The park is named for the Joshua trees (Yucca brevifolia) native to the Mojave Desert. Originally declared a national monument in 1936, Joshua Tree was redesignated as a national park in 1994 when the U.S. Congress passed the California Desert Protection Act. Encompassing a total of 790,636 acres (1,235.37 sq mi; 3,199.59 km2)—an area slightly larger than the state of Rhode Island—the park includes 429,690 acres (671.39 sq mi; 1,738.89 km2) of designated wilderness. Straddling the border between San Bernardino County and Riverside County, the park includes parts of two deserts, each an ecosystem whose characteristics are determined primarily by elevation: the higher Mojave Desert and the lower Colorado Desert. The Little San Bernardino Mountains traverse the southwest edge of the park.