Under the worst-case scenario, plant diversity would decrease everywhere by as much as 25%, and 66% of all species unique to California would suffer more than an 80% decrease in range.
In the most optimistic scenario, under which governments move to rapidly decrease greenhouse gas emissions globally, and plant species prove able to move into new habitats, diversity might increase along the state's northwest and central coasts, the study concluded.
But even under this scenario, many species would disappear from Southern California and the Northern Sierra.
The authors steered clear of predicting specific extinctions.
"If a plant loses 80% of its range and goes from 100 to 20 square kilometers, it is hard to say if that plant is extinct or not," Loarie said. "In a hot year, that plant's gone."
Native plants often support 10 to 50 times as many species of native wildlife as nonnative plants, and biologist Philip Rundel, a California plant specialist at UCLA, noted that the effects measured by the study "will surely be paralleled by what we can expect to occur with animal species."