Tropical rainforest biome of Biosphere 2: Structure, composition and results of the first 2 years of operation

Abstract

The tropical rainforest biome in the Biosphere 2 mesocosm was managed with rainfall and temperature conditions to simulate a natural rainforest typical of the new world tropics. The establishment of the biome was based on the introduction of excessive numbers of species allowing self-organization of an ecologically unique rainforest. Over 282 species of plants from rainforest areas were planted within the topographically diverse rainforest biome (area of 1900 m², volume of 35 000 m³), just before the Biosphere 2 closure in 1991. Approximately 61% of these species survived when counted in 1993, representing a plant species richness reduction to 172 species in 0.19 hectare. Rank order graphs show that a high diversity community resulted not unlike insular rainforests. The plants of the rainforest mesocosm, however, grew under anomalous conditions of soil (amended desert grassland soil), atmospheric composition (CO₂ up to 4500 ppm by volume (ppmv)) and rainwater composition (high salinity and nutrients). Stem growth rates of a dominant canopy tree, Cecropia, were up to four times higher but had reduced diameter at breast height compared to natural counterparts. Human intervention in plant succession was also an important factor in shaping the ecology of the rainforest biome of Biosphere 2.

Introduction

The tropical rainforest biome in Biosphere 2 was designed to be functionally analogous to planetary rainforests. One of five ecological systems of Biosphere 2 collectively called the ‘wilderness’ (Fig. 1a, b), it was built and its climate managed to emulate the general structure and function of a new world tropical rainforest. There were four design goals of the rainforest: (1) to produce a sustainable system that would help manage the atmosphere of Biosphere 2; (2) to contribute to the diversity of the Biosphere 2 laboratory; (3) to provide occasional food and other extras (cacao, flavors, medicines) to the inhabitants, and (4) to provide an attractive landscape for the inhabitants. The rainforest was excessively species-packed for self-organization over time under minimal influence of human management, with the expectation that new ecosystems would emerge representing ‘signatures’ of interactions among the physical properties of the system and the biota, including human residents. It was hypothesized that the rainforest structure would change over time from that of a recently cleared ecosystem to a complex primary forest (Prance, 1991, Finn, 1996, Silverstone and Nelson, 1996, Atkinson et al., 1999). Details of other areas in the wilderness and the agricultural area are given by Petersen et al. (1992), Finn (1996), Silverstone and Nelson (1996), Atkinson et al. (1999)and Marino et al. (1999).

The conceptual design for the rainforest biome began in 1985; plant collections were made from 1986–1991; planting began in 1989; and a total survey and mapping of every plant was completed prior to the first material closure, which lasted from September, 1991 through September, 1993. A total re-survey of plants in the rainforest was made after the 2 year closure and again in 1996. This paper describes the rainforest area, its habitats, the species planted, and the characteristics of the ecosystem that developed.