Scientists at the University of California, Santa Cruz, writing in the journal Evolution, said California, with more than 5,500 native plant species, 40 percent of which occur nowhere else, has more species and more endemic species than any other U.S. state.
California has served as a refuge for plant species that might have gone extinct in other regions during climatic shifts in the distant past, they said.
"It seems that California has been an important refuge for plant lineages for a long time," ecology and evolutionary biology Professor Kathleen Kay said in a university release Wednesday.
"These findings speak to the importance of protecting areas in California so that it can continue to be a refuge for biodiversity in the future."
Scientists had long thought high speciation rates were the cause of the state's plant biodiversity, but the researchers said that rate was no higher than elsewhere.
"Instead, features of California's climate, topography, and latitude have preserved these species, allowing us to see them today, when they may have simply gone extinct if they had arisen elsewhere," study author Lesley Lancaster said.