SEEDLING CROWN ORIENTATION AND INTERCEPTION OF DIFFUSE-RADIATION IN TROPICAL FOREST GAPS

In forest gaps, the geometry of canopy openings above a plant determines the spatial distribution of diffuse and direct radiation, which may be received from different sectors of the sky. We examined crown orientation and light interception by seedlings of four species of pioneer trees, Cecropia obtusifolia, Heliocarpus appendiculatus, Piper auritum, and Trema micrantha, transplanted into natural forest gaps at the Los Tuxtlas Tropical Biology Station, Veracruz, Mexico. The four species differed in the number and mean size of leaves, but total leaf area was not significantly different among equal age plants. The mean azimuth and zenith angle of diffuse and direct radiation were determined from hemispherical photographs taken above each plant. Mean diffuse radiation vectors were distributed around the zenith, with a significant clustering in an easterly direction, while mean direct radiation vectors were distributed along the solar track in the southern portion of the sky. The mean orientation of the crown of each seedling was determined from detailed, three-dimensional reconstruction of leaf display using a canopy architecture model. A directional correlation test indicated that seedling crowns were oriented toward diffuse radiation received from the gap, and not toward direct radiation. The mean orientation of the crown, at the whole plant level, was primarily determined by nonrandom orientation of individual leaves. The mechanisms of this differential response to diffuse vs. direct radiation are not known. For diffuse radiation, the total light capture capacity integrated over all sky directions increased with seedling leaf area, while efficiency of capture per unit area decreased. Based on simulations, the light interception efficiency of each plant in its own microsite was 5-25% greater than would be achieved in the sites of conspecifics or in symmetric canopy openings centered on the zenith. These analyses provide a quantitative estimate of the value of nonrandom leaf and crown orientation for whole-plant light interception in natural environments.