Weathering the Heat 

In the decades ahead, as temperatures rise and droughts intensify, Northern California's climate, vegetation, and wildlife may look more like Southern California does today.

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click to enlarge UC Berkeley ecologist David Ackerly and his team discovered that some of California's oldest oak trees are dying. - PHOTO BY MELATI CITRAWIREJA
  • Photo by Melati Citrawireja
  • UC Berkeley ecologist David Ackerly and his team discovered that some of California's oldest oak trees are dying.

In California, an estimated 62 million trees died in 2016, bringing the state's total tree mortality from the five-year drought — which modeling has shown was made worse by climate change — to more than 100 million.

Drought didn't directly kill most of the trees, but it weakened them to the point that they succumbed to beetle infestation. Still, the die-off was massive, and it could prompt a landscape-scale change in the Sierra Nevada's ecosystems, especially if another comparable drought occurs before seedling trees have a chance to become adults.

"This is how plants move — they die off in one place and regrow in others," explained Nate Stephenson, a forest ecologist with the U.S. Geological Survey. "To me, the drought was a preview of what could happen if it continues to warm."

The majority of California's drought-induced tree deaths occurred in the Sierra Nevada's conifer forests, where many species are physiologically adapted to living in soils kept moist through the summer by high mountain snowmelt. It's in these mountains and their foothills where impacts from warming will materialize first, said UC Berkeley's Ambrose. That's because the ecosystems there depend on snowmelt that originates in the highest summits.

"As temperatures keep increasing, the loss of snowpack will mean less recharge of the groundwater they depend on," he explained.

Droughts are natural and frequent events in California. So are fires, which in October killed countless more trees throughout the state. "But when they occur, their intensity is increasing," noted Ackerly. "The low rainfall of a drought is one thing, but to have higher temperatures on top of that means greater intensity."

This means dry spells that California's native vegetation could once withstand will, in a warmer future, push many species past their thresholds for survival. Oaks will die off in low valleys — as Ackerly and his students have seen happen — and conifers will retreat up the mountains. "We're already seeing a pine-to-oak transition as oaks basically move uphill," Ackerly said. "It's a signature of climate change that we see in the fossil record through the ages."

Bill Stewart, a forestry specialist at UC Berkeley's Department of Environmental, Science and Policy Management, said that, during this century, Northern California's landscape will begin, if it hasn't already, to resemble that of Southern California. "It's like we're moving one or two counties south every decade," Stewart said.

Ackerly said the North Bay fires, along with the Sierra Nevada tree mortality and the forest clearing effects of sudden oak death, could also result in new plant communities. "We don't know what recovery will look like," he said. "Some systems might rebound as the same species grow up underneath the dead adult trees. Others might transition into new habitat types."

Williams, at Columbia University, said that, in some areas of the Sierra Nevada, the gaps left by fires and drought-related mortality "are so large that natural reseeding may not be possible. These gaps could take centuries or even thousands of years to naturally recolonize."

Just what recolonization will look like is unclear, he said, because of how quickly climatic conditions are changing. "Temperatures are rising so rapidly that it's likely the same places will burn over and over again in the next few decades," said Williams, who is a California native. "These forests might not have time to recover."

In northern New Mexico, destructive wildfires have transformed conifer-dominated landscapes into ecosystems dominated by shrubs. A drought in the 1950s similarly allowed pinyon pine and juniper to recolonize an area previously home mostly to ponderosa pine.

LeRoy Westerling, professor of Management of Complex Systems and co-director of the Center for Climate Communication at UC Merced, also believes California's burned woodlands could see a dramatic shift in biodiversity. "When the ecosystem begins to recover after these disturbances, they're recovering in a new climate regime, and so they may not go back and reset to what they looked like before," he said.

The loss of moisture also seems to be affecting biomes in the Bay Area. Clark, the ecologist in San Francisco, said "creeping, perennial vegetation" — like California blackberry, honeysuckle and woodland strawberry — has retreated and become less prevalent in Golden Gate Park, the Presidio, and the Marin Headlands. The Labor Day heatwave, he noted, caused an abrupt, widespread die-off.

"There's less moisture in the ground, and it gets used by the trees first," he explained. Such conditions exert "a selective force that directly selects for species that can tolerate drier conditions."

Animal communities are also responding to the changing environment. For example, in San Francisco, the Nuttall's woodpecker — a species well suited to drier conditions — has largely replaced the Downy woodpecker — a species that favors environments more frequently dampened by fog and rain, Clark said.

Other native animals will decline as brush thins and the land grows drier, he explained. "Ecological productivity declines the drier it is," he said. "There is less food, less forage, and less shelter. Animals have fewer places to hide in a simplified environment."

But how dry will California be in 50 or 100 years? Williams said essentially that all climate models agree that the bands of desert that circle the planet at about 30 degrees of latitude will shift poleward with global warming (the East Bay's latitude is about 38 degrees). This could push landscape conditions that we see in northern Mexico into California, where forests would likely dry out, burn up in huge wildfires, and fail to grow back.



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