California’s perennially drought-parched Central Valley bears little resemblance to the vibrant landscape of the pre-Gold Rush days, when wild rivers sustained lush woodlands and floodplains teeming with life. 

Trees at the center of these biodiversity hotspots evolved in an arid landscape sculpted by finely tuned exchanges between free-flowing rivers and the shallow aquifers below them. Streamside, or riparian, trees hitched their reproductive cycles to the rhythms of these water bodies, which ebbed and flowed through the seasons and with recurring floods and droughts.

But California’s massive system of dams and canals has robbed riparian trees of the resources they need to regenerate, recent research shows. 

Every chance a tree has to spawn new life is tied to the variable flow of water on the landscape. As late-spring Sierra Nevada snowmelt sends water coursing through rivers, willows, cottonwoods and other riparian species release all their seeds at once. Spring floodwaters deposit their short-lived seeds in shallow bends in rivers or floodplains, where they germinate, and quickly send out roots to follow the water table as it drops over the summer. This reproductive strategy allowed riparian woodlands to propagate and thrive for millennia. 

In theory, these adaptations should also help riparian woodlands weather unpredictable water access under a changing climate. 

But some 95 percent of the Central Valley’s riparian woodlands, along with the conditions they evolved in, have already been sacrificed, mainly to make the Central Valley an agricultural powerhouse. The scattered remnants face multiple threats, including droughts and floods intensified by climate change; manipulated streamflows  that favor human over ecological needs; and shrinking aquifers left critically overdrawn by decades of unregulated groundwater pumping.

Now, researchers warn, these intensively managed water flows may be undermining the resilience of the state’s remaining riparian forests to both climatic changes and the increasing water demands that will come with a warming world. 

Their study, published earlier this month in PNAS, showed that managed waterways allowed riparian woodlands to stay greener later into the summer than those along natural rivers. This happened even in the driest parts of the state where groundwater overpumping has depleted aquifers. But that means the trees are relying on unnatural water flows from conveyance structures, said coauthor John Stella, a professor of sustainable resources management at the State University of New York’s College of Environmental Science and Forestry. And that, he added, is a serious problem.

“It’s not sustainable,” Stella said. “If we ever have to change the water delivery schedule, these trees are going to be left high and dry.”

Messing with Evolution

To understand how managed surface waters affect common riparian species, such as willow, cottonwood and valley oaks, the team drew on five years of groundwater, streamflow and satellite data. Satellite images detect canopy “greenness,” a measure of tree vitality that detects reduced photosynthesis and canopy dieback in response to drought-induced water stress. These datasets allowed the researchers to determine whether greenness varied with groundwater depth across woodlands near natural and managed waterways.

Streamside woodlands should be most sensitive to receding groundwater levels in late summer, when soil is drier and rivers low. As expected, the researchers found a tight connection between canopy health and the depth of groundwater along natural streams. All three tree species showed stress responses at greater groundwater depths.

To their surprise, the researchers also found that artificial subsidies, from agricultural and municipal water deliveries and treated wastewater discharges, disrupted the dependence on groundwater of riparian forests that had been optimized over evolutionary history. 

“We didn’t really appreciate how much the altered streams were influencing the seasonal growth of these riparian woodlands,” said coauthor Michael Singer, a researcher at the University of California, Santa Barbara, and deputy director of the Water Research Institute at Cardiff University in the United Kingdom. 

“It made us think about what happens to these forest trees over the long term,” Singer said. “Are they at risk because they are defying their natural genetic programming, which was set to respond to a declining water table from spring to summer?”

While healthy canopy growth during a drought in regions where groundwater basins are perilously low might seem like a good thing, the implications are ominous. 

Research indicates that faster annual growth may doom trees to an early death, Singer said. “We suspect this may be the untimely fate for these riparian woodlands along streams with altered flow regimes and along artificial canals.” 

Even more troubling, decoupling the trees’ ancient reliance on groundwater from their vegetative growth means there will be no way for dying forests to regenerate. 

The altered flows of water and the channels that straightjacket the rivers limit opportunities for new trees to establish, Singer said. Stream flows are decreased in the spring and elevated in the summer, the opposite of what the trees have adapted to. So when trees release their seeds in the spring, the riverbank lacks the wet sediments they need to germinate in. Plus, structures along riverbanks limit seeds’ access to soil. 

Once the existing trees die off, Singer explained, they probably will not be replaced because the cycle of flows and the natural features of the landscape won’t support the recruitment of new seedlings.

That’s what has happened along the San Joaquin River and its tributaries, said Stella. Canals and other diversion structures have cut off the rivers from their floodplains, leaving nowhere for seedlings to grow.

Even when seedlings do manage to get established along the water’s edge during the summer, when water managers deliver high flows to irrigate farms, they die after those deliveries suddenly stop in the fall and the water drops below their roots.

Although natural systems can be very resilient, said Stella, we’ve undermined the processes that drive riparian forest regeneration by damming major rivers, altering their flows and shackling their banks. “If the mature trees suffer, there isn’t a backup for them to regenerate.”

Losing Biodiversity

Climate change will place even more stress on the few remaining natural woodlands. As the globe warms, earlier snowmelt may disrupt the timing of spring flooding and seed release. 

And if California loses more riparian woodlands, it will also lose their capacity to absorb carbon emissions.

“The Central Valley will sequester a lot less carbon than the Sierras because you don’t have as many trees,” said Stella. “But where you do have trees is in the riparian zone, and they sequester a lot more carbon than the valley’s grasslands and shrublands.”

Stella and his colleagues showed in a 2018 study that riparian woodlands accumulate carbon faster than most other forest types over 30- and 100-year periods.

But what may be even more important, Stella said, is the role riparian trees like valley oaks, cottonwoods and willows play in the ecosystem. “These are massive trees in an ecosystem that has few species of trees,” he said. “They’re what we call a foundation species. If they’re gone, the whole ecosystem unravels because of the number of functions these trees provide.”

California’s remaining riparian woodlands provide critical habitat for scores of the state’s imperiled species, including the red-legged frog, Chinook salmon and Swainson’s hawk. They harbor so much biodiversity they’re known as the rainforests of California. 

Yet for all the ecosystem services these woodlands provide, no single piece of legislation protects them from the diverse impacts that ultimately threaten their survival.

“We protect these areas piecemeal,” Stella said. “But this piecemeal approach is not deliberate nor is it effective in protecting these hotspot ecosystems.”

With more than 90 percent of California’s unique, native freshwater species vulnerable to extinction within the next 100 years, Stella and his colleagues wrote in the PNAS study, “it is imperative that groundwater and surface water are managed to protect the broad ecohydrologic niches within natural riparian environments that support biodiversity and ecosystem function.”

That means adjusting water deliveries to better mimic the natural seasonal flows of rivers and their aquifers, Singer said. “Where possible, modifications to river channels could be made to support recruitment of new riparian forests.”

And groundwater managers need to make sure water levels stay within nine to 16 feet of the surface so riparian trees survive, Stella said. “If we’re drawing the water down far below the surface, they’re not going to make it.”