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Texas trees teach California

Texas Hill Country is a far cry from urban California, with spicy barbecue, worn dance floors, pin-dot towns and ranches that stretch to the horizon.

But Texan trees could become the urban forests of the Golden State’s hotter and drier future — succeeding, rather than struggling, in an era of climate change.

A 20-year UC Davis research study called “Climate-Ready Trees” is exploring whether species native to our red-state rival can replace more familiar trees ill-suited to California’s coming reality.

“How a tree grew in the past is no longer a good predictor of its future success,” especially in California’s inner valleys, said project leader Emily Griswold, who is overseeing the living lab, a young campus forest of more than 200 tiny saplings, representing 40 different species, native to the undulating plateau of central west Texas. Most of those have rarely been grown here before.

The project comes at a time when cities seek not only to save but also to expand their urban forests.

That is because trees are uniquely valuable in helping cities adapt to climate change. They sequester carbon, removing CO2 from the atmosphere and reducing the greenhouse effect. They also provide shade to homes, streets and people, making urban environments more livable.

Many of California’s city trees, planted decades ago in post-World War II suburban developments, are aging and will die. Others were toppled by this past winter’s storms and need replacing.

“Municipal tree managers and other urban residents, especially those in the inland cities of California, should begin reconsidering their palettes of common street tree species,” said Joe McBride, professor emeritus of Landscape Architecture & Environmental Planning at UC Berkeley.

Trees take decades to mature. So seeds planted today will grow up in an environment very different from current conditions, he said. For example, Berkeley can expect a climate at the end of the century  more like Santa Ana’s today. Eureka’s climate will resemble today’s Berkeley, and Fresno’s climate will resemble El Centro in California’s Imperial Valley, McBride said.

In his survey of 16 cities across California, published in the journal Urban Forestry & Urban Greening, he evaluated how the common street tree species might fare in a warming climate.

He found that city trees in most of Northern California and along the coast are relatively suited for increased temperature and periodic droughts.

But trees in the interior cities in the Bay Area, as well as across all of Central and Southern California, will feel greater impact. “The change in temperature is going to be more significant, particularly as we move away from the coast and the potential availability of irrigation water is going to be more limited,” he said.

This means letting go of many thirstier cherished species, such as redwoods, magnolias, Tulip trees, Chinese hackberries and London plane trees.

Cities are trying to adapt.

“Over time, we adjust the tree palette as we learn more from research or as the needs of our community partners change,” said Laura Gronek of Canopy, a nonprofit dedicated to nurturing urban trees in the Bay Area’s Peninsula communities.

An effort called Re-oaking Silicon Valley, created by the San Francisco Estuary Institute, is prioritizing the planting of oak trees to restore the region’s local ecosystem and increase climate resilience. Google and Apple have landscaped their new campuses with thousands of oaks.

A redwood tree called El Palo Alto has long served as the 120-foot-tall symbol of Palo Alto. Now city arborists “are avoiding planting redwoods or any other high-water-user species as street trees,” said Meghan Horrigan-Taylor, city spokesperson.

It’s tough to say goodbye. Many cities have “Heritage Tree Ordinances” that prohibit the removal of redwoods and any other tree with a diameter of 15 inches or more without a permit.

Longtime Menlo Park resident Kimberly LeMieux, frustrated by her city’s 125 thirsty redwoods, is leading a citizens task force to reform Menlo Park’s restrictive ordinance.

“We are a group of environmentally conscious residents who are looking to take down inappropriate trees,” she said, “and replace them with trees that will grow to be heritage trees and provide shade and beauty for the next 40 to 60 years.”

Could today’s sentimental favorites be replaced by the Texas Mountain Laurel, Lacey oak and Lindheimer’s hackberry, native to Texas’ Uvalde County? Ashe’s blackjack oak and Texas madrone, from Real County? Bigtooth maple, from Bandera County? Emory oak from Brewster County? Mexican plum from Travis County?

That’s what the UC Davis project hopes to learn. About half of the 20,000 trees now on campus may be unsuitable by the end of the century, according to campus climate models and a tree assessment included in the UC Davis Living Landscape Adaptation Plan.

Griswold, senior staff horticulturalist for the UC Davis Arboretum and Public Garden, knew Texas has native trees that can survive through extreme weather events such as heat, drought, floods and winds as well as summer irrigation and alkaline groundwater.

But she didn’t want sparse desert trees for California’s cities. Rather, she sought species that look lush but are actually water-thrifty.

Her team collected seeds from trees in west central Texas and brought them back to the Davis Arboretum to germinate. Now the trees are growing along with California natives on a three-acre plot on Old Davis Road, as well as smaller plots on campus, in Sacramento and in Southern California. They get annual physical exams, like people, and are inspected to make sure they don’t turn invasive. Most are thriving. A few have perished.

Over the next two decades, her team will learn how these many rural Texas species best fit into California’s urban forests. But the team is also making current recommendations, based on what’s known about more proven species, to guide arborists.

“We need trees for different kinds of situations, with a range of sizes and characteristics,” said Griswold, “that are going to be better adapted to our future climate.”

Source: redbluffdailynews