Epiphyte Biomass and Nutrient Capital of a Neotropical Elfin Forest

Abstract

The epiphyte communities of a Costa Rican cloud forest make up a conspicuous portion of the canopy, especially on large canopy dominants. Non-destructive sampling methods were used to assess the composition, biomass, and nutrient concentration of live and dead epiphytes on representative host trees to determine the mineral capital contained in the epiphyte components of the standing vegetation. Epiphyte standing crop on a single large Clusia alata (Guttiferae) tree is 141.9 kg. The nutrient capital (in g) is: N = 3062; P = 97; K = 678; Ca = 460; Mg = 126; Na = 207. Using information on forest structure and epiphyte distribution, stand-level estimates of epiphyte mat nutrient capital were made. Although epiphyte biomass constitutes less than 2 percent of total elfin forest ecosystem dry weight, the nutrients they contain are equivalent to up to 45 percent of nutrients contained in ecosystem foliage of similar ecosystems. Assessment of epiphyte nutrient capital gives a more complete and accurate idea of the aboveground vegetation pools, and supports the idea that epiphytes may play a greater role in ecosystem nutrient dynamics than has been previously considered. ALTHOUGH THE IMPORTANCE OF MINERAL NUTRITION for plants and animals has been recognized for centuries, only recently has there been a systematic approach to mineral element cycling in entire ecosystems (Pomeroy 1970, Jordan et al. 1972, Golley et al. 1975). Estimates of the total amount of mineral elements and the rate of elemental cyding within a complete landscape unit during a period of time have resulted in the formulation of several basic mineral cycling concepts (Rodin and Bazilevich 1967, Jordan and Kline 1972, Bormann and Likens 1979). The general approach has been to divide the ecosystem into a series of compartments, or pools, and to measure the quantity and chemical composition of each pool and the pathways and rates of flux between each component in as great detail as possible. A small number of ecosystemlevel studies (e.g., Ovington and Madgwick 1959, Cole et al. 1968, Woodwell and Whittaker 1968, Duvigneaud and Denaeyer-DeSmet (1975), an even smaller number of which were tropical (e.g., Nye 1961, Odum and Pigeon 1970, Golley et al. 1975, Grubb 1977, Cole and Johnson 1978, Jordan 1982), have been important in the development of theories of community stability, nutrient use efficiency, and ecosystem resilience. Because of the greater diversity and complexity of tropical forests, and the greater logistical problems encountered there, the sizes of many of the compartments that make up tropical ecosystems remain poorly known. In most whole-ecosystem studies, epiphytes-plants deriving support but not nutrients directly from their host trees-have been discounted or ignored, as their biomass was considered insignificant in proportion to other forest components. However, vascular and non-vascular epiphytes make up a conspicuous portion of many rain forest canopies, reaching their greatest diversity and abundance in neotropical cloud and elfin forests, which are regularly enshrouded in mist and lack a prolonged dry season (Richards 1964, Sanford 1968, Madison 1977, Sugden and Robins 1979). Although the epiphytic flora of these forests has attracted a good deal of botanical attention, most of it has been focused on aspects of taxonomy (Dressler 1979, Benzing 1981a), phytosociology (Eggeling 1947, Sanford 1968, Johnson and Awan 1972, Russell and Miller 1977, Madison 1979, Sugden 1981, Yeaton and Gladstone 1982), and physiology (Hosokawa and Odani 1957, Medina 1972, Benzing and Ott 1981, Huber 1978). These canopy-dwelling plants must overcome greater extremes of insolation, temperature, humidity, and wind than their terrestrial counterparts. They lack organic connection to the bank of nutrients and water stored in forest soils. Resources are pulse-supplied from atmospheric sources, and canopy surfaces may be characterized by frequent and/or prolonged deprivations of moisture and nutrients (Benzing 198 1b). Many aspects of epiphyte morphology, physiology, and life history contribute to their efficiency at garnering and retaining airborne nutrients (Benzing 198 1b, 1982). By virtue of their powers of mineral accretion and their location along primary nutrient flux routes, epiphytes can be major participants in the impoundment and movements of mineral elements in a forest ecosystem (Nadkarni 1983). The effects of epiphytes on ecosystem-level interactions have been investigated in only a few studies, all in temperate forests (Denison et al. 1972, Pike 1972, 1978, Lang et al. 1976, 1980, Schlesinger and Marks 1977, Benzing and Seeman 1978). Assessing the mineral IReceived 3 November 1983, revised 3 February 1984, accepted 7 February 1984. BIOTROPICA 16(4): 249-256 1984 249 This content downloaded from 157.55.39.104 on Sun, 19 Jun 2016 06:38:59 UTC All use subject to http://about.jstor.org/terms capital contained in epiphytes is an important first step in determining their role in rain forest nutrient dynamics and in obtaining a more complete and accurate picture of all aboveground components. Minerals contained in their living and dead tissues constitute a nutrient pool, distinct from host tree minerals, which are immobilized for some span of time within the canopy. These can be transferred to other ecosystem pools via litterfall, crown wash, and in some cases, by host tree canopy root systems (Nadkarni 1981). In this paper, as part of a comparative study of within-canopy nutrient dynamics in temperate and tropical rain forests, I assess the composition, biomass, and nutrient content of epiphyte communities on large canopy trees in a neotropical elfin forest, compare the nutrient capital contained in epiphytes with other ecosystem components of similar forests, and discuss some of the processes by which epiphytes accrue and retain their nutrient capital. Some of the terms used in this paper have multiple or ambiguous meanings and are defined here: biomassthe dry weight of living matter present in a given plant community; standing crop-the dry weight of living and dead components of a given plant community; epiphyte mat-the composite unit of living arboreal plants and their associated detrital matter found within host tree