Current issues

To learn about arboricultural issues of current interest in Southern California, click on a link below:

What about global warming and trees?
Sudden oak death
Bark beetles, drought, and fire
Eucalyptus trees and red gum lerp psyllid
Trees and energy conservation
Tipu psyllid
Goldspotted oak borer
Thousand cankers disease
Opportunities for education

Please note: We are not affiliated with any organization listed in this website. We do, however, share a common professional interest in educating the general public about trees.


What about global warming and trees?

One of the most pressing issues in the news these days is global warming. It is an important issue, and trees play a role in it, but it may not always be clear exactly what that role is. We offer a simple discussion below for those who are interested in understanding more about the subject.

Global warming is probably a natural occurrence to some extent, and some people would have us believe it is nothing more than that. Most scientists, however, think that we are contributing to global warming through our activities, and that we are accelerating the rate at which global warming occurs. The “greenhouse effect” is widely believed to be a major factor in global warming. So-called greenhouse gases such as water vapor, carbon dioxide, and methane allow sunlight to pass through them and warm the earth. As the earth’s surface warms, energy is radiated back into space in the form of heat. Because this radiant heat has a longer wavelength, some heat is absorbed by greenhouse gases in the atmosphere and reradiated back toward the earth causing additional warming of the earth’s surface. As the concentration of greenhouse gases in the atmosphere increases, the warming effect increases.

One greenhouse gas that is increasing in the atmosphere is carbon dioxide (CO2). Internal combustion engines used in most cars produce CO2, and contribute significantly to the gain of this greenhouse gas. But there are also many other sources: Aircraft engines, coal-burning electricity-generating plants, home fireplaces, small engines, and forest fires also contribute to the release of CO2 into the atmosphere. All of these processes burn carbon-based materials (gasoline, jet fuel, coal, wood) in oxygen and release energy, along with CO2 and various other byproducts. Other sources of greenhouse gases include thawing tundra (frozen tundra decays when it warms and thaws, releasing CO2), livestock farming (methane, CO2), and vented landfill emissions (methane, CO2).

Emissions of Green House Gases in the US

The carbon that is released in the burning of fossil fuels, wood, and wood products is the same carbon that can be “sequestered” by the growing of trees and plants. When a plant grows, it combines CO2 with water, using solar energy (photosynthesis), creating complex hydrocarbons (glucose, wood, etc.), and releasing oxygen into the atmosphere. So, in effect, sequestration is the capture and storage of carbon atoms in the production of living tissues.

When trees die and fall to the ground, they usually decay within a few years to decades, releasing CO2 slowly back into the atmosphere. By contrast, when a forest fire occurs, or when forested land is cleared and burned for agriculture, huge quantities of CO2 are released into the atmosphere very quickly. Removing a single tree and turning it into firewood will have a similar rapid conversion, although on a much smaller scale. Under the right conditions, trees in a forest or jungle may become buried or submerged and eventually converted into crude oil and/or coal. That process takes millions of years, but that is where the carbon in our fossil fuels came from.

Trees are one of the best carbon sequestration vehicles, since they are large and long lived, and they decay slowly when they die. The more trees that are growing on the planet, the less free carbon there is likely to be in the atmosphere. Think about that when you consider removing a tree. We do!

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Sudden oak death

Sudden oak death (SOD) is a disease caused by the fungus-like pathogen Phytophthora ramorum, found in the cool, wet forest communities of coastal California. At present, the disease has proved fatal only to certain species (tanoak, coast live oak, canyon live oak, Shreve oak, and California black oak), usually by causing cankers on the trunk of an infected tree. On many other types of plants, SOD is a foliar disease, causing leaf spots and dieback of twigs and branches. The disease cannot yet be definitively diagnosed in the field (without the use of laboratory tests).

Although SOD is considered an epidemic in Northern and Central California, it has not yet been identified in oak trees in Southern California and current research models predict continued low risk here in the future. You can follow the gradual spread of SOD online at oakmapper.org.

The most immediate source for up-to-date information on SOD (including how recreational users can prevent the spread of SOD) is the California Oak Mortality Task Force. To learn which plant species are known to be affected by SOD, consult this list maintained by the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service.

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Bark beetles, drought, and fire

Large numbers of pine trees in the mountains of San Bernardino, Riverside, and San Diego counties have succumbed to damage from a complex of western, mountain and Jeffrey pine bark beetles. While bark beetle activity has decreased somewhat in recent years, with mortality at its peak in 2003–2004, the problem persists in Southern California’s mountain forests.

Yet beetles are just one piece of the puzzle, and generally are not a primary cause of death: Prolonged drought has weakened these trees and reduced their ability to withstand invading beetles. Air pollution in the form of high ozone levels mimics the effect of drought, altering trees’ chemical processes and causing water loss. Air pollution in the form of nitrogen oxides causes additional problems, stunting root growth and changing how trees store energy. Beyond leading to drier, weaker trees and more leaf and branch litter on the ground, these pollutants inhibit decomposition, leaving more plant material on the ground to burn in the event of a wildfire. Decades of fire suppression have led to very dense forests made up of trees of similar age and species, competing for limited resources and resulting in less vigorous trees overall.

The environmental stresses on trees can benefit beetles, as dense stands of weakened trees serve as convenient food sources for expanding beetle populations. In addition, climate change has led to warmer winters in places where extreme cold once kept beetle numbers down. As a result, we have had more beetles, reproducing more often, over a wide range of elevation and latitude.

Even as populations of some beetle species decline, others persist, and standing dead trees remain a fire hazard and a potential danger to human safety. For more information, visit the Mountain Area Safety Taskforce (MAST) or the State of California Department of Forestry and Fire Protection.

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Eucalyptus trees and psyllids

Eucalyptus and Corymbia* trees throughout California have been affected by several types of psyllids, each specific to one or a few species growing here. Psyllids are insects that can be free-living or lerp-forming: The latter produce and live in small shelters (“lerps”) on the leaf surface. The feeding activity of psyllids can inhibit development of new shoots and distort the shape of new leaves, and, in large numbers, psyllids may cause defoliation (loss of leaves) that can lead to weakening of the tree, branch dieback, and sometimes death. While smaller infestations may be less harmful to a tree, they create a nuisance by excreting sticky honeydew, which drops from affected trees and encourages the growth of unattractive sooty mold.

The blue gum psyllid, first discovered in Monterey County in 1991, caused significant damage to eucalyptus foliage and impacted the floral industry. This pest is now under complete biological control.

The red gum lerp psyllid first appeared in Los Angeles County in 1998 and then spread throughout California, typically causing mortality where trees did not receive supplementary irrigation. By 2005, this pest was largely under control in most coastal regions of the state, but continued to affect California’s inland areas. The success of red gum lerp psyllid biological control continues to vary between populations and regions, and current research suggests that a type of bacteria may protect the psyllid from being parasitized by the biological control agent.

Biological control of the spotted gum lerp psyllid is underway, and is considered reasonably successful. This psyllid affects lemon-scented gum (Corymbia citriodora) as well as spotted gum (C. maculata), and its abandoned lerps are sometimes colonized by the lemon gum psyllid, which does not form lerps of its own. No tree mortality has yet been attributed to these psyllids, but the large amount of honeydew they produce can be a nuisance.

To learn more about psyllids affecting eucalyptus and the continuing research being done, visit the University of California Statewide Integrated Pest Management Program (psyllids in general, or red gum lerp psyllid) and follow the work of Dr. Timothy Paine at the University of California, Riverside.

*DNA research has prompted the reclassification of some tree species that were once considered part of theEucalyptus genus. Lemon-scented gum and spotted gum (noted above) are now considered part of the Corymbiagenus.

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Trees and energy conservation

In Southern California, the need to conserve energy is more critical today than ever. With careful planning, trees can be effectively planted to reduce energy needs on both residential and commercial properties.

A tree’s placement and species, in combination with the climate and season, will affect that tree’s contribution to energy conservation. Buildings under shade will absorb and retain less heat, particularly when trees shade a building’s east and west walls from direct summer sun. In winter, unshaded south walls allow more warmth inside a building. While evergreen trees provide the same degree of shade year-round, deciduous trees provide shade in spring and summer but expose portions of a building to the sun’s rays in fall and winter.

Shade is just one way that trees can modify climate. The air around trees stays cooler, because a tree’s natural processes intercept solar energy and convert water from liquid to vapor form. Where high-speed winds force outside air into a building (for instance, through poorly sealed windows), tree canopies serve as windbreaks.

The information provided here draws from the research of Greg McPherson, James Simpson, and others at the U.S. Forest Service (USFS) Pacific Southwest (PSW) Research Station. Follow the work of the USFS PSW Urban Ecosystems and Processes team, including a beta project that quantifies the value of individual trees: Visit iTree Design to see how much energy—and money—your trees could be saving.

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Tipu psyllid

The tipu psyllid, an insect native to South America, was found in 2008 infesting Tipuana tipu trees in San Diego County and soon afterward in coastal portions of the Los Angeles Basin. Like the psyllids currently affecting someEucalyptus species, the tipu psyllid excretes large amounts of honeydew, which encourages the growth of unattractive sooty mold on leaves and branches. Heavily infested trees experience defoliation, and surfaces below (such as sidewalks or parked cars) become covered in sticky honeydew.

While the tipu psyllid is not yet under biological control in California, high-density populations in South America are known to be somewhat controlled by ladybugs. For more information, visit the University of California, Riverside’s Center for Invasive Species Research.

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Goldspotted oak borer (GSOB)

Goldspotted oak borer (GSOB) is a beetle responsible for a tremendous amount of oak mortality in San Diego County within the last decade. With a preference for coast live oak (Quercus agrifolia) and California black oak (Q. kelloggii)—but also documented to a lesser extent in canyon live oak (Q. chrysolepis) and Engelmann oak (Q. engelmannii)—this beetle has the potential to decimate oak populations throughout California if its spread continues. GSOB is believed to have been introduced to Southern California by firewood that was transported from Arizona, or from southern Baja California, Sonora, or Chihuahua, Mexico.

Because GSOB favors mature or old growth trees, GSOB-caused mortality typically leads to the loss of aesthetically valuable trees whose absence is sorely felt—whether that loss is contained to one large, old tree or extends to all mature or old growth oaks in a stand. If GSOB spreads into Central and Northern California—as researchers warn that it could—we may be facing ecosystem-level consequences that could alter the food webs of associated species and increase the amount of available fuel in places susceptible to wildfire. In the meantime, California landowners with potentially susceptible oak trees can start collecting information to be better prepared for future GSOB infestation.

While research on potential biological control agents is underway, GSOB is now spreading uncontrolled within San Diego County. Current management priorities include educating people not to transport firewood and planting acorns and oak seedlings to mitigate the loss of older oaks. In addition, the University of California Cooperative Extension has established an Early Warning System that includes support for “citizen scientists” trained to watch for and report symptoms of GSOB infestation. Further information can be found at www.GSOB.org.

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Thousand cankers disease (TCD)

Thousand cankers disease (TCD), caused by Geosmithia morbida, a fungus that is spread by the walnut twig beetle, can kill an infected tree within three years. In Southern California, TCD is believed to affect only walnut trees, occurring mostly on native black walnut (Juglans californica). While the beetle has been known in California for over 50 years, the beetle’s role as a vector for this aggressive, often fatal fungus was first identified in 2008. In 2010, TCD had been found in 15 California counties, including Los Angeles. The commercial walnut (J. regia) may also be threatened by TCD, as native walnut is often used as rootstock in California walnut orchards.

In the process of making tiny entrance holes in the bark that lead to “galleries”—tunnels where eggs will be laid–the beetle causes little physical damage, but it carries the fungus under the bark and into the wood. There, the real damage occurs, as the fungus infects, stains, and eventually kills tissue around the galleries, resulting in small cankers. When beetle populations are high, numerous beetles may enter the bark of a single tree at different locations, and a large number of small cankers result. As the many small cankers combine into fewer larger cankers, the infected tissue begins to girdle parts of the tree, effectively cutting off transport of water and nutrients needed for survival. Twigs and branches die first, and cankers continue to form and coalesce until the tree trunk becomes girdled, leading to death of the whole tree.

The current management priority is to prevent spread of TCD to new geographical areas, by detecting new outbreaks early and by being vigilant about not moving infected logs or burls. The University of California Statewide Integrated Pest Management Program provides detailed information on monitoring and identifying the walnut twig beetle, and advises that any possible detections be reported to the local agricultural commissioner’s office or to the local University of California Cooperative Extension office. While we currently can neither prevent TCD infection nor save affected trees, ongoing scientific research is refining detection and monitoring methods and testing pesticides that would target the walnut twig beetle.

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Opportunities for education

To learn more about trees, arboriculture and urban forestry, visit the following websites:

  • The International Society of Arboriculture (ISA) maintains the Trees Are Good website to provide user-friendly information to the general public.
  • The American Society of Consulting Arborists (ASCA) maintains this website both for ASCA members and the public at large.
  • TreePeople, a well-known non-profit organization in the Los Angeles area, offers general tree-related information on their website, and offers training programs for people interested in organizing tree-planting and tree-care projects.

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