Humboldt Redwoods State Park is a world-class forest preserve containing over 53,000 acres, including some of the world's most impressive redwood forests, many miles of river frontage, and nearly 100 miles of hiking, riding, and mountain bike trails. Rockefeller Forest, west of the Dyerville and Founders' Grove area, is one of the largest remaining ancient redwood forests on earth. This 10,000-acre forest is part of the park's 17,000 acres of old-growth redwoods, and almost one in five of the remaining coast redwoods are here. Measured as living and once-lived organic material, Rockefeller Forest has over seven times the biomass of tropical rainforest, more than any other forest. The Visitor Center, 1.5 miles south of Weott, offers information, exhibits, maps, and programs. A pole to the left marks the 33-foot crest of the 1964 flood that obliterated Weott's downtown. Groves throughout the park honor individuals or groups whose donations helped protect these trees. Since 1918, the Save-the-Redwoods League has used private donations, often matched by public park funds, to purchase redwood lands from willing sellers and deed them to local, federal, and state agencies such as California State Parks for redwood parks and preserves. Poison oak, Toxicodendron diversilobum, grows here as a shrub or as a vine climbing into trees; its shiny foliage changes from red to yellowish green in spring, green in summer, and flaming red in fall, and even casual contact or bare winter limbs may cause an itchy rash or blisters.