Physiological evolution and association between physiology and growth form in Aeonium (Crassulaceae)

Aeonium (Crassulaceae) comprises species that display a remarkable degree of morphological diversity, including rosette trees, candelabrum shrubs, highly branched shrubs, and woody rosettes. Greenhouse studies have documented that the genus is also diverse in basic photosynthetic metabolism, with C₃, CAM, and C₃-CAM intermediates present. However, the degree of photosynthetic diversity present in natural populations and between species has never been surveyed. We analyzed stable isotopes of carbon from field-collected species of Aeonium and confirmed that these species are diverse physiologically. Using these data, each species sampled was coded as CAM, C₃, or intermediate. This trait was optimized using parsimony onto one arbitrarily selected cladogram for Macaronesian Crassulaceae that had been previously published. These analyses indicated that the ancestral physiological condition for the entire Macaronesian clade of Crassulaceae is C₃ photosynthesis. Within Aeonium, one subclade consists largely of species with obligate CAM, one is exclusively intermediate, and two subclades are more variable in photosynthetic metabolism. The ancestral character state for the Aeonium clade is equivocal when the intermediate carbon isotope values were coded as a third character state; however, when these intermediates are considered polymorphic (i.e., facultative CAM), C₃ photosynthesis also was ancestral in Aeonium. Concentrated changes tests were conducted to explore associations between CAM and three growth-form attributes, including the candelabrum shrub and rosette tree growth-forms and monocarpy. These tests revealed no significant association between CAM and monocarpy. The candelabrum shrub growth-form is also not linked with CAM, but CAM has evolved twice within taxa displaying this growth-form. In contrast, the hypothesized correlation between CAM and the rosette tree growth-form was neither rejected nor supported.