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UC study seeks street trees that can cope with climate change

The changing climate predicted for California – including less rain and higher day and nighttime temperatures – is expected to cause chronic stress on many street tree species that have shaded and beautified urban areas for decades.

Realizing that popular trees may not thrive under the changing conditions, UC Cooperative Extension scientists are partnering with the U.S. Forest Service in an unprecedented 20-year research study to expand the palette of drought-adapted, climate-ready trees for several of the state's climate zones.

“The idea is to look at available but under-planted, drought-tolerant, structurally sound, pest resistant trees for Southern California that do well in even warmer climates,” said Janet Hartin, UCCE horticulture advisor in San Bernardino County.

Hartin, a 34-year veteran advisor, said the project is her first to stretch to 20 years; it will likely extend past her tenure with UCCE.

“I'd like to retire in five or six years,” she said. “But I'm very excited about being a pioneer in a study that will continue with my successors. I think it's important for our children and our children's children, as well as for the environment.”

At the end of 2019, with three years of data on tree health and growth rates, the scientists expect to be able to publish the first results and make them available to arborists, urban foresters and residents throughout the regions of the study.

Twelve tree species were selected for each climate zone in the comparative study, with several area parks used as control sites. Hartin and her Southern California research collaborators – UCCE advisors  Darren Haver of Orange County and Jim Downer of Ventura County – worked closely with UC Davis plant biologist Alison Berry, UC Davis research associate Greg McPherson and USFS research urban ecologist Natalie van Doorn to select promising species.

They looked for trees that are already available at local nurseries, but are underutilized. The trees in the project exhibit drought tolerance and disease resistance, plus produce minimal litter. The researchers also sought trees that would provide ample cooling shade for a long time – ideally 50 years or longer.

The varieties come from areas around the world with climates similar to California. Two trees planted in replicated plots at the UC Riverside Citrus Field Station are native to Australia, two are native to Oklahoma and Texas, one is native to Asia and two are non-native crosses of other trees. Three of the trees are native to California: the netleaf hackberry, Catalina cherry and island oak.

“Trees are a long-term investment,” Hartin said. “A tree will live 50, 70, 90 years. The proper selection is very important to help ensure longevity.”

Making the long-term investment with the proper selection yields considerable returns. In a warming world, trees are natural air conditioners.

“Urban areas create heat islands, with dark asphalt surfaces reradiating heat. Cities can be 10 to 20 degrees warmer than the surrounding environment,” Hartin said.

Other tree benefits include soil health and stability, wildlife habitat and aesthetic beauty.

Following are a sampling of trees that are part of the comparative study:

Acacia – A 20-foot-tall, 20-foot wide evergreen that is drought resistant, and withstands moderate irrigation.  Native of Australia.

Palo verde is a drought-resistant tree. (Photo: Bri Weldon, flickr, CC BY-SA 2.0)
Blue palo verde – A tree that reaches about 25 feet in height, the Blue palo verde is drought resistance and lives 50 to 150 years. Its trunk, branches and leaves have a blue-green hue. Native to the southwestern U.S. and Mexico.

Brazilian cedarwood – A native of Brazil and Paraguay, the deciduous tree grows to 50 to 65 feet. The tree produces pale yellow tubular flowers in the spring.

Catalina cherry – Native to the chaparral areas of coastal California, the Catalina cherry grows to 30 feet high. The evergreen tree tolerates drought when mature. It produces sweet purple-to-black edible fruit.

Chinese pistache – A deciduous tree with beautiful fall color. Grows to 35 feet tall, 30 feet wide. Drought resistant, but tolerates moist soil. Native to central and western China.

Desert willow – Growing to 30 feet tall and living 40 to 150 years, the desert willow tolerates highly alkaline soil and some salinity. A deciduous tree, it boasts large pink flowers all summer that attract hummingbirds and other wildlife. Native to the southwestern U.S. and northern Mexico.

Escarpment live oak – Native to west Texas, this tree is cold hardy and drought tolerant. Typically evergreen, it can be deciduous in colder climates.

Ghost gum – Very tall at maturity and drought tolerant. An Australia native.

Indian laurel – Commonly called a ficus, this is a 35-foot-tall, 35-foot-wide tree at maturity that is drought resistant and tolerates highly alkaline and saline soils. Shade potential is high. Native of Asia and Hawaii.

Ironwood – A southwestern and northern Mexico native, Ironwood is semi-drought resistant once mature and tolerates alkaline soil. Ironwood, which grows to about 33 feet tall, can live 50 to 150 years.

Island oak – This tree is native to five of six California off-shore islands. Drought tolerant, it grows to nearly 70 feet tall when mature.

Maverick mesquite – Native to the southwestern U.S. and northern Mexico, this tree does well in full sun and is drought resistant once established. The tree grows to 35 feet tall. The Maverick mesquite is a thornless variety.

Mulga – A versatile and hardy tree that grows 15 to 20 feet in height, the mulga – a Western Australia native – tolerates hot and dry conditions. The leaves are evergreen and the tree has yellow elongated fluffy flowers in spring.

Netleaf hackberry – A California native, the netleaf hackberry grows to 30 feet. Its deep root systems and heat resistance makes the tree idea for urban conditions.

Climate-ready trees planted in 2016 at the UC South Coast Research and Extension Center.

Rosewood – Native to southern Iran, Indian rosewood grows to 65 feet tall, and 40 feet wide. Evergreen. Semi drought resistant and intolerant of alkaline soil.

Shoestring Acacia – Evergreen and 30 feet tall when mature, shoestring acacia is drought resistant and thrives in slightly acidic to highly alkaline soils. Native to Australia.

Tecate cypress – A native of Southern California and Mexico, the Tecate cypress is very drought tolerant. Its foliage is bright green. Young trees are pyramidal in shape, becoming more rounded or contorted with age.

Partners in the tree study are Los Angeles Beautification Team volunteers, LA Parks and Recreation team, Chino Basin Water Conservation District, and Mountain States Wholesale Nursery.

Funding and other support is provided by LA Center for Urban Natural Resources Sustainability, ISA Western Chapter, Britton Fund, USFS Pacific Southwest Research Station, and the UC system.

Posted on Wednesday, April 18, 2018 at 11:35 AM
Tags: climate change (89), Earth Day (6), Janet Hartin (7), trees (9)
Focus Area Tags: Environment

Landscape trees also suffer drought consequences

Water-stressed landscape trees tend to shed their leaves to conserve resources.
Low rainfall in the state of California is stressing landscape trees in urban areas, reported the San Jose Mercury-News.

Igor Lacan, UCCE advisor in Santa Clara, Contra Costa and Alameda counties, says it's difficult to make blanket statements about which species are in trouble, since a tree's water- and heat-related health depends on its location.

Some species on a south-facing slope might show drought stress, while the same species on a north-facing slope doesn't. Stress symptoms also show up on trees planted near paved surfaces, though not near other surfaces that don't reflect heat, Lacan said. Since many tree species are fairly resilient, damage comes on slowly and may take months or years to become apparent.

But Lacan was able to conclude with a positive comment.

"The good thing is that in all likelihood mature, established trees, if climatically appropriate (for their location), will make it through ... just fine," Lacan said.

Posted on Friday, October 3, 2014 at 1:38 PM
Tags: drought (163), landscape (11), trees (9), urban horticulture (1)

Diagnosing Tree Problems Can Be Colorful

As a Master Gardener we are frequently presented with problems that various gardeners face. Sometimes people will bring in pictures or even samples of the plant they are dealing with. Sometimes figuring out the problem is simple, other times definitely not.

We have learned to ask a series of questions to help tease out clues which may help us diagnose the problem. We also have many references and resources which we can use to help us narrow down the possibilities.

When we were in Germany we came upon what appeared to be an outbreak. First describe what you see. The manifestation seems to  involve only the trunks of trees not the branches. The   problem seems to be an enveloping, raised, almost furry growth.  The coloration is quite variable, not only  from trunk to trunk, but within a single trunk.  We also saw that the outbreak did not limit itself to a single species but seemed to cross into several genera in the area of older Wiesbaden.

Next find out the history or progression of the symptoms.Find out what the gardener has done or not done.  I didn't speak German well enough to ask questions, but I suspect if I had the answer would have been that the manifestation appeared overnight. Lastly, hit the books! In the last few years there have been similar outbreaks in cities and towns across the globe.  They have even experienced an outbreak on the campus of UC Davis.

The manifestation is called..... yarnbombing.  Some industrious knitters knit up multicolor sleeves which are generally put up and sewn together in the dead of night.  The decoration is not necessarily limited to trees; we saw a few lamp posts adorned as well.  It's a wonderful surprise for a town's inhabitants.  In the dead of winter, under generally gray skies, it made for a wonderful pop of color.

Yarnbombed trees. (photo by Keith Arrol)
Yarnbombed trees. (photo by Keith Arrol)

Posted on Tuesday, March 12, 2013 at 7:38 PM
Tags: diagnosing problems (1), Germany (4), trees (9), yarnbombing (1)

Trees Toppled by Space Shuttle Trip

Old. Sick or dying. Creating a havoc on sidewalks. Non-native. Not heritage species. These are some of the reasons given for chopping down 400 trees that once grew along the 12-mile stretch of road traveled by the space shuttle Endeavor on October 12-13.  From Los Angeles International Airport to Exposition Park, Eucalyptus (Eucalyptus spp.), Jacaranda (Jacaranda spp.), and others were toppled to make room for the NASA transporter carrying the 75-ton, five-story tall spaceship that measured 122 feet in length and 78 feet wide. 

Photos of the gaping spaces and enormous trunks left behind after tree removal can be viewed online at the link listed below. Some trees were more than 80 years old. While I don’t label myself a “greenie,” I am a softie for all things living, especially big and tall, old trees. For me, something seems amiss when a single two-day road trip trumps a survivor with almost a century of growth.

Certainly, the California Science Center expects the shuttle, which CSC president Jeffrey Rudolph called “a historical piece” and “a national treasure,” to draw 800,000 visitors a year. And by 2017, the center plans to build the shuttle’s permanent home—the Samuel Oschin Air and Space Center complete with launch pad and replica fuel tanks and rocket boosters. Also, CSC pledged $500,000 for road improvements and landscaping, promising to replant more trees than were removed.

But I wonder about the arboreal impact on people in communities (Westchester, Inglewood, Los Angeles) now living without wind protection, air filtration and summer shade from those big and tall, old trees. In the dialogue of history, will anybody mention the measure of their worth?

Click below for link to a slide show of post-removal median strip in Inglewood, CA: 

http://www.reuters.com/article/2012/09/19/us-usa-space-shuttle-idUSBRE88I01D20120919

Tree removed by space shuttle. (c) LA TIMES
Tree removed by space shuttle. (c) LA TIMES

Posted on Thursday, November 15, 2012 at 10:57 AM
Tags: greenie (1), space shuttle (1), toppled (1), trees (9), trunks (1)

Purdy Macadamias

I got the chance to visit a farm recently on Molokai.  Amongst other things, they grow macadamia trees, Macadamia integrifolia.  They had about fifty trees on their farm that had been there since the 1920s. Farmer Purdy explained that macadamia nuts start off as pale, slender, bottle-brush shaped flower clusters.  The flower cluster is pollinated by bees and later forms small green nuts.  The nuts grow and when they are ripe, they fall to the ground.  The family gathers the nuts daily from the orchard floor.  Harvest is essentially year round.  The family then husks them, dries them, cracks them and roasts them without any preservatives. 

The farmer said that because his is a small multi-crop farm, he has not found the need for any insecticides.  The only thing he spreads around the trees is ashes from the burned leaves that he rakes up daily. He stressed that because the crop is picked up from the ground, the orchard must be kept very clean.  It was so unique to look up into the tree and see every stage of the nuts' life cycle all growing at the same time. And cool to think of a year round harvest.

Macadamia nuts on the tree. (photos by Karen Metz)
Macadamia nuts on the tree. (photos by Karen Metz)

Clusters of macadamia flowers.
Clusters of macadamia flowers.

One lonely macadamia nut on the ground.
One lonely macadamia nut on the ground.

Posted on Wednesday, May 30, 2012 at 8:54 AM
Tags: farm (4), harvest (4), Macadamia integrifolia (1), Molokai (1), nuts (2), trees (9)

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