Evolutionary Changes in Crassulacean Acid Metabolism ( CAM ) and Related Traits During the Diversification of Aichryson (Crassulaceae) on the Macaronesian Islands

ABSTRACT Crassulacean acid metabolism (CAM) is a highly plastic photosynthetic pathway with ecological and evolutionary significance, ranging from weak inducible to strong obligate forms. While most Crassulaceae taxa may be capable of performing CAM, the Macaronesian genus Aichryson has not traditionally been associated with CAM. We integrate phylogenetic, physiological, isotopic, anatomical and bioclimatic data to investigate the distribution, plasticity and evolutionary history of CAM and related traits in Aichryson . Our study includes all 15 accepted species, combining over 1100 occurrence records, carbon isotope (δ 13 C) data, nocturnal acid titration and a CAM performance experiment under temperature and drought gradients. Multivariate analyses of bioclimatic variables show clear ecological differentiation among the Azores, Madeira and Canary Islands, with life form strongly associated with climatic niche. Annual species are generally restricted to cooler, wetter climates, while the perennial A. tortuosum lineage, endemic to the arid eastern Canaries, exhibits increased succulence, lower minimum leaf conductance and higher CAM performance. Ancestral state reconstruction of δ 13 C data suggests that the ancestor of Aichryson possessed a predominantly C 3 physiology with low‐level CAM capacity, from which independent shifts towards stronger CAM expression or reversions to predominant C 3 photosynthesis occurred in response to local climatic conditions. Our CAM performance experiment revealed pronounced interspecific differences in nocturnal acid accumulation and plasticity. Some annuals, such as A. bollei , exhibited high CAM inducibility under stress, while others, like A. dumosum, maintained low ΔH + across treatments, likely reflecting relaxed selection in mesic habitats. These physiological traits align with environmental niche and life history, supporting two main strategies: fast‐growing annuals with flexible CAM and slow‐growing perennials with more constitutive CAM and investment in leaf longevity, cuticular properties and water storage. These findings support a ‘CAM continuum’ and highlight the roles of ecological differentiation and climatic filtering in shaping CAM evolution. Aichryson emerges as a model system for understanding CAM plasticity and the interplay between photosynthetic pathways, life history and insular biogeography.