The Vertical Component of Plant Species Diversity in Temperate and Tropical Forests

Forests are widely observed to contain two or more discrete superimposed strata of plants. In the eastern United States, deciduous forests growing between 30⚬ and 42⚬ north latitude are commonly constructed of three strata: a high canopy of sun-adapted trees, a shade-tolerant herbaceous ground layer, and a midstory of shrubs or small trees, of which the flowering dogwood (Cornus florida) is a prime example. Flowering dogwoods and other woody species occupying ecologically equivalent positions in the forest midstory must depend heavily on sunflecks because up to 80% of the light penetrating the forest interior is this kind of direct-beam illumination. The light field in the forest interior is simply modeled. Immediately beneath the canopy, the light field (integrated over the daily sun cycle) is characterized by a high horizontal variance, because any point is either in a gap and directly illuminated or under a crown and continuously shaded. Farther down in the forest interior the horizontal variance becomes greatly diminished because the expanding cones of light beneath each gap eventually intersect. Where the cones of light from alternate gaps intersect, the light field becomes horizontally uniform. Plants of the woody middle stratum should grow up to, but not exceed, the upper limit of the horizontally uniform light field. The position of the upper limit of the horizontally uniform light field can be predicted from measurements of the canopy and the angular distribution of light gaps overhead. Such predictions for five climax stands in the southeastern United States agreed quite closely with the median height of trees composing the midstory of these stands. Finally, the shapes of tree crowns can be expected to vary adaptively with the systematic latitudinal decrease in the angle of the sun above the horizon. Consequently, tree crowns at high latitudes are narrowly conical in profile, but those in tropical latitudes tend to be planar or shallowly dome-shaped. As a result, direct light is admitted to high-latitude forests only at sharply inclined angles that do not permit the establishment of a woody midstory. By contrast, light is able to penetrate the canopy of tropical forests at relatively shallow angles, allowing the development of a second tree layer closely beneath the crowns of the first. The vertical dimension of plant species diversity thus seems to be controlled by adaptive responses to physical parameters of the latitudinal gradient.