For the last two centuries American elm and ash, which both belong to the ancient Elm-Ash-Cottonwood Bottomland ecosystem, achieved distinction as North America's two most popularly planted urban species, used primarily for their superior survival traits and slowly maturing 180–300 year majestic natural beauty. Today used as living national monuments, the National Park Service is protecting Thomas Jefferson's 200-year-old planted example, and George Washington's 250-year-old white ash which has a 600-year possible lifespan. Green ash had been widely used as a primary ornamental and long lived monument tree until the elm fad of the 1880s, and regained top position once again after Dutch elm disease arrived.