Two-thirds of California's unique plants, some 2,300 species that grow nowhere else in the world, could be wiped out across much of their current geographic ranges by the end of the century because of rising temperatures and changing rainfall patterns, according to a new study.
The species that cannot migrate fast enough to higher altitudes or cooler coastal areas could face extinction because of greenhouse gas emissions that are heating the planet, according to researchers.
California's flora face a potential "collapse," said David Ackerly, an ecologist at UC Berkeley who was the senior author of the paper. "As the climate changes, many of these plants will have no place to go."
Half of the plant species that are unique to the continental United States grow only in the Golden State, from towering redwoods to slender fire poppies. And under likely climate scenarios, many would have to shift 100 miles or more from their current range -- a difficult task given slow natural migration rates and obstacles presented by suburban sprawl.
The study, published Tuesday in the peer-reviewed on-line journal PLoS One, is the first to analyze the effect of climate change on all of the plants unique to one of the world's most biologically diverse areas. Previous models have focused on fewer species in areas such as the eastern United States, Europe, South Africa and Australia.
"The climate is changing 10 times faster than it did during the last ice ages," said ecologist Scott Loarie, who has a doctorate from Duke University and who conducted the study over five years with Ackerly and other collaborators. "The first thing we need to do is to reduce the pace of change."
The study, which was based on more than 80,000 specimens, was hailed as groundbreaking by leading scientists in the field. "It is a timely analysis of the likely fate of the plants of California in the face of climate change," Peter Raven, president of the Missouri Botanical Garden and coauthor of seminal texts on California flora, said in an e-mail.
And in Southern California, given water shortages and habitat disruption, he added, "lots of the populations are right on the edge. . . . The balance could easily be tipped so we could lose many of them in a very short period of time."
As California's unique species migrate, they could be separated from the creatures that pollinate them. Animals could be divided from the plants on which they depend, the researchers noted.
"Individual plants can't pick up and fly away like birds," Ackerly said. "A seed grows into a tree. Then the adult tree drops another seed, which can be carried by the wind or an animal. And that seed grows into another tree."
The state may also have to set aside new refuges and corridors, and prepare to move some plants if necessary. "Planning for plant refugees will become a new but important concept for natural reserves to think about," said biologist Brent Mishler, director of the University and Jepson Herbaria at UC Berkeley, the state's most important flora collection.
The study is likely to add urgency to a decades-long movement to protect the state's flora. The California Native Plant Society, which has 33 chapters, warns that less than 10% of the state's original coastal sage-scrub land and less than 1% of its native grassland remain intact.
But the paper foresees even more dramatic changes. Coast redwoods may range farther north, it said, while California oaks could disappear from Central California in favor of cooler weather in the Klamath Mountains along the Oregon border. Many plants may no longer be able to survive in the northern Sierra Nevada or in the Los Angeles Basin.
It also predicts that plants of northern Baja California will migrate into San Diego County ranges. Meanwhile, the Central Valley could become the preferred habitat for plants of the Sonoran Desert.
And what would replace Southern California's native plants? "We don't know what will move into the void," Loarie said. "Possibly desert plants similar to those in Nevada and Arizona, but more likely unpleasant agricultural weeds."
Coauthor Katharine Hayhoe, a Texas Tech University scientist who serves on the United Nations' Intergovernmental Panel on Climate Change, prepared projections under a scenario of a relatively rapid rise in global temperature of 3.8 to 5.8 degrees Celsius by the end of the century, and under a conservative estimate of 2.3 to 3.3 degrees Celsius.
The study looks at eight scenarios that used different rates of warming and of species mobility. Loarie cautioned that there were uncertainties in the analysis, such as the known range of individual plants, the precise microclimate each plant prefers, and the magnitude of predicted changes in rainfall patterns.
"But there is a clear trend," he said. "The climate is outpacing these plants."