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Joshua Trees (Yucca brevifolia) under rainbow in Joshua Tree National Park, created 1994 with passage of California Desert Protection Act (previously National Monument Est. 1936). 790,636 acres (319,959 ha) or 1,235 sq. mi. Joshua trees are fast growers for the desert; new seedlings may grow at an average rate of 3.0 in (7.6 cm) per year in first ten years, then only about 1.5 in (3.8 cm) per year. Trunk made up of thousands of small fibers and lacks annual growth rings, making it difficult to determine a tree's age. Name Joshua tree from Mormon settlers in the mid-19th century; shape reminded them of a Biblical story in which Joshua reaches his hands up to the sky in prayer. Mojave Desert, Riverside County, CA.