The Key tree-cactus (Pilosocereus robinii) is unusual because it is a tall cactus in southern Florida, far from the large tree cactus of Arizona (saguaro) that we are all familiar with. The Key tree-cactus is found on a few of the Florida Keys and in Cuba. It is actually a real tree-like plant with a trunk and distinct branches and may reach 20-30 feet. Though they are large and pretty, the flowers apparently smell like garlic.
The Key tree-cactus is found on tropical hammock communities in the Florida Keys; these are areas of islands that are a bit higher than surrounding habitats. For a variety of reasons, the Key tree-cactus is declining in numbers. By one estimate the numbers of Key tree-cactus have declined by 80% in the last decade. Collection may have played a role in the decline of this species, but habitat destruction is thought to be a more important cause; development of the islands has caused direct loss of habitat but has also allowed salt water to invade cactus growing areas.
Several groups are involved with conservation of the Key tree-cactus and its habitat including the Fairchild Tropical Botanic Garden, the Conservation Fund, and the Center for Plant Conservation. You can read more about the Key tree-cactus here.