Most people know that trees tells us stories — about age, climate, and other events — through the rings in their trunks. Did you know that saguaro cactus spines have rings, too? The spines also hold important information about the plant’s physiology and growth processes, information that’s helping geochemists like David Dettman piece together the historical climate record in the Southwest. In this episode Dettman talks about his groudbreaking studies of the saguaro cactus spines and what they are telling us about climate in this region.