Seedling Establishment in Vertical Gradients of Sunlight

Abstract

Although the amount of solar energy intercepted by a community of herbaceous species may be comparable with that intercepted by forest (Monsi & Saeki 1953), the height of the shaded stratum will certainly differ. In grassland the shaded stratum is low and is usually renewed annually by extension of shoots and individual leaves from positions near the ground. In forests, however, the shaded stratum is high and arises by expansion of foliage in situ. Many seedlings grow out of the shaded stratum of grassland within a few days after germination, while in forests shading persists beyond the seedling phase and may extend over many years. Within low vegetation, small differences in height are associated with large changes in intensity, direction and quality of radiation, and establishment may depend upon height, aspect or inclination of the first leaves produced by the seedling. Because vertical gradients are less pronounced near the forest floor, initial growth in height may be unimportant; here tolerance rather than avoidance of shade would appear to have ecological significance. Hence, radical differences in adaptation to shade may be expected between woodland and grassland plants. To recognize mechanisms by which shade is avoided or tolerated we have examined the reaction of a variety of seedlings to standardized gradients of light intensity.