Leaf Decomposition Rates in Costa Rican Lowland Tropical Rainforest Streams

Whole leaves of five species of riparian plants and five species of upland plants were placed in a tropical stream in a wet, lowland, Costa Rican forest. Leaves of the five riparian species disappeared faster than did the leaves of the upland species. Whole leaves of four species of riparian plants were placed in three streams and on the forest floor. Two species, Ficus glabrata and Trema micrantha, decomposed within 32 days; Ardisia sp. and Pithecelobium longifolium did not decompose over that period. For the two species that decomposed, decomposition occurred at a slower rate on the forest floor than in a stream 50 m from the forest floor site. Leaves of one species, F. glabrata, decomposed faster in water in the wet than in the dry season. Decomposition rates were similar for leaves of F. glabrata and T. micrantha placed in three streams during the wet season. Results differ markedly from previously published results for tropical streams. Differences in the dynamics of allochthonous leaf nutrient inputs into heterotrophic mid-latitude and tropical streams are discussed. BECAUSE STREAMS AND RIVERS running through forested watersheds receive very little sunlight, nutrients from primary production of aquatic origin contribute little toward the accumulation of basic nutrients by which the biogeochemical cycle operates. Nutrient inputs come primarily from ground water, tributaries, precipitation runoffs, or from terrestrial insects and plants parts, especially leaves, falling into the water (Boling et al. 1975, Fittkau 1969, Cummins 1974). In addition to serving as nutrient input sources, leaves serve as substrata for aquatic invertebrates. Consequently, considerable literature has developed concerning the role leaves play in heterotrophic mid-latitude streams (Peterson and Cummins 1974, Reice 1974, Davis and Winterbourn 1977, Kaushik and Hynes 1971, Cummins et al. 1973, Iverson 1973). In tropical areas where soil nutrients are very low (Janzen 1974), essential elements from soil and rain are concentrated by both terrestrial and streamside emergent plants (Howard-Williams and Junk 1977). In lowland tropical areas studied, leaves have the highest content of total elements of all plant parts (Klinge and Rodrigues 1968, Stark 1971a, b). It is probable that plant tissue, especially leaves, provide heterotrophic streams with a source of essential element unattainable to any major degree by runoff. As such, the roles that any plant parts play in tropical rainforest streams may be accentuated, in particular, in tropical environments low in soil nutrients. Not only do all plants concentrate essential elements from nutrient-poor soils, but some defensive compounds produced by secondary plant metabolic pathways are found at higher levels within leaves from plants grown on nutrient-poor soils (Janzen 1974, Davies et al. 1964, McKey et al. 1978) than in plants on good soils. Defensive compounds, in general, may be allelopathic to decay organisms or may be more difficult to break down by those organisms than other nutrient-rich compounds in the plant