Thornscrub Biome

The wilderness area of Biosphere 2 was designed to represent a gradient from a humid equatorial to arid subtropical climates and vegetation, and to maintain the maximum possible biodiversity. Consistent with these goals, the thornscrub area was designed to simulate the arid limit of the winter-dry, summer-rainy savanna biome, and to contrast with the desert biome, which was designed to have cool-season rainfall and intense summer droughts.

The thornscrub was intended originally to be part of the savanna biome, but its design goals and history have progressively distanced it from the rest of the savanna. The position of the thornscrub next to the desert biome required that it be divided into two parts: the Upper Thornscrub above the scrubber room and the Lower Thornscrub between the scrubber room and the freshwater marsh. Upper Thornscrub soil simulates a relatively mature deposit derived from volcanic parent material which might be found in Sonora, Mexico. Most of the Lower Thornscrub soil simulates a stabilized coastal dune deposit typical of southern Sonora and Baja California, Mexico, or southwestern Malagasy (Madagascar). Simulated volcanic talus cones were built along the west side of the Lower Thornscrub to create extra habitat for animals and a logical transition to the artificial rock cliffs below the Upper Thornscrub.

Plants were collected from the vicinity of Alamos, Sonora, near the boundary between Sinaloan Thornscrub and Sinaloan tropical deciduous forest. Because of their availability, most Biosphere 2 thornscrub plants are Sonoran species. Bursera grandifolia (torote mulato), Jatropha cordata (torote de vaca), Fouquieria macdougalii (palo adan), and some Erythrina flabelliformis (coral bean) trees up to 6 m tall were dug from wild stands, transported with bare roots, and planted into the Biosphere 2 thornscrub. Other species were brought in as nursery-grown seedlings or smaller transplants from Sonora. Several thornscrub plants from Malagasy and other areas were available in the succulent plant trade, and these were added to increase biodiversity. Examples are Alluaudia ascendens, Uncarina sp., Xerisicyos danguyi, and Aloe vaombe.

Currently the vegetation canopy structure has developed to resemble prototype sites, and the woody plant community seems able to resist potential invasions of savanna grasses. Biodiversity appears to be high; although there have been no formal comparisons with other thornscrub sites.