Epiphytic orchids on arborescent Velloziaceae and Cyperaceae: Extremes of phorophyte specialisation

Abstract

Vascular epiphytes are widespread throughout the tropics where they are usually not restricted to particular phorophytes. In contrast to this the trunks of arborescent Cyperaceae and Velloziaceae are regularly colonised by vascular epiphytes with certain orchids showing a remarkably high degree of phorophyte specificity. Arborescent, desiccation‐tolerant Cyperaceae (e.g. Afrotrilepis) and Velloziaceae (e.g. Vellozia, Xerophyta) form characteristic elements of rock outcrop vegetation in the Neotropics and Paleotropics where they frequently occur with mat‐forming species. Their woody‐fibrous stems mainly consist of persistent leaf bases and adventitious roots which possess a velamen radicum. In tropical Africa several species of Polystachya occur in mats and on stems of Cyperaceae (e.g. P. microbambusa on Afrotrilepis pilosa) and Velloziaceae (e.g. P. johnstonii on Xerophyta splendens). In Brazil a higher number of orchid species (e.g. Constantia cipoensis, Pseudolaelia vellozicola, Sophronites brevipedunculata) are largely restricted to the stems of Velloziaceae growing on rock outcrops. Moreover, in Brazil a number of non‐phorophyte specific orchids (e.g. Epidendrum saxatile, Prosthechea vespa) grow on the trunks of Velloziaceae. Orchids restricted to the stems of Cyperaceae and Velloziaceae were derived from non‐phorophyte specific, epiphytic/lithophytic ancestors and originated independently in South America and Africa. The exact nature of the relationship between arborescent Cyperaceae and Velloziaceae and epiphytic orchids growing exclusively on their trunks is not yet clear. In a number of areas both the highly host specific orchids and the arborescent Cyperaceae/Velloziaceae are seriously threatened by human lit fires and commercial collection.