That is a warning from Canopy, the nonprofit organization that is the recognized guardian of the "urban forest" of Palo Alto, long known as "the city that loves trees" — if not their leaves that clog storm sewers and roots that lift sidewalks.
The concern relates to a relatively high level of salinity in Palo Alto's recycled water from the regional wastewater treatment plant. In addition to Palo Alto effluent, the plant processes sewage from Stanford, Mountain View, Los Altos, Los Altos Hills, Menlo Park and East Palo Alto. A small fraction of the wastewater is reclaimed and used for watering lawns and planting areas, including hundreds of trees.
At 950 parts per million (ppm), the recycled water falls a bit short of being sea water, at about 35,000 ppm. But over a decade or two does the water harm the trees — the longest-lived plants on which it is used? That is the question that worries Canopy and tree fans.
Fewer than a thousand trees are presently irrigated with recycled water, most of them at the Palo Alto Municipal Golf Course and Greer Park. About 400 trees in Greer Park are irrigated and roughly the same number at the golf course and treatment plant. Mountain View's Shoreline Links golf course and multiple commercial sites use recycled water.
A plan to replace more than 30 ailing, dying or dead trees at Greer Park elevated the salinity concern to a higher level when city arborists and Public Works staff members met with worried (and some angry) residents about the plan Oct. 18.
The staff did a remarkable job of answering resident concerns during a discussion at the Palo Alto Friends meeting hall. The meeting culminated in an informal poll of attendees, who indicated unanimous support for the staff plan following a tree-by-tree illustrated run-down of why each tree should be removed.
But Canopy remains worried about long-term effects on other trees as use of recycled water spreads to other parts of the community. There is a continuing push to expand its use to conserve the pure Hetch Hetchy water from the Sierra.
Catherine Martineau, Canopy's executive director, sums up the problem: "Plants, unlike animals, cannot process salt. Trees are particularly sensitive. The issue is complex, as soil composition is also a factor.
"Very few tree species are tolerant of recycled water, even less of the quality of recycled water available in Palo Alto," she said.
The city's Public Works tree section "has been removing and replanting hundreds of trees in the park in the last 15 years," Martineau said.
The issue is a growing one, she warns.
"As water-conservation measures apply pressure on the availability of potable water for landscape irrigation, efforts are made to deliver recycled water to more areas of town and the Stanford campus." An environmental impact report on extending the recycled-water pipeline is already underway (www.cityofpaloalto.org/depts/utl, search for "recycled water system").
"Canopy's concern is that increased use of recycled water will threaten large portions of the Palo Alto urban forest because a majority of the species planted in the last 100 years need irrigation during summer.
"Trees are the most valuable elements of the landscape and their health should be a priority when making policy decisions regarding the landscape," she said.
Efforts are underway to transition the "urban forest" to less-thirsty, "drought resistant" trees — such as the thousand trees Canopy and volunteers planted along the sound walls of East Palo Alto in 2007 and 2008. Once mature enough, in three to five years of growing deep roots, they won't need extra watering.
Palo Alto's Urban Forest Master Plan embraces that concept.
Prompted by a City Council directive two years ago (the "Salinity Reduction Policy") Palo Alto is moving to lower the salinity level, according to Phil Bobel, assistant director of Public Works. The policy calls for reducing salinity in recycled water by a third, to about 600 ppm.
Bobel is skeptical of the threat, however: "I would estimate that the recycled water itself has had a very limited negative impact on the (Greer Park) trees' health," he said. "More likely to be a major factor are poor soil conditions, given the park sits on former salt marsh."
He said the salinity is higher than in some cities (but comparable to many) partly because pipelines that bring wastewater to the plant from Mountain View and East Palo Alto, and within Palo Alto itself, go through existing or former salt marshes. The older pipelines allow seepage in from the salt-water-laden soils.
The three cities are collaborating to reduce the "reverse leakage" problem, either by lining pipelines, or repairing or replacing pipelines.
Public Works staff members are even conducting an experiment within the 25-acre treatment-plant site at the end of Embarcadero Road. They have planted a small grove of redwood trees — redwoods are particularly sensitive to salt — and some grass. They water half with recycled water and half with salinity-free Hetch Hetchy water.
"Our redwood trees are doing fine," Bobel reports — all of them. Tree health, he adds, "is a combination of the water and the soil," and some trees have a higher salt-tolerance. No one has suggested planting mangroves, yet.
Both the golf course and Greer Park were once marsh. In addition, for many decades there was a paved drive-in movie theater along Highway 101 — how much that might have sterilized the underlying soil is a question.
There's also a wider perspective: "One thing I was going to try to have a conversation about is the big picture, to get it out there. It's not really a sustainable practice to bring water 250 miles from the Sierra to water landscapes," Bobel summed up. "As California grows it's not a sustainable practice. Sooner or later we will have to stop doing it."
So it's both an environmental and an engineering "innovation challenge": Can we make recycling water economically feasible on a large scale to the point it doesn't cause concerns about safety to trees? Can existing or new technologies be developed, such as reverse osmosis or some other technique?