Leaf Litter Disappearance Rates in Puerto Rican Montane Rain Forest

Abstract

The time course of leaf litter disappearance of six rain forest tree species was monitored for 32 weeks at the El Verde lower montane rain forest study site in northeastern Puerto Rico. Dacryodes excelsa, Sloanea berteriana, and Drypetes glauca were chosen to represent primary climax species, while Cecropia peltata, Inga vera, and Ixora ferrea were selected to represent secondary successional species in this forest. The study was designed to elucidate differences in nutrient release associated with the successional position of different tree species to determine what properties of leaf structure influenced the observed rates of nutrient disappearance. The string, nonconfined, tethered leaf method was employed. Dry weights and concentrations of nitrogen, phosphorus, potassium, calcium, and magnesium were determined over a period of 32 weeks. The secondary canopy species exhibited slower decay rates than did the primary species. The same pattern emerged between the secondary and primary understory species. The interacting effects of the leaf structural characteristics such as percentage lignin and percentage fiber correlated most strongly with observed decay rates. Nutrients were considered important in establishing organic matter resource quality but did not appear to influence decay rates; instead, nutrient dynamics reflected intervals of element immobilization, mineralization, and importation. Secondary species exhibiting specific combinations of structural properties may act to conserve nutrients by their slower rates of leaf litter disappearance. THE BREAKDOWN AND MINERALIZATION OF DEAD ORGANIC MATTER through the actions of decomposer organisms and fluctuating environmental conditions results in the progressive disappearance of litter from the forest floor. Mineralization and the subsequent translocation of biologically important elements among the various ecosystem components insures maximal reutilization and minimal cycling loss. The study of this process, particularly as it pertains to evergreen tropical forests, contributes to the understanding of nutrient relations in these ecosystems. The present study determined the time course of leaf litter disappearance from six different tropical forest tree species and discusses properties of leaf structure that influence the observed rates. Prior research has centered on specific aspects of nutrient release and the factors affecting the overall rate of litter disappearance. Such contributing factors have included differences among species, season, and location (Wiegert & Murphy 1970), plant material (Jenny et al. 1959, Shanks & Olson 1961, Olson 1963, Cornforth 1970), environmental conditions (Bocock & Gilbert 1957, Nye 1961, Hopkins 1966, John 1973), or microflora and -fauna (Witkamp 1963, 1966, 1969; Witkamp & Crossley 1966; Edwards et al. 1970). Other efforts have considered nutrient release in terms of the structural properties of the decaying substrate. These studies have concentrated on the structural effects and modifications of organic materials during the disappearance process and on the influences that individual substrate constituents might have on decomposer communities and overall decay rates (King & Heath 1967, Bailey et al. 1968, Minderman 1968). Disappearance dynamics provide an insight into the mechanism of nutrient release. Rainfall and temperature in tropical rain forest ecosystems do not affect the decomposition rate of leaf litter (La Caro 1974). Constant rapid rates exist because the biological activity of decomposer organisms is never hindered or inhibited. The rate of disappearance is ultimately dependent on the nature of the organic matter resource. Each plant part, due to differences in chemical and physical properties, will yield different disappearance rates (Odum 1970). Litter resources can be further subdivided to distinguish between different species or forest types. Categorizations distinguishing species by successional life-support strategies may help identify physiological mechanisms in specific ecosystems. Identification of some specific nutrient cycling mechanisms may shed light on the dynamics of nutrient maintenance in tropical rain forest ecosystems. Secondary successional vegetation is characterized as having the ability to live in disturbed forest sites under widely ranging environmental conditions. Secondary species live comparatively short lives, are fast growers, good seed disp rsers, and generally shade tolerant. Secondary vegetation has also been reported to decay at a faster rate than primary because of the differing structural characteristics of their decaying materials (Ewel 1976). In contrast, climax species generally exhibit opposite properties (Richards 1952, Smith 1970). Leaf litter exhibiting a specific combination of structural characteristics that tends to imI Received 8 September 1983, revision accepted 20 August 1984. BIOTROPICA 17(4): 269-276 1985 269 This content downloaded from 207.46.13.149 on Mon, 03 Oct 2016 06:15:48 UTC All use subject to http://about.jstor.org/terms mobilize nutrients for longer periods of time in disturbed forest sites may provide a valuable nutrient conservation function.