- Author: Saoimanu Sope
In a drought-prone region like Southern California, working with Mother Nature is not only wise but necessary, according to Janet Hartin, UC Cooperative Extension horticulture advisor for Los Angeles, San Bernardino and Riverside counties, who studies climate-ready trees.
In 2020, Governor Newsom launched the California Climate Action Corps, empowering Californians to protect their communities from the impacts of climate change. Newsom's call to action emphasizes the need for long-term and sustainable solutions like Hartin's research, which urges Southern California to care for existing trees and plant new ones.
In collaboration with the U.S. Forest Service and other UC Cooperative Extension scientists, Hartin is amid a 20-year research study identifying trees suitable for California's different climate zones. Her work provides a comprehensive understanding of trees and their benefits related to human and environmental health, particularly as Californians navigate climate change's evolving challenges.
One of these concerns is urban heat islands. UHIs are areas in which heat is reradiated from paved concrete or asphalt surfaces. In cities covered in asphalt, like Los Angeles, average temperatures can become six degrees hotter than surrounding areas.
To reduce urban heat islands, she has been working with community organizations to plant trees. In March, for example, Hartin teamed up with the Inland Empire Resource Conservation District to increase tree canopy in the Inland Empire.
Trees keep cities cool
To keep the city cool, some Los Angeles neighborhoods are repainting pavements with reflective coating. According to a 2020 study published in Environmental Research Letters, reflective coating can decrease pavement temperatures up to 10 degrees. As helpful as this is, augmenting urban landscapes to include heat-, drought- and pest-resistant tree species, whether native or not, can significantly reduce the impacts of urban heat islands too.
“Trees can cool impervious surfaces by 40 to 65 degrees,” Hartin said. During a 2021 study, in May and June Hartin discovered that unshaded asphalt could be more than 60 degrees hotter than shaded asphalt during late spring and early summer in inland and desert cities.
Other than providing shade, trees are effective at deflecting the sun's radiation and cooling the atmosphere through evapotranspiration. Given that they absorb and store carbon as well, trees lessen the impacts of pollution from fossil fuels.
“A well-tended mature landscape tree can absorb 40 tons of carbon over its lifespan,” said Hartin.
In a 2021 blog post, Hartin suggests trees be selected based on their adaptation to the “micro-climate” in each particular landscape, noting factors to consider like shade, proximity to buildings, space needs below and above ground, soil type and water source. She also recommends the Sunset Western Climate Zone maps for reference, noting that they are “more precise than USDA zones for our warmer climates.”
Based on the study with the U.S. Forest Service examining the performance of 12 species of underplanted but promising landscape trees at UC Riverside, favorable candidates include bubba desert willow and maverick thornless honey mesquite for their drought resistance, and red push pistache for its drought and heat resistance.
Tamara Hedges, executive director of UC Riverside Palm Desert Center and member of the Board of Directors for the Oswit Land Trust, agrees that trees are important in our fight against climate change:
“Through our partnerships with the UC California Naturalist and the Master Gardener Programs and many other nonprofits in the Coachella Valley, natural ecosystems are being protected and expanded and built environments cooled through the planting of appropriate tree species. These UC/USFS studies go a long way in identifying new underrepresented tree species."
General tips for planting
For California, planting in early fall through late winter provides ample time for trees to establish a strong root system before enduring the summer heat. Doing so also means that natural rainfall can fulfill water needs, as opposed to solely relying on irrigation systems.
Unlike newly planted trees, mature trees should be watered infrequently but deeply. Watering too often can reduce the level of oxygen in the rootzone and result in waterlogged soils prone to crown and root rots. 
During the fall, trees only need about 15% of the water they would require in the summer. When watering, keep the tree trunk dry. Because the roots of the tree grow outward and are usually a foot deep into the ground, Hartin recommends watering the area around the trunk rather than the trunk itself. This will also help avoid water waste.
“Trees not adapted to the climate they're planted in and not receiving proper care are much more susceptible to invasive pests like shothole borers and diseases,” said Hartin. “Even the loss of one front yard tree can significantly reduce shade, increase the surrounding temperature, and diminish energy savings.”
- Author: Anne E Schellman
The Benefits of Trees
Trees provide so much more than shade. Here are a few reasons trees are important, courtesy of the International Society of Arboriculture (ISA):
- Air-conditioning costs are lower in tree-shaded homes & businesses; heating costs are reduced.
- In workplaces with trees, people report decreased workplace stress and fatigue.
- Cleaner air: leaves filter the air we breathe by removing dust and other particles.
- Cooler environment: trees reduce heat in and around parking lots and paved areas.
Trees and the Drought
California residents are being asked to lower their water use by 15%. One easy way to do this is to turn off your sprinklers. But wait! What happens to the trees? Although they may look okay for now, the stress of going without water will take a toll. Lawns can be easily replanted and replaced, but trees take many years to become established.
What do Trees Need?
How Should I Water My Tree?
There is no “silver bullet” on how often or how much to water. This depends on many factors. Instead, after you water, take a shovel (away from tree roots) and check your soil. How deep did the water penetrate? Make sure water penetrates to a depth of 18” by digging with a shovel. Once that area is dry, water again.
Prioritize Watering Trees with the TRIC
For more information about the value of trees, visit the following resources.
Resources
California Center for Urban Horticulture. UC Davis. Tree Ring Irrigation Contraption. https://ccuh.ucdavis.edu/tric
International Society of Arboriculture (ISA). Trees are Good. https://www.treesaregood.org/Portals/0/TreesAreGood_Benefits%20of%20Trees_0321_1.pdf
Water Talk. Janet Hartin. Podcast Episode 21. https://water-talk.squarespace.com/episodes/episode-21
This article was originally published on July 26, 2021.
- Author: Denise Godbout-Avant
sake of his children and his children's children, who are to sit beneath the shadow of their spreading boughs.”
Author: Hyacinthe Loyson
I love to wander among trees, to see and hear the leaves swaying in the breeze, observe birds darting about the branches, squirrels running up and down the trunk, and insects flitting about. I have spent many contented hours sitting under a tree reading a good book. Trees mark the seasons of our lives, blooming in the spring before producing green leaves, producing fruits or nuts in the summer, changing colors in the fall, and stand bare and stark during the winter months. I particularly love the Valley Oak tree (Quercus lobata) with its distinctive lobed leaves, the acorns it yields, the fascinating oak galls produced by tiny wasps, and their historical importance to the original people of California as a staple food source.
Arbor Day
Along with 50 countries around the world, the USA celebrates trees on Arbor Day. The day is celebrated during the spring tree planting season. In the USA, the date is typically the last Friday of April, which this year is Friday, April 29th. The date varies around the world depending on geography, weather, and if in the Northern or Southern Hemisphere.
But why do we commemorate trees and how did Arbor Day get started?
Benefits of Trees
From the beginning, trees have provided us with the oxygen we breathe, along with food, shelter, medicine, and tools. To list a few of their other benefits:
- Trees help combat climate change – Trees absorb carbon dioxide (CO2), which is contributing to global warming. They remove the CO2 from the air and store the carbon. In one year, an acre of trees can absorb the same amount of CO2 produced when the average car is driven 26,000 miles.
- Trees clean the air – Trees absorb odors and pollutant gases such as nitrogen oxides, ammonia, sulfur dioxide and ozone and filter particulates out of the air by trapping them on their leaves and bark.
- Trees provide habitat for wildlife – Sycamores and oaks are among numerous trees that provide homes for many species of birds, mammals, insects, amphibians, and reptiles.
- Trees cool cities – Trees deflect the sunlight, cooling the air up to 10°F by shading our homes, buildings, and roads.
- Trees prevent soil erosion – Trees reduce water runoff by allowing the rainwater to flow down the trunk, onto the earth below. Their roots also slow runoff and hold the soil in place.
- Trees provide wood and paper – Trees help us build our homes and the paper we write on.
- Trees beautify urban spaces – Trees can mask unsightly concrete walls, parking lots, and unsightly views. They help muffle the sounds of the city and create eye-soothing canopies of green. They absorb dust and reduce glare.
- Trees provide personal and spiritual values – During our busy lives, trees can give us a piece of nature and moments of tranquility.
Arbor Day History
The tradition quickly spread and within 20 years the day was celebrated in every state but Delaware. On April 15, 1907, President Theodore Roosevelt, a conservation supporter, issued an “Arbor Day Proclamation to the School Children of the United States.” In 1970, Arbor Day became nationally recognized due to efforts by President Richard Nixon.
People can celebrate Arbor Day by planting a tree (which can be dedicated to a loved one) and spending time caring for the trees we have. Morton's words resonate strongly today as climate change becomes a serious threat: “Other holidays repose on the past: Arbor Day proposes for the future.”
How Can We Help?
- To reduce the demand for paper, check out books from your local Stanislaus County Library instead of purchasing new ones.
- Help protect existing forests – Encourage reduced cutting down of healthy forests by supporting sustainable reforestation.
- Afforestation – Support the planting of new forest plantations, which can enhance existing forest cover and help reduce global warming with carbon sequestration.
- Continue to water your tree, even during a drought. If you shut off lawn water, don't forget to deep water your tree! You can use a hose or soaker hose to water under the drip line.
- And, of course, plant a tree! You can volunteer for local organizations when they have tree planting days or plant a tree in your garden.
An excellent resource on care and selection of trees is Stanislaus County's Master Gardener “Trees in Your Home Garden,” https://ucanr.edu/sites/CEStanislausCo/files/341553.pdf
Additional resources include on how to plant a tree: https://ucanr.edu/sites/UrbanHort/files/80160.pdf, and on pruning trees: https://ucanr.edu/sites/UrbanHort/files/80116.pdf
You can learn more about trees from the National Arbor Day Foundation at https://www.arborday.org/trees/treefacts/
Favorite Tree?
Do you have a favorite tree? Write in our comment section what your favorite tree is and why?!
- Author: Kathy Keatley Garvey
Are you tired of the triple-digit temperatures? Wish someone would throw a breeze your way and provide a little shade?
A honey bee foraging on a Mexican sunflower (Tithonia rotundifola) probably felt a slight breeze when a Western tiger swallowtail (Papilio rutulus) fluttered down and landed next to her.
This is a two-insect blossom now.
Butterfly: "Bee, what are you doing?"
Bee: "Sipping some nectar, same as you."
Butterfly: "Bee, don't get any closer."
Bee, edging closer. "But I was here first. The nectar is excellent."
Butterfly: "Go away."
Bee: "No."
Butterfly: "Then I will." The butterfly lifts off.
Bee: "Thanks for the shade. You make a good umbrella, Madam Butterfly. Come back anytime."
- Author: Susan Flaherty
I recently heard a comment that really surprised me! A home buyer did not want trees in the yard! They thought raking leaves was too much work and the potential of a fallen limb during a storm was dangerous.
If they asked for my opinion I would have told them about some of the benefits of trees.
Trees are produce a healthy environment. They purify the air we breathe by taking in carbon dioxide and releasing oxygen. A tree's roots help to keep groundwater clean.
Properly planted and cared for shade trees can reduce home energy costs by up to 40%. Trees cool the air by releasing water vapor and cool the earth by giving shade which helps with climate control.
Also, healthy, mature trees can add an average of 10% to a property's value and give neighborhoods an established look.
Trees can be very beneficial.
