Mangroves are trees that live along tropical coastlines, rooted in salty sediments, often underwater. Like seagrasses, mangroves are flowering plants, but unlike seagrasses, most of the plant lives above water. The upper trunk and all of the branches and leaves of a mangrove tree live completely above the water line, while the lower trunk and the very large system of aboveground roots (called prop roots) are often covered by seawater.