different growth forms of Nothofagus pumilio (Poepp. et Endl.) Krasser (Nothofagaceae) were studied along an altitudinal gradient. We evaluated the primary growth and branching pattern of annual shoots belonging to the main axis categories (trunk, main branches, and secondary (short) branches) of four growth forms developed at different altitudes (1200–1600 m a.s.l.) in northern Patagonia: (i) low shrubs at the high-altitude timberline, (ii) high shrubs at high altitude, (iii) low trees at mid-altitude slopes, and (iv) high trees at low-altitude single-species forests. Shoot structure and branch distribution along shoots were similar for all axis categories and growth forms. Significant differences between axis categories were found regarding the sizes of shoots, which decreased from trunk to secondary/short branches. For each axis category, shoots of similar size differed between growth forms in stem slenderness and internode length. The extent of differentiation between axis categories is key in the structural distinction between growth forms of N. pumilio.