As wetlands are formed and riparian habitats enlarged, aquatic plants colonize the newly available watery habitat. One study in the Adirondacks found that beaver engineering leads to a 33 percent increase in the number of herbaceous plant species on river banks. Another study in semiarid eastern Oregon found that the width of riparian vegetation on stream banks increased several-fold as beaver dams watered previously dry terraces adjacent to the stream. Riparian ecosystems in arid areas appear to maintain more vegetation productivity when beaver dams are present. Beaver ponds act as a refuge for river bank plants during wildfires and provide them with enough moisture to resist such fires. Introduced beavers at Tierra del Fuego have been responsible for destroying the indigenous forest. Unlike many trees in North America, trees in South America rarely regenerate when cut down.