The Sierra Nevada of California and Nevada is the longest continuous mountain range in the United States. Covering about 20 percent of California's land base and part of western Nevada, it offers an exceptional variety of topographic relief and environmental conditions that allow for a high diversity of plant species, many of them endemic to the range. The Sierra contains over 50 percent of California's total flora, approximately 405 plant taxa native to the Sierra, and 218 taxa considered rare.