Before European settlement, the area was prairie and open parklike forest, maintained by Native Americans via burning. Trees are mainly oak, maple, and hickory, similar to the forests of the Ozarks; common understory trees include Eastern redbud, serviceberry, and flowering dogwood. Riparian areas are heavily forested with mainly American sycamore. By the 1920s most of the timber in the county was harvested. Since that time, large parks and undeveloped areas in the western and southern parts of the county have grown dense forest cover. Old pastures are usually colonized with Eastern red cedar. Most of the residential area of the county is planted with large native shade trees. In autumn, the changing color of the trees is notable. St. Louis County has the most recorded native species of plants in the state, but this is probably due to the intensive botanical research done in the area. Most species here are typical of the Eastern Woodland, but some southern species are found in swampland, and typical northern species survive in sheltered hollows. Invasive species, most notably Japanese honeysuckle, are common in some homesteads converted to parks; these are actively removed.