"The good news is there is no catastrophic disease striking the University Avenue magnolia trees," Passmore said.
However, "University Avenue is a well-used and developed corridor where trees have significant challenges to thrive. They compete with road, sidewalk, utilities, landscapes and buildings for space," he said.
Passmore said the University Avenue trees are mostly between three and 80 years old, with the average age of 40 to 50 years. Their roots have outgrown their space, and soil is poor in some areas, making it difficult for roots to absorb water and nutrients. To accommodate traffic, some trees require more frequent or more extensive pruning, Passmore said. The stress weakens trees' immune systems, making them less able to defend against decay organisms, insects and diseases.
What could improve their situation?
"More volume of high-quality soil combined with comprehensive maintenance will grow better trees," Passmore said.
City staff is currently assessing all the trees along University Avenue. This winter workers will start on maintenance. Most trees will be pruned, but some in poor condition will be removed. New magnolias will be planted where space is available, he said.
The trees will be assessed on a list of factors including condition, risk, size, location and possibility to replant.
Passmore asserted that, based on public input, workers could treat trees with questionable health or defer their removal, after risks to the public are reduced. The trees would be taken out and replaced one at a time rather than en masse, he said.
He plans to reach out to the community once assessment is complete.
Catherine Martineau, executive director of Canopy, which is dedicated to protecting the urban forest, said the iconic University Avenue trees are one of the few stands the city replaces with the same species, she said.
"They create such an amazing sense of place," Martineau observed.
Palo Alto has a total of 589 Magnolia grandiflora trees throughout the city.
The trees are rumored to have been planted because the wife of the first public-works director was from the South, which could explain why there are so many planted. But Martineau said she has never been able to verify the story.
Native to the southeastern United States from Virginia to central Florida and west to eastern Texas and Oklahoma, the so-called Southern magnolia is found on the edges of waterways and swamps in moist but well-drained soils. It can grow to a large tree, with some magnificent specimens in Mississippi having reached 114 feet in height, according to the U.S. Department of Agriculture. There are at least 100 cultivated varieties.
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