Current issues

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

What about global warming and trees?
Drought in southern California
Summer/sudden limb drop
Bark beetles, drought, and fire
Shot hole borer (SHB) and fusarium dieback (FD)
Asian citrus psyllid (ACP) and huanglongbing (HLB)
Goldspotted oak borer
Thousand cankers disease
Opportunities for education
Tree Tales

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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Drought in southern California

We are in the midst of a long-standing drought in the west. Scientific research (paleo climatology)* indicates that what we have experienced in the west for the past 150 years may be unusual and this drought may well be the “new normal”. As arborists, we are interested in helping our clients to avoid excessive irrigation while keeping their trees (and landscapes) healthy. Of course, over the long term this may mean a complete revisualization of what a typical landscape will be comprised of.

Among other concerns will be transitioning to more xeric (drought tolerant) landscape plants and tree choices. We are encouraging our clients who are considering planting new plants and trees to make choices that reflect this new reality. But, as we have witnessed in many cases, the rapid shift in Los Angeles County to irrigation reductions (often mandated), and the rapid changes in the landscapes to plants that require much less water (or no plants at all) has potentially dire consequences for the trees that remain in the landscape. Trees have declined and even died in some cases due to these changes. For more information about how to deal with trees in drought stressed landscapes, please see Dealing with drought stressed trees in the Useful Info section.

* The West Without Water: What Past Floods, Droughts, and Other Climatic Clues Tell Us about Tomorrow, B. Lynn Ingram, Frances Malamud-Roam, UC Press, 2013

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Summer/sudden limb drop

Summer/sudden limb drop (SLD) is a phenomenon that occurs across the world, in various landscape types and tree genera. SLD describes an event in which a limb fails or breaks from a tree seemingly for no reason, and occurs most commonly on hot, calm days (no wind). The trees which experience SLD are most commonly mature or over-mature trees. The limbs that fail are typically more horizontal than diagonal or vertical, and may or may not show evidence of decay or defect. The limbs most often break at a distance from the trunk and may even create a loud sound upon cracking (unlike most breaks due to wind, which occur where a branch attaches to the trunk, a break due to summer branch drop usually occurs 3 to 12 feet away from the trunk, along the length of the branch*).

SLD is still largely unexplained, though there are some theories as to why these events take place. It is commonly thought that hot days without wind cause changes in the properties of wood, making branches more susceptible than usual to failure under heavy end weights. Some have suggested that SLD may result from pre-existing defects in the wood, related to old wounds and/or flush cuts, which become exacerbated under such conditions. Still others have suggested that the main determinant in SLD is branch positioning and orientation combined with heavy end weight rather than defects. It is entirely possible that SLD results from a mixture of these factors when combined with hot, calm conditions, and the true cause will remain unknown.

If you are concerned about limbs on your tree breaking during hot summer months, it may be wise to have your tree pruned by a professional to lighten end weights. In some cases, it might even be practical to remove a horizontal limb entirely. If you want to read more about Summer/sudden limb drop, this University of California Agriculture and Natural Resources (UCANR) blog is a good place to start.

* “Summer Branch Drop in Oaks”, Brent McGhie, The Real Dirt Blog, 8/14/15

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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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Shot hole borer (SHB) and fusarium dieback (FD)

The shot hole borer (SHB, Euwallacea sp.) is an invasive boring beetle that broke onto the scene in southern California around 2012. Female SHB beetles bore tunnels into host trees and create galleries in the sapwood and heartwood. Females then lay their eggs in these galleries and cultivate a fungal disease complex known as fusarium dieback (FD). The hatched SHB larvae feed on the cultivated fungi until they mature, at which point siblings mate with each other and pregnant female beetles leave the host tree to either: re-enter the same host tree at a new location, or find a new host tree entirely. Male beetles do not fly, so they remain inside the original host tree. The life cycle repeats itself until FD kills the original host tree, or all females leave the tree in search of new hosts. SHB attacks can be difficult to identify without training, but are usually evidenced by staining, sugary exudate, gumming, or frass, and an entry or exit hole roughly the size of the ballpoint on a pen. For more detailed information about SHB and how to identify it, visit the University of California Agriculture and Natural Resources (UCANR) website.

Unfortunately for our southern California trees, SHB is not a picky pest. This beetle finds itself at home in a wide range of host tree species, with the list numbering in the 200’s. Of these 200+ host species, over half are susceptible to FD; and of those species susceptible to FD, 57 are known to be “reproductive hosts” for SHB, meaning these trees facilitate the spread of the beetle and the disease it carries. Included among the reproductive hosts are many of Southern California’s most important native and cultivated species, including but not limited to California sycamore (Platanus racemosa), coast live oak (Quercus agrifolia), avocado (Persea americana), southern magnolia (Magnolia grandiflora), and many maples, willows, and other riparian trees. For a complete list of the known reproductive host species, visit the University of California Agriculture and Natural Resources (UCANR) host species page.

The SHB beetle in itself is not the main concern for our trees. While the galleries created by SHB have the potential to compromise the structural integrity of a tree in severe cases, it is the fungal disease complex (fusarium dieback, FD) they cultivate that is of utmost concern. FD plugs the vessels of a tree, interrupting the transport of water and nutrients inside the tree. The result of this disease is that entire branches, or the whole tree, may die. Furthermore, limbs affected by the FD fungus may be weakened and break off, putting tree climbers and anything below at risk. Treatment for SHB/FD may not always be possible or necessary, but can be effective in some cases. We can advise on management and treatment options.

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Asian citrus psyllid (ACP) and huanglongbing (HLB)

Asian citrus psyllid (ACP) is a relatively harmless, invasive pest that was first discovered in southern California in 2008. It is a sucking insect that feeds primarily on the flush growth of all types of citrus and a few closely related ornamentals. In 2012, ACP was discovered to be spreading the deadly disease known as huanglongbing (HLB) across southern California, making it a pest of extreme concern. HLB is an incurable disease that affects all citrus species (and the closely related ornamentals), killing afflicted trees in as little as five years. ACP/HLB threatens to destroy the California citrus industry, as well as privately owned citrus trees, if an effective control is not found.

ACP can be easily identified by inspecting newly forming leaves for eggs, nymphs excreting white waxy tubules, and adults with brownish mottled wings feeding with their head down and their bottoms in the air at a 45-degree angle. HLB can be more difficult to identify, but is characterized by yellowing of the leaves asymmetrically across the central vein. HLB can also be identified during fruiting season as fruits will be small, lopsided, bitter, and often remain green towards the bottom of the fruit (this disease is sometimes called “citrus greening” due to the appearance of fruits turning green instead of coloring). For help learning how to identify ACP and HLB, visit the University of California Agriculture and Natural Resources (UCANR) Asian citrus psyllid (ACP) website.

If you are a citrus owner, or plan to purchase a citrus tree in the future, it is important to be aware of ACP and HLB quarantine boundaries in southern California. These boundaries affect citrus production, movement, and sales, including within the nursery trade and private arrangements. Always be sure to purchase your citrus plants and fruits from a responsible seller who abides by the quarantine restrictions. The most current map of quarantine boundaries can be found at the University of California Agriculture and Natural Resources (UCANR) Asian citrus psyllid (ACP) website. Current methods for limiting the spread of the incurable HLB disease involve adhering to quarantine boundaries, immediate removal of HLB infested plant materials, and controlling ACP populations. If you think your tree might have ACP and/or HLB, please contact the California Department of Food and Agriculture (CDFA) Exotic Pest Hotline at 1-800-491-1899 for confirmation and management recommendations.

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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 a reporting 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 Western Chapter of the ISA (WCISA) maintains this website with event listings, publications, and industry news, all of which is available both to WCISA members and the general public.
  • The American Society of Consulting Arborists (ASCA) maintains this website both for ASCA members and the general public.
  • 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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Tree Tales

For further current issues, visit Jan’s editorial blog Tree Tales.

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