Forested wetlands in temperate and subtropical climates undergo periodic flooding. One or two tree species dominate the canopy. Trees shape the flow of flood waters, the ground surface, and the understorey, as well as animal habitats. With flooding, complex aquatic food webs support turtles, frogs, fish and birds, but can produce microbial blooms with nutrients flushed from the floodplain. Many vertebrates use these wetlands as refuges during dry times.