Abstract
Two species of Ephedra: E. trifurca and E. torreyana inhabit shrub and grassland habitats in the northern Chihuahuan Desert. E. torreyana is limited to black grama grasslands where grasses are taller than the shrub. E. torreyana is heavily browsed by vertebrates and E. trifurca is browsed during some years. We established an experiment with cylindrical exclosures that excluded rabbits and rodents, rabbits but accessible to rodents, for comparison with E. torreyana plants available to all herbivores. Plants accessible to all vertebrate herbivores were significantly smaller with shorter stem lengths than plants in exclosures. We concluded that E. torreyana in black grama grassland are largely hidden from vertebrate herbivores and that intense herbivory reflects the degraded state of the study site which makes the E. torreyana evergreen shrubs apparent to vertebrates.
Highlights
The genus Ephedra (Grymnospermae; Gnetales, Ephedraceae) is composed of approximately 50 species in arid and semiarid ecosystems worldwide [1]
In a livestock enclosure around the grassland, the average leaf length of E. torreyana was 18.4 ± 7.4 cm [4]. In this black grama grassland we identified three potential vertebrate species that were able to access the interior of the exclosure: the pronghorn (Antilocapra americana), the black-tailed jackrabbit (Lepus californicus) and the desert cottontail (Sylvilagus auduboni)
In 2008, green stem lengths of the plants exposed to all herbivores were essentially the same as those of E. torreyana stems on plants in the black grama grassland on the Armendariz Ranch, while the stems of plants protected from rodents and rabbits were longer than those reported by [6] (Figure 1)
Summary
The genus Ephedra (Grymnospermae; Gnetales, Ephedraceae) is composed of approximately 50 species in arid and semiarid ecosystems worldwide [1]. Two species (Ephedra trifurca and E. torreyana), which are common in the northern Chihuahuan Desert of the USA, produce dry winged cone bracts and the seeds are wind dispersed. The Ephedra species in the northern Chihuahuan Desert developed from two pairs of sister species distributed in southwestern North America: E. californica-E. trifurca and E. torreyana-E. viridis and probably occurred in the Late Miocene to Pliocene [2]. They concluded that genetic and climatic changes documented for these regions related to the expansion of arid lands, contributed to the diversification in North American Ephedra, rather than adaptations to new climatic conditions
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