Competition and Growth Form in a Woodland Annual

(1) Distributions of several variables of above-ground growth form, including plant height, vertical distribution of leaf area and vertical distribution of primary and secondary branch length, were compared for crowded (naturally occurring) and uncrowded (naturally occurring and experimentally thinned) populations of Impatiens pallida in south-eastern Pennsylvania. Growth form was examined at the population, individual plant and sub-individual levels. (2) There were major differences in the growth form of crowded and uncrowded plants. Although they were smaller in stem diameter and had less total leaf area and branch length, crowded plants were taller than uncrowded plants. (3) Uncrowded plants had significantly more leaf area, and this leaf area was located lower along the main axis of the plant than in crowded plants. Similarly, uncrowded plants had more, longer and lower branches than crowded plants. Uncrowded plants had more second-order than first-order branches, and the vertical distributions of both first and second-order branches were similar. Crowded plants had very few second-order branches, and the secondary branches were located higher on the plants than the more numerous and longer first-order branches. (4) In uncrowded populations there was a linear relationship between the height of an individual and its total leaf area or branch length, but these relationships were curvilinear or discontinuous for crowded populations. This suggests different patterns of growth for canopy and sub-canopy individuals within crowded stands.