Nutrient Scavenging of Rainfall by the Canopy of an Amazonian Rain Forest

Fluxes of nutrient elements entering the forest canopy by rainfall are compared with fluxes to the forest floor by throughfall, for two tropical rain forest types growing on soils of low nutrient content in southern Venezuela. In contrast with other forests, total yearly rainfall fluxes of calcium, sulfur, and phosphorus were greater than corresponding throughfall fluxes. For other elements, rainfall fluxes were occasionally greater than throughfall fluxes. We hypothesize that these nutrients are intercepted in the canopy by algae and lichens growing on leaf surfaces, resulting in nutrient conservation in this nutrient-limited ecosystem. THE RAIN FOREST WHICH COVERS much of the central and eastern portion of the Amazon Basin grows on soils low in essential nutrient elements (Sioli 1975, Klinge and Ohle 1964). Despite the low nutrient content of the soils, the forests appear large and vigorous. The question of how the forests survive in the nutrient-poor environment has interested ecologists for many years, and it has been hypothesized (Richards 1952) that nutrient-conserving mechanisms evolved in these ecosystems which have enabled these forests to survive and grow in this nutrient-poor environment. In order to study the structure and function of the Amazon rain forest, including nutrient-conserving mechanisms, an ecological project has been established near San Carlos de Rio Negro, Venezuela, lat. 20N, 67?W (Medina et al. 1977). The area is within the northern part of the drainage basin of the Amazon River. Some of the initial results of the project have shown that there is virtually no nutrient input to the ecosystem from the mineral soil and subsoil (Klinge et al. 1977, Stark and Jordan 1978, Stark and Spratt 1977). This finding means that atmospheric input must be a major source of nutrients for the central Amazon forests. In this paper we report that nutrients are filtered out of the rainwater as the water passes through the forest canopy, and we hypothesize that this interception is an important nutrient-conserving mechanism for the Amazonian