Climatic niches provide insights into the evolutionary origins and ecological significance of the succulent CAM syndrome around the world

Abstract

Although distributed globally, plants possessing the succulent syndrome are thought to have evolved to adapt to arid climates, because they possess modifications that increase their water use efficiency. Here we study the evolution and the ecological nature of the succulent CAM syndrome at a global scale by analyzing the climatic niches of succulents within the Caryophyllales, testing the hypothesis of a climatic niche specialization by comparing them with their non-succulent, non-arid adapted relatives. We assembled and carefully curated a worldwide dataset of 5447 species in 28 families, and analyzed the current and evolutionary trajectories of climatic niches with an array of statistical methods including ecological niche modeling, phylogenetic regression and divergence dates estimation. Our results confirm the Core Caryophyllales tend to inhabit drylands probably since their origin in the Early Cretaceous. However, the succulent syndrome appeared later with some lineages diversifying profusely afterwards. The climatic niche of succulents is not differentiated from their non-succulent relatives, but narrower, and contained within the non-succulents, showing no relationship with extreme conditions such as high aridity or temperatures. Our results support alternative interpretations of the origin of the CAM syndrome and the ecological significance of succulence, as well as the prolific radiation of richest lineages.