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

Summary 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 ¹³C values. Other species occur in warm dry habitats and exhibit ¹³C values which indicate strong Crassulacean Acid Metabolism. A third group of species shows intermediate ¹³C 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.