This year is the second in what fire officials and climatologists have said is a severe drought. Historically, this May was Santa Clara County's ninth driest in 127 years, according to National Integrated Drought System data.
"As summer progresses, we'll start to see more (redwoods dying) as it gets hotter and drier," new city Urban Forester Peter Gollinger said during a joint interview with outgoing forester Walter Passmore earlier this month.
The decline of the redwoods is not a huge story — yet, Passmore said. But if drought persists, it could be. It generally takes three to five years or more before drought affects healthy redwoods. Weakened trees and those without irrigation would be the first to go.
Gollinger said redwoods make up 1% of Palo Alto's street tree population. There are 923 redwoods under the city's direct care, and about 1,500 private trees that the city prunes to keep utility lines clear. Many more redwoods are located on private property, and he said the city doesn't know about their health.
Looking toward a drier and hotter future, over time, the city's treescape is likely to change inalterably. The drought and climate change likely "won't wipe out but will diminish the population of redwoods to pre-development Palo Alto. There were very few (naturally occurring) redwoods; El Palo Alto is a notable exception," Passmore said.
Palo Alto and the Bay Area have always been marginal places for redwoods to survive in, given that they didn't occur naturally here. Seeds from the city's namesake tree, El Palo Alto, likely floated from the Santa Cruz Mountains down San Francisquito Creek and deposited in the fertile soil bank. Fed and watered by nutrients and creek flow, the young tree was able to flourish and put down roots that took advantage of the moisture.
Today, a less robust El Palo Alto is still holding its own, Passmore said. But it now sports a mister at the top to help spray its leaves with water, a necessary man-made dew fall.
Adapted to coastal environments, the trees rely heavily on fog and dew fall for their water source, making them less resilient to climate change, Passmore said.
In a three-year study of redwood forests in coastal California, U.C. Berkeley biologist Todd Dawson found that redwoods capture tremendous quantities of moisture from fog.
For their own hydration, the redwoods used about 13% to 45% of the fog water for their annual transpiration, according to Dawson's 1998 paper in the journal Oecologia.
With climate change and hotter and drier winds and less rainfall, the redwoods, particularly those living under stress conditions, will continue to fare more poorly, Gollinger said.
A visual scan of the tree canopy over Palo Alto's downtown neighborhoods on Wednesday showed most of the redwoods still looked verdant, but looks can be deceiving. Here and there, tucked into side and back yards, on closer inspection some of the redwoods appear to be faring poorly. Passmore and Gollinger said that's likely due to a combination of issues.
"Rarely is stress or decline related to a single factor. It's almost never one, such as drought or construction. It's usually a series of factors of stress that the tree exhibits in a spiral of decline," Passmore said.
The trees near Caltrain were likely harmed due to construction and vegetation management to clear areas for electrified lines. At Palo Alto Square, the trees are confined to restricted soil spaces and are less able to store water, Passmore noted. Then add drought into the mix, and the trees start to die.
"Even a minor drought will diminish their root systems. Trees adapt to situations slowly. Any quick change is difficult for trees to adapt to," he said.
The redwood's local survival is due in large part to irrigation. Passmore said all trees adapt to their sites by storing water in their root systems. In suburban landscapes, they will grow their roots into irrigation systems and collect water that way. They can use the stored water to compensate for persistent drought.
The city is slowly converting its urban forest to more drought-tolerant trees. To replace the stately redwoods, it is looking to use trees that will maintain a similar stature and form to the redwoods, such as the incense cedar, but which are more drought-tolerant.
There is a way to prolong the redwood's life and help it through an extended drought on residential and commercial properties. Gollinger and Passmore recommend slow, deep watering at the tree's drip line during the summer months: about 30 to 60 minutes of watering each week and additional mulch if the tree is in bare earth, they said.
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