Variation in carbon isotope ratios of Sempervivoideae species from different habitats of Teneriffe in the spring.

According to carbon isotope ratios, species of the Sempervivoideae from Teneriffe show in general a tendency for increased participation of dark CO2 fixation via PEP-carboxylase in total carbon fixation as habitats become drier and warmer. Certain species are found in cool moist habitats and exhibit C3-like δ13C values. Other species occur in warm dry habitats and exhibit δ13C values which indicate strong Crassulacean Acid Metabolism. A third group of species shows intermediate δ13C values which are more C3-like in cool moist habitats and which indicate increased dark fixation in warmer and drier situations. Included in this group is Aeonium holochrysum, which of the Sempervivoideae of Teneriffe is thought to be most closely related to the common ancestor (Lems 1960). Comparison of CO2 gas exchange of several species under identical environmental conditions reveals differences among species in the ability to regulate CO2 fixation in the light and in the dark which may have arisen in the process of adaptive radiation.