California's native plants are stuck between a rock and a hard place. Hotter temperatures, shorter winters, and drier soils will become fixtures of California's climate. And plants will struggle to weather the shift.
This is not the first time California's flora has faced environmental upheaval. The most recent retreat of the glaciers roughly 10,000 years ago inflicted a comparable magnitude of change. Plants used two main survival strategies to cope. Individuals located near mountain slopes dispersed their seeds to cooler, higher ground, while those that were able evolved characteristics better suited to the new conditions.
The current environmental changes projected to culminate in the next century are of a different stripe. Greenhouse gases are accumulating so rapidly in the atmosphere that many plants aren't expected to keep pace. They won't be able to move their seeds far enough nor evolve their genes fast enough to adjust to new climes.
Species that fall behind will pay a steep price. In 2008, UC Berkeley professor of integrative biology David Ackerly and colleagues found that many of the state's native plant species — including its iconic redwoods, fragrant bay laurels, and day-glo poppies — will disappear from most of their current ranges by 2100. The researchers identified the conditions now favored by more than 2,000 native plants and mapped out where these environments are projected to occur in the future. They found climate warming would displace two-thirds of species from 80 percent of their present locations. No one has any idea what types of plants will take their place. For example, conditions in the Central Valley are expected to become similar to those found currently in the Sonoran Desert of Arizona. But how saguaro cactus, ironwood, and other desert species will travel the hundreds of miles that separate the two areas remains to be seen.
The inability to travel as fast as climate changes leaves slow-growing species in a bad position. While rapid evolution may be possible for annual plants, it is out of the question for such long-lived plants as trees, which measure their generational spans on the order of 50 to 100 years. Instead, survivors will need to rely on traits they already possess.
Plant species that are widespread are most likely to have evolved such adaptations. The shrub toyon, for example, is found from Humboldt County to Baja California. It grows in both dry inland chaparral as well as moister coastal scrub. Plants at the extremities of the range already experience baking summers and icy winters. That suggests some populations currently possess characteristics they will need to tolerate this century's weather trends.
Many examples of such local adaptation are known to have evolved. For example, the herb yerba buena, when found in redwood forests, produces distinctive aromatic compounds lacking in plants of this species that grow in oak woodlands. The trait is thought to help yerba buena repel the hungry banana slugs living in moist forest duff.
Victoria Sork is examining whether such population variation exists in a mainstay of California's Mediterranean ecosystems: valley oaks. "I thought it would be a good idea to study a keystone species and one highly in jeopardy," says Sork, a professor of ecology and evolutionary biology, and dean of the division of life sciences at UC Los Angeles.
Like toyon, valley oaks (Quercus lobata) are found along a broad swath of California, from the Coast Ranges to the foothills of the Sierra Nevada, as far north as Redding to as far south as San Diego. Capable of living 400 to 600 years, California's largest oak is the signature species of grasslands across the state.
Samples from UC reserve system trees have formed the backbone of Sork's research. "Every time I do a rangewide experiment, it's my rule of thumb to sample in every reserve that has my species. The system focuses people on sampling in the same places and leveraging each other's information," Sork says. In this case, her oak studies have involved trees from Sedgwick, Quail Ridge, Blue Oak Ranch, Hastings, McLaughlin, Jepson Prairie, and Stebbins Cold Canyon.
Sork believes that at least a few far-flung valley oak groups carry the genes to handle a warming climate. In previous research, she established the existence of genetically distinct populations of valley oaks up and down California.
She is now studying the variation within 50 genes associated with climate adaptations. These genes are involved in bud-burst timing, osmotic stress, cold tolerance, and other traits. Using next-generation genomic tools, Sork can determine when these genes differ by a single base pair. Such small nucleotide polymorphisms (SNPs) could represent distinct alleles, or versions, of each gene. If certain SNPs tend to cluster in areas with similar climate, that suggests they are associated with functional genes adapted to local conditions.
The past experience of oak populations, Sork predicts, will dictate their future performance. Oaks closer to the coast "might not have the genetic variation to tolerate the tremendous seasonality or high temperatures they might encounter in the future, because they have not experienced them in the past," she says. But all is not lost for the species. "In the Sierra foothills, which we know from genetic work are very different from coastal populations, oaks are already in habitats that can get very, very hot and very, very cold."
Sork plans to verify her SNP findings through traditional garden studies. She will grow acorns sampled from sites throughout the species' range in two sites under dissimilar climate conditions, then document their growth and survival. The experiment will ascertain whether these distinct genotypes translate into observable characteristics, or phenotypes. After all, natural selection acts on phenotypic traits through their interaction with their environment.
If climate-linked variation exists in valley oaks, these findings have major implications for oak conservation. Such genomic information may provide new tools to identify populations that will be more likely to tolerate future conditions. By the same token, the technique can highlight populations that may need help with assisted migration. Biologists and land managers can then prioritize these populations for conservation, acquiring land to establish reserves and migration corridors. Oaks and other native plants could use that help on the rocky road ahead. — KMW