The diverse diaspora of CAM: a pole-to-pole sketch.

This article provides a comprehensive review of Crassulacean acid metabolism (CAM) photosynthesis, a water-conserving carbon fixation pathway present in about 5% of vascular plants across diverse lineages including angiosperms, gymnosperms, ferns, and lycophytes. CAM plants inhabit a wide range of global environments—from arid deserts to moist forests, and from sea level to high elevations—and exhibit varied life forms such as perennials, annuals, epiphytes, and aquatics. The review highlights the phylogenetic diversity, biogeography, and evolutionary history of CAM across multiple plant families, noting that most extant CAM lineages diversified after the Oligocene/Miocene in response to global aridification and declining atmospheric CO2. It also discusses the variability in CAM expression, including strong CAM, facultative CAM, and C3 + CAM intermediates, and emphasizes gaps in understanding the order of biochemical and anatomical trait evolution, as well as the ecological and evolutionary roles of CAM in different plant groups.