In plants with rhythmic growth, a branch segment extended in one event is known as growth unit (GU). GU structure, resulting from the resources allocated to stem length, volume and mass, and to leaf area and mass, is relevant for understanding branch functioning in the context of plant development. This study compares GU structure between main branches and short branches positioned at low and high positions on nursery-grown trees of three closely related genetic entities: Nothofagus nervosa, N. obliqua and natural hybrids between these species. GUs of short branches, compared to those of main branches, had lower length, diameter and number of leaves, and higher specific leaf area (SLA), stem density and proportional mass in leaves than in stems. GUs at high position on the trees had a higher proportion of their mass in stem than in leaves and a lower SLA than those at low position. Stem density was higher for N. nervosa and the hybrid trees than for N. obliqua. Most other GU traits did not differ statistically between the considered genetic entities. The three genetic entities exhibited distinct patterns of variation in leaf size with leaf position along main-branch GUs. The individual tree had a significant effect on most variables. GU structure would have a major ontogenetic component and would play a relevant role in the architecture of Nothofagus species and their adaptation to different environmental conditions.

• Species distributed along wide environmental ranges are expected to be either plastic or adapted to local optima. The elucidation of which of these alternatives prevails, is crucial in planning breeding and conservation strategies for not yet domesticated species. Austrocedrus chilensis (Cupressaceae) is the most commercially important conifer of the temperate forests of Argentina and the target of a domestication program. A steep precipitation gradient characterizes its Argentinean range. • Variation within and differentiation among four natural populations of this Patagonian cypress representative of two contrasting precipitation regimes (>1 300 and 330 mm per y) were assessed by analyzing several morpho/architectural traits in one-year-old seedlings grown in a greenhouse. • Progenies from one of the two xeric populations did not differ from those corresponding to both humid-site populations. The two most variable populations in terms of additive genetic variance belonged to contrasting precipitation regimes. Differentiation among populations was low as measured by each and every variable (average Q st = 0.088). Morphological traits related to the main axis size would have a dubious adaptive meaning. • The results suggest that the Patagonian cypress would have not evolved genetic pools adapted to local optima, and thus it appears to be a phenotypically plastic species, at least regarding growth at juvenile age.

Summary: Growth and branching in Nothofagus alpina and Nothofagus obliqua (Nothofagaceae) under different light conditions. Architectural traits of entire main branches and their growth units (GUs) were assessed for trees of Nothofagus alpina and N. obliqua developed at three sites with different levels of natural shading (low: roadside, intermediate: nursery, and high: understory). The hierarchical structure of main branches was evaluated by means of: (1) the ratio between the diameters of the secondary branches and that of the parent main branch, and (2) the angles formed between secondary branches and main branches. Understory trees had GUs with less leaves and a thinner and shorter stem compared to those of trees at the less shaded sites. These differences were less notable for more recently-extended GUs. Branching systems became less hierarchical as they aged, but were not significantly different in their hierarchical structure among sites. Main branches of understory N. alpina and N. obliqua trees may develop for years by producing small and scarcely branched GUs. Main branches of N. obliqua trees tend to develop less hierarchical branching systems and to be more negatively affected by shading than those of N. alpina.Key words: Architecture, branching pattern, hierarchies, growth unit, morphogenetic gradient, Nothofagus alpina, Nothofagus obliqua, shade tolerance.Resumen: Se compararon las características arquitecturales de ramas principales completas y de sus unidades de crecimiento (UC) en árboles de Nothofagus alpina y N. obliqua desarrollados en tres sitios con diferente nivel de sombreo natural (bajo: banquina, intermedio: vivero, y alto: sotobosque). Se evaluó la estructura jerárquica de las ramas principales mediante: (1) la relación entre el diámetro de las ramas secundarias y el de la rama principal portadora y (2) los ángulos formados entre ramas secundarias y ramas principales. Las ramas principales de los árboles del sotobosque presentaron UC más delgadas y cortas y con menos hojas y ramas que aquellas de los árboles menos sombreados. Estas diferencias disminuyeron para UC extendidas más recientemente. Con el incremento de la edad de los sistemas de ramificación, se observó una disminución en el desarrollo jerárquico de los mismos, pero no se evidenciaron variaciones notables en su jerarquía entre sitios. En ambas especies, las ramas principales de los árboles de sotobosque pueden desarrollarse por años produciendo UC cortas y poco ramificadas. Las ramas principales de N. obliqua tienden a ramificarse en forma menos jerárquica y a perder vigor en forma más notable al desarrollarse en la sombra que las de N. alpina.Palabras clave: Arquitectura, gradiente morfogenético, jerarquías, patrón de ramificación, Nothofagus alpina, Nothofagus obliqua, tolerancia a la sombra, unidad de crecimiento.