As the climate heats up, those iconic crooked oaks stand to lose nearly half their range by the end of the century. Much of the remaining suitable habitat likely will shift northward, leaving a very different California in its wake and threatening the work of conservationists.
Many of the state's native trees and plants, from the towering redwood groves along the coast to the alpine forests that frame hulking granite domes in the Sierra, could be evicted from the territory they have evolved to occupy for thousands, if not millions, of years.
Climate modelers foresee alpine forests shrinking, pine forests being replaced by a mixture of evergreen trees, woodlands and shrublands giving way to grasslands and deserts expanding.
The relocation of each species could have ramifications well beyond a single tree or flower moving to new territory. Each plant is part of an intricate network of relationships between different species in an ecosystem. If one tree leaves, other plants and animals that depend on that tree for food or shelter may be forced out as well.
Some plant species have already started relocating to the north and to higher elevations in search of the climate they are adapted to.
"Some species can live in a broad range of ecological niches, but most can't," said Lisa Sloan, an earth scientist and paleoclimatologist at UC Santa Cruz.
The blue oak, a native California tree found nowhere else, is one of those that can't.
"These need particular temperature and precipitation conditions, and not just annually. It's very important for their growing season that in the spring there are certain conditions so that acorns can sprout and so on," Sloan said.
Oaks losing ground
The blue oak currently thrives on a broad swath of land along the flanks of the coast ranges and the Sierra Nevada that circles the great valley like a bathtub ring. They are particularly well-adapted to the hot, dry summers and cool, wet winters in these areas.
Sloan's research, published in the Proceedings of the National Academy of Sciences in 2005, suggests the oaks may be headed for trouble as California warms up.
Previous studies using global computer models to project future climate showed a 19 percent decline in the oaks' range by the end of the century with unrestricted greenhouse gas emissions. But these models produce average results over large areas, and in a state with so many mountains and valleys this doesn't provide a very realistic picture.
So Sloan used a more detailed regional climate model that showed more intense and faster warming at higher elevations, an effect that was lost by the global models.
The higher resolution produced a much grimmer result: The blue oaks could lose 41 percent of their range, and the closely related valley oak could suffer a 46 percent reduction. Around a third of the trees' remaining acreage would be north of their current habitat, in territory not currently inhabited by the oaks. The trees largely will be gone from the southern and central portion of their territory.
And there is no guarantee that the trees will be able to get to these new areas.
Californians have worked hard to preserve the state's natural heritage, one of the most biologically diverse in the world. National, state and county parks, as well as marine protected areas, have been painstakingly pieced together and defended during decades against pollution, development pressure, poaching and other misuses.
But global warming is a far more formidable foe threatening to make these places unsuitable for the plants and animals they were intended to protect.
"That's going to add an extra layer of complexity to conservation," Sloan said. "For all the agencies and people buying up land for conservation of the red-legged frog or the blue oak or whatever, they need to think about, will the climate in that region in the future be hospitable to that species? And the other thing is do they have migration corridors to get from plot A to plot B?
"We mean well, but do we know what we're doing?"
Redwoods on the run
Oaks aren't the only trees that could be in trouble.
Climate change biologist Lee Hannah of Conservation International is studying the future of more than 30 endemic California tree species using three different climate models and several different scenarios of future greenhouse gas emissions.
According to Hannah's research, most tree species will suffer the greatest range losses under the business-as-usual scenario in which future emissions are unrestricted. But even the more conservative scenarios caused some trees to lose ground by 2050.
"Many of the species are responding as we would expect by moving upslope and northward," Hannah said.
The lodgepole pine, common in the higher elevations of the Sierra Nevada, could take a particularly big hit. In some scenarios, the model predicts the pine will lose most of its range. In the most extreme scenarios, there is no more suitable habitat for this tree left in California.
"That's a species that's already being hit by beetles," Hannah said.
The pine beetle is busy decimating a variety of pine species throughout the intermountain west, leaving a trail of brown, dying trees behind.
In addition to reducing habitable territory, the higher temperatures probably also will help the beetles. Longer summers mean more generations of beetles will live and die each year, putting more trees a risk.
"Climate change can definitely ramp up these pest attacks," Hannah said. "We're seeing literally millions of acres and different species being wiped out from Colorado up through Canada."
Hannah's work shows that many tree species likely will decline as the temperature rises, and others won't be as affected, and a few could even see their ranges grow a bit. But it's clear that the warming predicted by the climate models will be beneficial for very few of the trees at best, he said.
Some of the most vulnerable species are those that are already fairly rare, particularly those in the higher elevations such as the Monterey pine and the bristlecone pine.
"If they're in a tiny area, it only takes a small change to wipe them out or push them off the top of a mountain to where there's no suitable climatic area anymore," Hannah said.
Another emblematic California tree that could be in jeopardy is the coastal redwood. Hannah's models show the range for the trees retracting northward significantly. The groves along the coast near the Bay Area, including those in Santa Cruz, could disappear by the end of the century if greenhouse gas emissions are not curbed.
Neither Hannah's nor Sloan's models is good at predicting the fate of the coastal fog the redwoods depend on.
In fact, the fog is the reason the trees are able to grow so tall. They have developed the ability to absorb moisture directly from the fog and can get as much as 40 percent of their water this way. Without this adaptation, the trees would not be able to move water from their roots all the way to their tops.
And without the fog, the towering giants might not survive.
Although the models don't capture the fog's future, it's likely that the fog season will become shorter. Some models predict a shortening of the upwelling season when cold, deep ocean water is brought up to the surface near the coast.
The fog is by and large a function of upwelling. And less upwelling may mean less fog, which could mean fewer redwoods.
"Here in Santa Cruz, without the redwoods there are no banana slugs," Sloan said. "And what is Santa Cruz without banana slugs?"
The slugs, which are prone to dry out, depend on the redwoods for shade and moisture that collects from the fog and drips down to the forest floor. The trees in turn depend on the slugs to eat plants that could otherwise outcompete young redwood saplings. This is just one example of how shuffling California's trees and plants could have a cascade effect on many animals, insects and other living things.
Timing is everything
Another consequence of a warming climate may be to throw many different plants together that had not previously co-existed as they try to populate new territory. Species may have to compete with one another for limited resources.
Shifts in space probably will not be the only changes that bring new competition for species. Many plants have life cycles that are triggered by climate cues. For example, as temperatures rise, spring conditions arrive earlier in the season, causing some plants to flower ahead of their typical schedule. If every species does not shift its schedule in precisely the same way, plants that normally do not flower at the same time may find themselves competing with each other for resources such as attention from pollinating insects.
Those insects could be in short supply if they are not able to reschedule their life cycles to match that of the plants they depend on and that depend on them.
"Animals are really finely attuned to the seasons, and they rely on resources appearing at different times throughout their feeding and breeding and migration cycles," said ecologist Elsa Cleland of the National Center for Ecological Analysis and Synthesis at UC Santa Barbara. "And so when plant timing starts getting off, that can really negatively affect animals."
Cleland's research suggests that warmer temperatures earlier in the year will not be the only thing to trigger changes in timing. The picture could be far more complex.
In addition to its greenhouse effect, adding carbon dioxide to the atmosphere could more directly affect some plant species.
"We were really surprised to find that species responded in opposing directions to elevated CO2," Cleland said.
She has been studying the effect of increased carbon dioxide on plants for eight years at Stanford University's Jasper Ridge Biological Preserve. On a two-acre plot, she exposed the flowers and grasses to higher levels of CO2 as they are accustomed to with a ring around the perimeter designed to emit the gas from whichever side is upwind of the plants.
Many studies have focused on the effects of higher temperatures and found that most species respond in the same way, by getting started earlier in the year. But Cleland found that CO2 had the opposite effect on the grasses that dominate the California landscape.
Even when she used heat lamps to raise the temperature as well, the CO2 caused the grasses to flower later. But the wildflowers reacted to the combination by flowering earlier. The work was published in the Proceedings of the National Academy of Sciences in September.
"If the grasses shift their timing and they're starting to flower later, that really makes them overlap in this time window when generally the wildflowers are active in the California grasslands," Cleland said. "And that's really where the diversity lies in California grasslands is with these little wildflowers, and so that's one additional way we can expect that these grasses could be pushing them out."
With the delicate balance that plants and animals have established with each other over thousands, and even millions, of years being upset in a far, far shorter time period, many species will face multiple pressures to which they may not have the chance to adapt.
On longer time scales, these types of pressures encourage evolution of new adaptations and new species. But at the rate these changes are occurring today and likely will continue to occur in the future, the result could be extinctions rather than evolution.
For a biologically diverse place like California, this could leave the landscape looking dramatically different in just a few decades.
Betsy Mason covers science and the national laboratories. Reach her at 925-847-2158 or firstname.lastname@example.org.