LONG-TERM SURVIVORSHIP AND CROWN AREA DYNAMICS OF TROPICAL RAIN FOREST CANOPY TREES

Abstract

Lateral shading of direct-beam irradiation among neighboring canopy tree crowns in a nonequatorial tropical rain forest canopy was modeled as a function of solar position using a photogrammetric database derived from large-scale color aerial stereopairs (1:1500–1:3000 scale) acquired in 1976. The interception of direct-beam irradiation by the orthogonally projected crown area of each tree was computed at hourly intervals over a full calendar year using a Parameterization model of cloud-attenuated direct-beam availability. The annual totals of intercepted direct-beam irradiation (Ib) ranged from 1.81 to 4.13 GJ·m−2·yr−1. Expressed as a percentage of the available incident direct-beam irradiation, these values ranged from 44% to 100%. Approximately 20% of the sample population intercepted <70% of the available annual direct-beam irradiation. The long-term effects of lateral shading and the intertree differences in Ib were assessed using repeat aerial stereophotography of the same section of forest 18 yr later in 1994 for the determination of the mortality, survivorship, and crown growth of the canopy trees delimited in the 1976 stereopairs. Mortality over the 18-yr period amounted to 27.2%. Based on the lateral shading simulations, the mean annual Ib totals of the survivors and those that died were significantly different (P < 0.001). Approximately 40% of the survivors experienced crown area reductions. Although there was no significant difference in the Ib of survivors with crown growth and those with crown reductions, a relationship was established between Ib and the extent of crown area change. Canopy trees that intercept the most direct-beam irradiation and experience the least lateral shading have higher probabilities of survivorship and significant crown area changes that may be in the form of crown growth or crown reduction. Their laterally shaded neighbors have a lower survivorship probability, and those that survive persist in an inhibited state with limited crown area change. We conclude that the effects of lateral shading are not limited to the margins of treefall gaps and that lateral shading determined by crown position in the uneven upper canopies of nonequatorial tropical rain forests has a detectable effect on the long-term fates of neighboring canopy trees.