Granitic and gneissic outcrops (inselbergs) as centers of diversity for desiccation-tolerant vascular plants

Although desiccation tolerance is common in non-vascular plants, this adaptive trait is very rare in vascular plants. Desiccation-tolerant vascular plants occur particularly on rock outcrops in the tropics and to a lesser extent in temperate zones. They are found from sea level up to 2800 m. The diversity of desiccation-tolerant species as measured by number of species is highest in East Africa, Madagascar and Brazil, where granitic and gneissic outcrops, or inselbergs, are their main habitat. Inselbergs frequently occur as isolated monoliths characterized by extreme environmental conditions (i.e., edaphic dryness, high degrees of insolation). On tropical inselbergs, desiccation-tolerant monocotyledons (i.e., Cyperaceae and Velloziaceae) dominate in mat-like communities which cover even steep slopes. Mat-forming desiccation-tolerant species may attain considerable age (hundreds of years) and size (several m in height, for pseudostemmed species). Both homoiochlorophyllous and poikilochlorophyllous species occur. In their natural habitats, both groups survive dry periods of several months and regain their photosynthetic activity within a few days after rainfall. Other desiccation-tolerant species colonize shallow depressions, crevices and even temporarily water-filled rock pools on inselbergs. Desiccation-tolerant vascular plants occur in 13 families and are best represented within the monocotyledons and ferns. Only a few desiccation-tolerant dicots exist, in the Gesneriaceae, Myrothamnaceae and Scrophulariaceae. In total, about 330 species of vascular desiccation-tolerant plants are known, of which nearly 90% occur on inselbergs. With regard to morphological adaptations, the mat-forming monocotyledons are particularly remarkable due to the possession of roots with a velamen radicum, which is reported here in the genus Borya for the first time.