Tracking down the truth about blue gum eucalyptus

Last week, we told our readers about the California Invasive Plant Council’s (Cal-IPC) draft reassessment of blue gum eucalyptus, Eucalyptus globulus. (available HERE) Cal-IPC is accepting comments on its draft until July 31, 2014 (send to  Although Cal-IPC now acknowledges that blue gum has “low invasive potential” and its population is stable in California, it proposes to maintain its over-all classification of “moderately invasive.”  You might wonder how Cal-IPC manages to accomplish this sleight of hand, so we will tell you how its scoring system enables it to maintain its overall rating of blue gum.

Cal-IPC has three main evaluation categories:  “Impact,” “Invasiveness,” and “Distribution,” which are combined to produce an over-all rating.  Cal-IPC’s draft lowers the rating of “invasiveness” to “limited,” but it has changed its rating of “impact” from “moderate” to “severe.”  These changes cancel one another, enabling them to reach the same over-all rating of “moderately invasive.”  In this post, we will focus your attention on Cal-IPC’s opinion of the “impact” of blue gum so that you can see how they arrived at their conclusion. 

Drought is on our minds

Epicormic sprouts on trees in Glen Canyon Park, June 2014
Epicormic sprouts on trees in Glen Canyon Park, June 2014

As we told our readers recently, the drought in California is making water use an important issue that is getting a lot of attention, as it should.  Native plant advocates have not hesitated to jump on that band wagon.  For example, they now claim that eucalypts in San Francisco are dying of drought.  With the help of a highly qualified arborist, we evaluated that claim in a recent post and reported that the trees are reacting to the loss of their understory and their neighboring trees, as well as the herbicides used to kill the understory and prevent their former neighboring trees from resprouting from their stumps.

Given this recent experience, we weren’t surprised to find that Cal-IPC has introduced accusations of extreme water use into their assessment of blue gum for the first time, and this new issue helps them to claim that blue gums have a “severe” impact on the environment.  Now let’s drill down into this claim, examine the source of the claim and tell you why we believe this is another bogus issue used to vilify eucalyptus.

Tracking down the truth about water use by blue gums was a bit like a game of gossip.  Starting with the final version of the rumor in Cal-IPC’s draft reassessment, we tracked that version back to its original source.  We found exaggeration at each iteration of the rumor, from its source to its landing in the Cal-IPC draft.  The final version bore little resemblance to the original version.  The original version is over 100 years old and therefore describes circumstances that have since changed substantially.

Exaggerated and outdated description of root structures

Cal-IPC’s draft assessment of “impact on hydrology” is: “Eucalyptus globulus is adept at tapping into deep groundwater, even under drought conditions (DiTomaso & Healy 2007), altering water availability to depths of 45 feet and distances of 100 feet from the trunk.”

The first distortion of evidence occurs with Cal-IPC’s exaggeration of its cited source.  Here’s what DiTomaso & Healy actually said about the roots of blue gum:   “In deep soils with high water tables or other deep soil water source, lateral roots grow toward the moisture source and can deeply penetrate soil to 14 m deep.”  In other words, DiTomaso and Healy say that such deep roots occur only in certain conditions of “deep soil,” “high water table,” or a “moisture source.”

The source of DiTomaso & Healy’s description of the roots of blue gum is a similar statement in Bean & Russo, who wrote an evaluation of blue gum for the Nature Conservancy in 1989:  “Large roots have been discovered at a depth of 45 feet below the surface, and surface roots frequently spread over 100 feet away from the trunk. (Sellers 1910)”

Bean & Russo provide a reference for their statement, which takes us to the original version of this rumor.  A book about eucalyptus in California by C.H. Sellers was published in 1910.  Here is what Sellers actually says about the roots of E. globulus, grown in the State of California

“The root system consists mostly of numerous strong laterals; the tap root rarely penetrates to a depth of more than 10 feet.  Abundant supply of moisture is demanded and as the roots grow quickly toward water Eucalyptus globulus should never be planted near wells, cisterns, water pipes, irrigating ditches or similar water impounding structures.  In loose, sandy or gravelly soils the large lateral roots penetrate to great depth, and extend almost incredible distances.  In digging wells large roots have been discovered at a depth of 45 feet below the surface and the surface roots of large trees frequently spread over 100 feet from the trunk of the tree.” (1)

In other words, the extreme root lengths reported by DiTomaso & Healy are an anomaly, only found in specific conditions and unusual cases.  The roots of E. globulus are “rarely” deeper than 10 feet, according to this original source of information regarding the roots of E. globulus

Contemporary sources which are not trying to make a case against E. globulus describe its roots in a less extreme manner: “Bluegum eucalyptus generally does not form a taproot.  It produces roots throughout the soil profile, rooting several feet deep in some soils.” (Esser 1993)  Also:  Bluegum eucalyptus generally does not form a taproot. It produces roots throughout the soil profile, rooting several feet deep on soils that permit it, or shallowly otherwise.” (Skolmen & Ledig 1990)  Esser and Skolmen wrote their evaluations of blue gum for the US Forest Service.

Bringing Sellers up to date

We will assume that Sellers’s observations are accurate, but we will now consider the possibility that an observation that is over 100 years old may not be relevant to present conditions. For a more contemporary perspective on blue gum, we turn to R.G. Florence, an Australian academic scientist who studies eucalyptus and was also a visiting professor at UC Berkeley.  R.G. Florence helps us to understand why the observations of Sellers may be very different from more recent observations:  “As a tree develops through the sapling and pole stages it will tend to form a main root which penetrates vertically downward, but this main root is rarely significant in a large tree.“ (2) Florence reports that E. globulus uses more water during early stages of growth, when it is growing quickly, than it does as a mature tree.  Sellers was reporting in 1910 at a time when most eucalypts in California were young.  Their roots may have been longer during these early stages of growth.  Since E. globulus has not been planted in California for decades and is no longer available for planting, its root structure at early stages of development is irrelevant to evaluating its behavior in California presently.

Is blue gum drought tolerant?

Cal-IPC denies that blue gum is drought tolerant:  “E. globulus is able to withstand prolonged dry summers by tapping into deep water reservoirs; they do not economize in the use of water but have far-reaching root systems and can extract water from the soil at even higher soil moisture tensions than most mesophytic plants (Pryor 1976, Florence 1996).”

Once again, Cal-IPC misquotes its cited sources.  R.G. Florence does not make the statement that Cal-IPC attributes to him.  Rather, this is how Florence describes the water needs of eucalypts:  “…the eucalypt might be generally characterized as being a drought-tolerant mesophyte, that is, it tends to maintain transpiration and cell metabolism under conditions of developing drought.”  (2)

Florence says drought tolerance and water use varies by species of eucalyptus.  He places E. globulus in this category of drought tolerance:  “coastal zone species with high rates of growth and water use, but with somewhat wider environmental [drought] tolerance.”  He makes these observations about water use by E. globulus:

  • Water use is greatest when the trees are young and generally tapers off as the tree grows, between 10-15 years of age.  As we have said, only mature E. globulus exists in California presently or will in the future.
  • E. globulus has been observed to regulate water use by opening and closing its stomata in response to temperature and moisture:  “Water use may be regulated in this way.” (2)

Pryor’s description of drought tolerance of eucalypts was published 20 years earlier than Florence at a time when variations in drought tolerance of different species of eucalyptus had not yet been studied.  He therefore, does not report on the ability of E. globulus to withstand drought, as Florence does in 1996.  He acknowledges some variability with respect to drought tolerance in genus Eucalyptus, which is consistent with Florence’s later report specifically about E. globulus, although Pryor says nothing specifically about E. globulus.

Self-watering blue gums

Sutro forest on a typical summer day.  Courtesy Save Sutro Forest.
Sutro forest on a typical summer day. Courtesy Save Sutro Forest.

E. globulus was planted most widely along the coast of California.  Most of the coast of California is foggy during summer months, when there is little if any rainfall.  Tall eucalypts are known to condense the moisture in the fog which has been measured to double annual rainfall. (3) In these regions, eucalyptus is essentially self-watering.  Ironically, Cal-IPC acknowledges this self-watering feature of eucalypts:  “The volume of water channeled down the stem is about eight times more than that of falling rain, so soil at the base of trunks receives relatively large quantities of water…” (May & Ash, see “allelopathy” section of Cal-IPC draft).

A recent study conducted in 24 parks in San Francisco measured moisture in the soil in the late summer months (August to September), when the soil would be driest in the absence of fog condensation.  That study reported that the greatest amount of moisture (15%) was found in the parks with eucalyptus forests.  Soil in parks vegetated with grassland or scrub contained significantly less moisture.  In other words, even late in the dry season mature eucalypts were not draining all available moisture from the soil.  Their water needs were met by the condensation of moisture from the fog during the dry summer season. (4)

Please come to the defense of our urban forest!

We hope that we have made the case that the draft reassessment is not a fair evaluation of the predominant tree in our urban forest, blue gum eucalyptus.  We ask that you write to Cal-IPC by July 31st about their biased reassessment.  Tell them why you think our urban forest is important to the environment and the people and animals living in it.  Ask them to remove blue gum eucalyptus from their “hit list” which is being used by managers of our public lands to justify the destruction of our urban forest. 

Eucalyptus forest, Lake Chabot
Eucalyptus forest, Lake Chabot

Thank you for your help to save our urban forest from being needlessly destroyed.

Update:  On March 13, 2015, the California Invasive Plant Council published its final reassessment of Blue Gum Eucalyptus (available HERE).  Cal-IPC has downgraded its rating of invasiveness and ecological impact from “moderate” to “limited.”  Although the detailed assessment is less than perfect, the over all rating itself is an improvement.  Thanks to those who sent comments to Cal-IPC.




(1)    C.H. Sellers, Eucalyptus:  Its history, growth and utilization, published by A.J. Johnston, Sacramento, CA, 1910

(2)    R.G. Florence, Ecology and Silviculture of Eucalypt Forests, CSIRO, 1996

(3)    Page 37, Weather of the San Francisco Bay Area, Harold Gilliam, 2002

(4)    Kevin M. Clarke,, “The influence of urban park characteristics on ant communities,” Urban Ecosyst., 11:317-334, 2008


California Invasive Plant Council sticks to its guns aimed at eucalyptus

Monarch butterflies roosting in eucalyptus tree.
Monarch butterflies roosting in eucalyptus tree.

In December 2013, one of our collaborators in the effort to save our urban forest from pointless destruction submitted a request to the California Invasive Plant Council to reconsider its evaluation of blue gum eucalyptus (Eucalyptus globulus) as “moderately invasive.” 

The California Invasive Plant Council (Cal-IPC) has responded to that request with a draft reassessment which is available here:’s draft maintains the same over-all rating of blue gum as “moderately invasive.”   Cal-IPC is inviting “substantive comments and questions” by July 31st to

Today we are publishing with permission the cover letter of a public comment that will be submitted by one of our collaborators.  We hope it will inspire you to write your own public comment by the deadline of July 31, 2014. 

From the perspective of humans, there are pros and cons to most species of plant and animal.  E. globulus is no exception to this general rule.  Cal-IPC reaches a negative conclusion regarding blue gum by exaggerating negative issues and de-emphasizing or omitting positive issues.  Cal-IPC now acknowledges that blue gum has “low invasive potential” only in specific conditions and that its population in California is stable, but it has introduced new issues and intensified others so that it can maintain its overall rating of “moderately invasive.”  I remind Cal-IPC that its name is Invasive Plant Council, not fire council or hydrology council.

Cal-IPC also fails to take into consideration the negative side-effects of attempting to eradicate eucalyptus.  There are environmental benefits associated with leaving blue gums alone.  These damaging consequences of eucalyptus removal should appear on the “asset” side of the ledger:

It is also not in Cal-IPC’s strategic best interests to continue to advocate for the eradication of eucalyptus in California:

  • As the eradication projects get progressively more destructive, the public’s negative reaction to the destruction becomes progressively more aggressive.  There are now thousands of us all over the State and all over the country, working to stop this destruction and we are often effective in preventing these projects from being approved or funded.  An op-ed in the New York Times in September 2013, expressed support for our opinion that the word “invasive” has become a destructive tool and is inappropriately applied to eucalyptus in California.
  • The scientific community has also become progressively more critical of the attempts to eradicate eucalyptus.  Last fall, Nature  magazine quoted several well-known academic scientists in an article that criticized plans to destroy 30,000 eucalypts on Mount Sutro in San Francisco.  In May 2014, the CEO and Chief Scientist of the Nature Conservancy expressed their opinion on TNC’s website that destroying eucalypts in California is unnecessary.
  • Since blue gum eucalyptus is no longer available in nurseries in California and has not been planted for many decades, it has no long-term future in California.  To the extent that eucalyptus is a problem, it is a problem that will resolve itself in time.

Cal-IPC’s continued support for these projects is no longer in the mainstream of scientific or public opinion.  Removing eucalyptus from Cal-IPC’s “hit list” would significantly improve Cal-IPCs chances of success with the plants that remain on its inventory of invasive plants.  The public is unlikely to expend the same amount of energy opposing the eradication of broom, for example.

Cal-IPC has an opportunity to defuse a controversy that is handicapping the success of its venture.  Cal-IPC would be wise to abandon its crusade aganst blue gum eucalyptus.

Update:  On March 13, 2015, the California Invasive Plant Council published its final reassessment of Blue Gum Eucalyptus (available HERE).  Cal-IPC has downgraded its rating of invasiveness and ecological impact from “moderate” to “limited.”  Although the detailed assessment is less than perfect, the over all rating itself is an improvement.  Thanks to those who sent comments to Cal-IPC.

Methods used by land managers to control “invasive” plants

It’s not easy to find information about herbicide use by land managers.  We make inquiries and public records requests of all the managers of public lands in the Bay Area.  Despite these persistent efforts, we have never been confident that we have the complete picture.  We are therefore grateful for a recent survey conducted by the California Invasive Plant Council (Cal-IPC) of land managers—public and private—about the methods they use and for what purposes.  The following charts tell us what Cal-IPC learned from their survey.

Cal-IPC sent surveys to 100 land managers who described the lands they managed as follows:

Organization* Response
Local agency 54%
Federal agency 53%
State agency 48%
Land trust or other private landowner 40%
Private consultant 26%
Other (nonprofit, forestry company, utility, regional park district, conservation district) 27%
*multiple employers

How frequently are the following objectives part of land managers’ reason for managing invasive plants?

Cal-IPC Survey 5

Non-herbicidal methods used by land managers to control invasive plants






Pulling with hand tools





Digging with hand tools





Cutting with pruners or loppers





Weed whacking with string or plastic blade





Cutting with hand saw or chainsaw





Mowing with large equipment





Brushcutting with metal blade










How often do land managers use herbicides for invasive plant control?

Cal-IPC Survey 4

What methods do land managers use to apply herbicides?






Foliar spray – spray to wet





Foliar spray – thin line





Foliar spray – low volume/high concentration





Basel bark application





Cut stump application





Drill and inject application





Girdling or frilling application





Broadcast application





Wick application





Aerial application





What herbicides are used by land managers?

Active Ingredient Response Percent
Glyphosate (e.g. RoundUp, Aquamaster)


Triclopyr (e.g. Garlon 3A, 4A)


Aminopyralid (e.g. Milestone, VM)


Clopyralid (e.g. Transline, Reclaim)


Imazapyr (Chopper, Stalker, Habitat, Arsenal)


Chlorsulfuron (e.g. Telar)


Fluazifop (e.g. Fusilade)


2,4-D (e.g. Amine 4, Weedar)


Acetic acid


Clove oil (e.g. Matran


Pelargonic acid (e.g. Scythe)


These charts were shown by the Executive Director of the California Invasive Council (Cal-IPC) at a meeting of the Integrated Pest Management Program in San Francisco on February 6, 2014.  He explained that the survey of land managers was conducted to assist Cal-IPC in preparation for a new edition of best management practices for managing invasive plant species in wildlands.  That publication will include risk assessments of the herbicides being used by land managers.  Cal-IPC is collaborating with the author of the risk assessments of potential herbicide use for the Marin Municipal Water District. We look forward to the publication of this document, which is anticipated in June 2014.  We hope that land managers will have confidence in the risk assessments of the herbicides they use, given the source of the information.

We make note of …..

According to this survey of land managers:

  • Ninety-four percent of land managers are using herbicides to control plants they consider “invasive.”  Sixty-two percent are using herbicides frequently.
  • Seventy-four percent of land managers are using Garlon, which is one of the most hazardous herbicides available on the market, as rated by the Environmental Protection Agency.
  • Many land managers are using Milestone and imazapyr which are known to be mobile in the soil as well as persistent in the environment according to the manufacturer’s label and the Environmental Protection Agency.
  • Foliar spray is the method used most frequently by land managers to apply herbicides.  This method of application has the potential to drift into non-target areas.

These practices are not adequately acknowledged in the environmental impact reports for the ecological “restoration” projects in the Bay Area.  Some environmental impact reports have not acknowledged the types of herbicides being used or the methods used to apply them.  None of the reports have acknowledged the quantities used by the projects nor have they acknowledged the toxicity of the herbicides.  The public is therefore unaware of the extent to which herbicides are being used by these projects and the risks associated with using them.

Marketing creates a need where none exists

On December 14, 2013, the New York Times published an article entitled “The Selling of Attention Deficit Disorder.”  Attention Deficit Hyperactivity Disorder (ADHD) is a neurological disorder that was identified about 50 years ago.  The number of children taking medication for ADHD has soared from 600,000 in 1990 to 3.5 million presently.

Advertising in popular magazines such as People and Good Housekeeping suggest that medication for ADHD is needed to cure childhood forgetfulness and impatience and promises that schoolwork will improve and family tensions will be reduced.  These are empty promises for symptoms that are normal child behavior.  One of the manufacturers of ADHD medication recently paid to publish 50,000 copies of a comic book that uses superheroes to convince children that these medications will make life easier for them.

These seductive promises have proved very profitable for the pharmaceutical industry:  “Sales of stimulant medication in 2012 were nearly $9 billion, more than five times the $1.7 billion a decade before.”  But the industry is not satisfied with these results.  Now they are marketing these medications to adults.  Sixteen million prescriptions for ADHD medications were written for adults between the ages of 20-39 in 2012, nearly triple the number of prescriptions written just five years before.

As disturbing as this example of the insidious power of advertising is to convince us that we need something they are selling, it is hardly an isolated example.  On a typical evening in front of the TV, men are told that a variety of medications will improve their sex lives and women will be told that a good night’s sleep is just a pill away.

Advertising is also used to improve the image of industries that the public would otherwise think badly of.  For example, energy ratepayers are paying for a television campaign that tells us how much P.G. & E.– the monopoly provider of electricity and natural gas in Northern California–cares about our safety.  Yet, the mainstream media informs the public of the many breaches of public safety by P.G. & E.  In 2010, 37 homes and 8 lives were lost in San Bruno when an underground gas pipeline exploded.  The pipeline had been badly built, not inspected, and not repaired.  Subsequent investigations of P.G. & E.’s records proved that such neglect and incompetence is rampant throughout their system and continues to this day.  Money that could be spent on our safety is being spent on advertising.  Such manipulation of the public’s attitudes with advertising is the American industry standard.

Is the pesticide industry fueling the demand for ecological “restorations?”

Readers are surely wondering by now what this has to do with the mission of Million Trees.  Clearly the manufacturers of pesticides are the beneficiaries of the ecological “restorations” that destroy non-native vegetation with herbicides (herbicides are one type of pesticide).  From the standpoint of the industry, the more plants that are labeled “invasive” the better.  And since new plants are always being introduced—either intentionally or unintentionally—it’s a winning business model to label every new plant “invasive.”

National Invasive Species Council

We don’t have a lot of evidence to support our theory that the pesticide industry is one of the sources of support for invasion biology and ecological “restorations.”    Two of the 31 members of the National  Invasive Species Advisory Committee are employed by companies that manufacture pesticides.  One member is employed by Dow AgroSciences which manufactures Garlon, the most frequently used herbicide to prevent the resprouting of non-native trees after they are destroyed.  The other member is employed by Syngenta which manufactures pesticides and biocontrols which are another method used to destroy vegetation by introducing insects or plant diseases.

Is it inappropriate for the pesticide industry that benefits from the designation of “invasive” species to participate in setting federal policy regarding those species?  Undoubtedly there are arguments on both sides of that question.

We also know that pesticide manufacturers and other types of companies that engage in ecological “restorations” are financial supporters of the California Invasive Plant Council (Cal-IPC).  In its latest newsletter, Cal-IPC reports that Dow AgroSciences was one of the supporters of their annual symposium in 2013.  They also reported that Shelterbelt Builders is an “organizational member” of Cal-IPC.  Shelterbelt Builders is the company that does most of the major “vegetation management” projects for the so-called Natural Areas Program in San Francisco as well as doing many of their herbicide applications.

Shelterbelt began the eradication of non-natve vegetation in Glen Canyon in November 2011
Shelterbelt began the eradication of non-natve vegetation in Glen Canyon in November 2011

Admittedly, it’s a stretch to say that invasion biology was created to increase demand for pesticides and other products and services needed for ecological “restorations.”  We can’t and won’t say that.  But we invite our readers to wonder with us if some aggressive investigative reporting would find more evidence that it’s a factor.  Wouldn’t it help to explain why invasion biology persists despite the lack of scientific evidence to support it?

2013 Progress Report

As we approach the end of the year, let’s review the progress we’ve made in 2013 on our mission to save healthy trees and prevent the unnecessary use of herbicides in our public open spaces.  It’s been a good year:

  • University of California San Francisco (UCSF) has decided to scale back its plans to destroy about 30,000 trees and the forest understory on Mount Sutro.  They have also made a commitment to NOT use herbicides in the forest in the future.   (Visit Save Sutro for details.)
  • UCSF’s plans to destroy most of the trees on Mount Sutro were criticized by the mainstream press, i.e. the New York Times and Nature magazine.
  • Thousands of citizens in the Bay Area signed our petitions to object to the Mount Sutro project and the projects in the East Bay which FEMA is considering funding.  Likewise, critics of these destructive projects overwhelmed a handful of supporters at the public hearings about these projects.
  • Marin County Open Space District and Parks Department engaged a consultant who reported that “vegetation management” projects result in more non-native plants and that managers of public lands in the Bay Area no longer consider it feasible to eradicate all non-native plants in open spaces.

The California Invasive Plant Council has noticed the public’s opposition

Anise Swallowtail butterfly in non-native fennel
Anise Swallowtail butterfly in non-native fennel

Another barometer of our progress is the latest edition of the newsletter of the California Invasive Plant Council (Cal-IPC) which is available here.  Readers are immediately alerted to a change of attitude by the photograph on the cover of a native butterfly and a native bee feeding on a non-native thistle.  As anyone who has debated the issues with native plant advocates or read their propaganda knows, they usually deny that non-native plants are useful to native insects.

Bumblebee on Cotoneaster, Albany Bulb
Native bumblebee on Cotoneaster, Albany Bulb

The cover photograph makes a concession and sets the tone of the Cal-IPC newsletter.  The opening message from the Cal-IPC Executive Director begins with a quote from Invasive and Introduced Plants and Animals:  Human Perceptions, Attitudes and Approaches to Management“…intervention in conservation practice hides behind a veneer of pseudoscience and certainly challenges democratic processes.”    We told our readers about that book and ironically we selected the same sentence to describe its conclusion.  So, have we found some common ground with native plant advocates, as represented by Cal-IPC?

Not quite.  The title of the Director’s message is “A ‘cottage industry of criticisms.’”  This is the phrase used to describe those who criticize invasion biology.  Speaking for Million Trees and those with whom we collaborate, it is not accurate to call us an “industry” because we derive no economic benefit from our advocacy on behalf of non-native species.  In contrast, the ecological “restorations” that are based on the assumptions of invasion biology are an industry.  The economic interests of those who are employed by “restoration” projects are one of the reasons they cling desperately to the ideology that supports their employment.

The Cal-IPC Director tells us that, “Though they raise critical issues to address, such critiques underestimate the degree to which these issues are already being addressed.”  He claims that “Cal-IPC’s workshop asked participants to consider ecological services offered by top weeds of concern.  Weighing such information will become increasingly important as land stewards design management approaches to meet long-term conservation goals in an age of great environmental change.”

Cal-IPC can demonstrate this new management approach

What an excellent idea!! And we hope that Cal-IPC will start that new approach by revisiting its outdated assessment of Blue Gum eucalyptus which presently contains nothing but demerits, most of which are not even accurate.  If Cal-IPC takes into consideration the significant ecological services provided by Blue Gum eucalyptus, they will surely remove it from their long list of “invasive species.”  Here is a brief list of the ecological services provided by Blue Gum eucalyptus in California:

  • These large, hard-wood trees are storing millions of tons of carbon which will be released into the atmosphere as carbon dioxide when they are destroyed and as their wood decays on the ground, thereby contributing to climate change.
  • These trees are expected to live another 200-300 years, which means that this ecological service would be needlessly terminated by their premature destruction.
  • These trees are providing windbreaks on windy hills and for agricultural crops.
  • The roots of these trees stabilize the soil on hills that will erode when the trees are destroyed and roots die.
  • These trees provide the over-wintering roost of tens of thousands of monarch butterflies.
  • These trees are a source of winter nectar for bees, butterflies, and birds.
  • These trees are the nesting and roosting habitat of raptors and owls.

We urge Cal-IPC to demonstrate its professed willingness to consider the benefits of non-native species by revising their assessment of Blue Gum eucalyptus, which is rarely invasive and is providing valuable ecological services to animals as well as humans.  

Invertebrates such as insects are plentiful in the eucalyptus forest

Native plant advocates frequently claim that the eucalyptus forest is a “biological desert.”  We find no evidence to support that claim.  We are as likely to see a diverse understory in the eucalyptus forest as in oak woodland and more likely than in a redwood forest where there is considerably less light.  We have reported on several studies that found comparable diversity of wildlife in native and non-native forests.

The abundance and diversity of insects is particularly important in evaluating the health of an ecosystem because they are near the bottom of the food web.  We won’t find many birds in an ecosystem where there are few insects, for example.  We have reported on several studies that found comparable abundance and diversity of invertebrates such as insects in native and non-native landscapes.

Still, the myth persists that eucalyptus forest is devoid of life.  In this article we will address this specific statement in the assessment of the California Invasive Plant Council (Cal-IPC) of Blue Gum eucalyptus:  “loss of native plant forage and migratory disruptions may have greater long-term impact on wider diversity of wildlife species, including invertebrates and microorganisms in soil.”    Cal-IPC provides no studies to support this speculative statement.  Therefore, we will tell you about a specific study that refutes the assumption of Cal-IPC:  “Similar breakdown rates and benthic macroinvertebrate assemblages in native and Eucalyptus globulus leaf litter in Californian streams” (1)

First, we will provide a few definitions for our readers who may not have encountered some of the more esoteric jargon before.  The benthic zone is the sub-surface layer of bodies of water.  Here is a brief list of some of the common names of macroinvertebrates that lay their eggs in water that were found in this study:  mayflies, caddisflies, stoneflies, and midges.   These insects and their larva are food for fish and birds and in turn, fish are food for other animals.

Cerritos Creek. Not one of the creeks in the study, but typical of an East Bay creek with native vegetation.
Cerritos Creek. Not one of the creeks in the study, but typical of an East Bay creek with native vegetation.

Three small streams in Alameda and Contra Costa counties in the East Bay were selected for this study because they have sections of shore with eucalyptus forest and sections with native trees (oak, bay, big leaf maple, and alder).  Like many ecological studies we have read over the years, this study hypothesized that it would find reduced abundance and diversity of insect populations in the streams bordered by eucalyptus based on the assumption that eucalyptus is “lower-quality food resource for macroinvertebates than a mixture of native litter.”  As we will, see, they did not find evidence that supported their theory.  We are fortunate that their study was published, because the chances that a negative finding will be published are significantly smaller than studies with positive results.

We will briefly describe the methods used by this study because they establish the credibility of the study.  They sampled insect populations directly from the streams as well as using mesh bags of the litter of the two types of forest:  eucalyptus forest and an assemblage of native tree species.  The sampling was done in three different seasons and the litter bags were sampled after 26, 56, and 90 days.  They used two measures of diversity and two metrics related to pollution tolerance, as well as two measures of abundance of invertebrate species in litter bag samples to describe the insect population.

Here are their key findings:

  • “[Differences in y]early litter input rates in reaches bordered by Eucalyptus and by native vegetation were not statistically significant.”
  • Species diversity and pollution tolerance did not differ significantly between eucalyptus and native sites, with one exception.  There was a higher proportion of one complex of insects (Ephemeroptera, Trichoptera, Plecoptera) in the eucalyptus samples.
  • The abundance of the five most common taxa (species or genus) did not differ significantly between eucalyptus and native sites with the exception of mayflies which were on average twice as abundant in eucalyptus sites.
  • One metric of diversity (Shannon Diversity Index) found greater species diversity in eucalyptus sites compared to native sites.
  • The decay of litter in the bags of eucalyptus litter was similar to the bags of native litter, i.e., “leaf mass loss was not significantly different between eucalyptus and native leaves.”  Decay of litter is a proxy for the amount of litter consumed by insects and microorganisms in the litter and by extension the population of these organisms in the litter:  “…the importance of biotic factors (bacteria, fungi, macroinvertebrates) in litter breakdown is greater than that of the physical fragmentation.”

The study then compared these findings with similar studies conducted all over the world.  When they found differences between their results of those of other scientists, they explained them in terms of local differences in conditions.  For example, in European native forests, more deciduous trees are found than in Californian native forests.

Only one similar study was conducted in North America, specifically in two streams in southern California:  “… [it] compared the decomposition of Eucalyptus litter to native species and found it slower than that of Alnus [alder], faster than that of Rhus [sumac] and similar to Quercus agrifolia [coast live oak].  Both the decomposition rate and the biomass of macroinvertebrate colonizers differed much more between…two streams than among the litter species.”

Both the results of their study, and the review of the literature of similar studies, led the researchers to this conclusion:

“In coastal California, we conclude that presence of small patches of riparian Eucalyptus even though it influences the species composition of plant litter in streams, has no noticeable influence on diversity and composition of benthic macroinvertebrates that colonize the litter.  Furthermore, based on similarities in leaf decomposition, Eucalyptus litter appears likely to be as suitable a substratum for macroinvertebrate colonization as some of the components of the native litter in the three streams tested.  Thus, the overall condition of these small streams is not markedly degraded by the presence of patches of riparian Eucalyptus and is unlikely to be improved by their removal.”

Looking for Godot

Looking for evidence of the harm that eucalyptus does to our ecosystems is like waiting for Godot.  No one has found any evidence yet.  We venture to say that they can keep looking, but we think they are looking for something that isn’t there.  If we keep pointing out that there is no evidence to support their indictment against eucalyptus, will they give it up eventually?  All we can do is keep trying. 

We congratulate those with the tenacity to slog through this tedious post.  Your reward is more good news for our harmless eucalyptus.


Igor Lacan, Vincent Resh, Joe McBride, “Similar breakdown rates and benthic macroinvertebrate assemblages in native and Eucalyptus globulus leaf litter in Californian streams,” Freshwater Biology, 55, 739-752, 2010.

California Invasive Plant Council fails to make the case that eucalyptus is allelopathic

In this post we will continue to critique the assessment of the California Invasive Plant Council (Cal-IPC) that Blue Gum eucalyptus (Eucalyptus globulus) is invasive.  One of the arguments that Cal-IPC used to reach this conclusion is that chemicals in the leaves of eucalyptus suppress the germination of native plant species:  “[E. globulus] inhibits germination and growth of native plant species.”   This property is called allelopathy.

Many plants, both native and non-native have such allelopathic properties.  Therefore it is important both to determine if eucalyptus has such properties, and to compare eucalyptus to native tree species to determine if suppression of germination of competing species is any more likely under eucalyptus than native tree species.  One of the references provided by Cal-IPC compares germination success of three native plant species using both eucalyptus leaves and oak leaves:  “The Effect of Eucalyptus and Oak Leaf Extracts on California Native Plants” (1)

This unpublished master’s degree thesis does not prove that eucalyptus leaf extracts inhibit growth of native plants.  The study uses two different methods to test the hypothesis that eucalyptus leaf extracts inhibit growth of native plants.

In the first method, the seeds of three native species (two bunch grasses and a perennial forb) were germinated in petri dishes in sand soaked with a solution of the masticated leaves of eucalyptus and oak.  Two of the species of seeds grew shorter roots in the eucalyptus solution than in the oak solution.  The third species of seed grew longer roots in the eucalyptus solution than in the oak solution.  The percent of germination was lower in the eucalyptus solution than in the oak solution for two of three of the species of seeds and the same in the third species of seed.

The second method used by this study was to test germination success in the soil of eucalyptus compared to oak soil.  No significant difference was found in germination success when seeds were planted in the soil:

“The Eucalyptus soil treatment did not result in germination inhibition relative to the control which suggests that allelochemicals present in the leaves are reduced or absent in the soil.”  (1)

Since natural germination occurs in the soil rather than in petri dishes soaked in concentrated solutions, this study does not substantiate the statement that E. globulus “inhibits germination and growth of native species.”

Using our eyes to test the theory

We don’t doubt that the leaves of eucalyptus contain chemicals.  But the leaves of other trees do as well.  The question is not whether or not the leaves of trees contain chemicals, but rather do they prevent the germination and growth of other species of plants?  The fact is no study has proved that the chemicals in the leaves of eucalyptus are more likely to prevent the survival of native species of plants than any other tree species, whether native or non-native.  We can see with our own eyes that eucalyptus forests often have a thriving understory of both native and non-native plants.  Here are just a few examples of local eucalyptus forests that have such an understory:

The management plan for San Francisco’s Natural Areas Program describes the eucalyptus forest on Mount Davidson as follows:

“Although the overstory is dominated by eucalyptus, when all species were considered within the urban forest at Mount Davidson (point data), native species accounted for 36 percent of the understory cover and 21 out of 50 species were native…Pacific reed grass (Calamagrostis nutkaensis) does not have a state or federal special-status rating, but San Francisco is at the southern edge of this species’ range. This species can be found in several locations on Mount Davidson”

Native Pacific reed grass under girdled eucalyptus tree on Mount Davidson
Native Pacific reed grass under girdled eucalyptus tree on Mount Davidson

The 2011 “Albany Hill Creekside Master Plan” describes the understory of the eucalyptus forest on Albany Hill as follows:

“The eastern portion of the eucalyptus forest has a [native] toyon understory as identified in 1991.  The toyon appears to be a wider band than shown in 1991 and covers approximately 2.0 acres…It was noted in a 1972 article in the California Native Plant Society publication Fremontia that the toyon has been introduced by either man or birds.  Native species [in the eucalyptus forest] include toyon, coast live oak, coyote brush, blue wild rye grass, and poison oak.”  

Native toyon under eucalyptus on Albany Hill
Native toyon under eucalyptus on Albany Hill

Finally, the understory of the dense eucalyptus forest on Mount Sutro is the richest understory we have personally witnessed.  Its understory is composed of both native (most notably elderberry) and non-native species.

The lush, green understory on Mount Sutro.  Courtesy Save Sutro Forest.
The lush, green understory on Mount Sutro. Courtesy Save Sutro Forest.

We give the last word on the scientific question of the allelopathic properties of eucalyptus to R.G. Florence of the Department of Forestry at The Australian National University.  An Australian scientist is not under the same pressure to find a negative story to tell about eucalyptus.  Professor Florence reports that a world survey of 3,000 articles about allelopathy found “…that the phenomenon of direct chemical interaction in natural communities, in the face of natural selection pressure, must be regarded as rare.”  And further, “While [allelopathy] is an attractive concept, there is no certainty that this occurs to any appreciable extent in nature.” (2)  These observations are certainly consistent with the reality of the eucalyptus forest in the San Francisco Bay Area, where an understory of both native and non-native plants is often found.

If not allelopathy, then what suppresses understory growth?

We have hiked as often in oak woodland in California as we have eucalyptus forests.  We find the understory in the oak woodland as varied as any eucalyptus forest.  Sometimes we don’t find much understory in either type of forest.  A redwood forest has the sparsest understory of any of these three tree species.

What these forest types have in common is that there is a layer of leaf litter under them that suppresses germination and growth of other plants because it forms a physical barrier to the soil.  And the limited sunlight on the floor of both forests is surely a factor in suppressing the development of an understory.  When an understory persists through the limiting factors of low light and heavy leaf mulch, there are obviously mitigating factors such as more moisture, better soil, and other resources that understory plants need.  Furthermore, some species of native plants seem to be suited to conditions in the eucalyptus forest.

The leaves of eucalyptus contain chemicals–as do the leaves of all plants– but if they do not prevent the growth of an understory or they are not any more likely to suppress the growth of competing plants than chemicals in native tree species, this is not a legitimate argument against eucalyptus.  Cal-IPC has not provided any scientific justification for indicting eucalyptus based on its allelopathic properties.


(1)    Kam Watson, “The Effect of Eucalyptus and Oak Leaf Extracts on California Native Plants,” 2000.

(1)    R.G. Florence, Ecology and Silviculture of the Eucalypt Forest, CSIRO, 1992?, pgs 71 & 103

Photographic evidence that eucalyptus is NOT invasive

Our subscribers have probably noticed that we are studying the case the California Invasive Plant Council (Cal-IPC) has made to classify Blue Gum eucalyptus (Eucalyptus globulus) as “invasive.”  We have reported to our readers that Cal-IPC has made speculative claims about harm to wildlife that are unsupported by scientific evidence:

Is Blue Gum eucalyptus invasive?

In this post, we will look at the “evidence” provided by Cal-IPC that Blue Gum eucalyptus is invasive in California.  Here is how Cal-IPC described the “local rate of spread with no management” of Blue Gum eucalyptus:

“Once a tree matures and produces seed, it can produce a profusion of progeny within a few years; doubling of stand area within 10 years possible but not well documented Without quantitative data, this response is conservative; stands have certainly expanded far beyond initial plantings in many locations, based on unpublished photodocumentation (1, 2) and personal observations (3)”  [numbers refer to cited “references”]

And here is the “evidence” Cal-IPC provides in support of this rather dire prediction of the invasiveness of Blue Gum in California:

 “Potts, Michael. 2003. About this edition. Caspar News. Online @ 2. Site Stewardship Program, Parks Conservancy. Unpublished photographs of Oakwood Valley, Marin Headlands, Golden Gate National Recreation Area. 3. Warner, PJ. 2004. Personal observations from 1980-2004 working in and adjacent to Eucalyptus stands in Marin, Sonoma, and Mendocino Counties, CA. 707/937-9172;”

With the exception of an article in “Caspar News,” all evidence provided by Cal-IPC is unpublished.  Although the one written source is described as “Caspar News,” in fact its title is “Caspar Newsletter.”  The edition of this newsletter that is cited is the first unprinted edition of the “Caspar Newsletter.” Some of the unpublished “evidence” cited by Cal-IPC is described as “personal observations” of Peter Warner, who is the author of the Cal-IPC assessment for Blue Gum eucalyptus. 

Therefore, the only source of information about the invasiveness of Blue Gum that we can evaluate is the one that is available on the internet HERE.

First a word about the town of Caspar, which is located 4 miles north of Mendocino on the coast of California.  According to the 2010 census, it has a population of 509 souls.  We celebrated New Years Eve there many years ago in a rocking bar, so we have fond memories of it.  It is a lovely little town.  We mention its small size to put its newsletter into perspective.  It’s hardly mainstream journalism.

The article in the “Caspar Newsletter” starts with the recommendation of Peter Warner to eradicate all eucalyptus in Caspar:

“In this newsletter you find several articles written by strong advocates of dire means, including the authoritative Eucalyptus indictment written by State Parks’ expert on managing exotics Peter Warner, who advocates a draconian solution:  cutting and then careful application of a dire chemical to eliminate every tree.”

In other words, the “Caspar Newsletter” is merely a repetition of Peter Warner’s agenda to eradicate eucalyptus and poison them with herbicides to prevent them from resprouting.  It’s not an independent source of information.

Photographic evidence of invasiveness?

The only photographic evidence of the invasiveness of Blue Gum eucalyptus provided by Cal-IPC’s assessment is in the article in “Caspar News:”

"Eucalyptus encroaching on the ocean view"
“Eucalyptus encroaching on the ocean view”

There are three problems with this photograph with respect to the claim that it is evidence of the invasiveness of eucalypts:

  • We are asked to trust the memory of the photographer about the history of this eucalyptus grove.  Credible evidence of spread of the eucalyptus grove would provide dated photographs taken at each period of time represented in this photo, i.e., 1989, 1994, 1999, and 2003.
  • We see the ocean in the far distance, west of this grove of trees.  As the forest approaches the ocean, we see that the trees are smaller.  This is as we would expect, because the wind from the ocean has suppressed the growth of the trees on the windward side of the grove.  The fact that wind suppresses the growth of trees was established by Joe R. McBride in his study of trees in the San Francisco Presidio which the Presidio contracted with him to conduct:  “Wind at the Presidio affects tree growth, form, and mortality. Exposure to winds in excess of 5 mph usually results in the closure of the stomata to prevent the desiccation of the foliage (Kozlowski and Palhardy, 1997) Photosynthesis is thereby stopped during periods of moderate to high wind exposure resulting in a reduction in tree growth…Eucalyptus showed the greatest reduction in growth with trees at the windward edge being only 46 percent as tall as trees on the leeward side.” (1) (emphasis added)
  • The photographer asks us to believe that the eucalyptus forest is spreading towards the ocean.  Given that the seeds of eucalyptus are dispersed by gravity and wind and that the wind is coming from the ocean, we would not expect the eucalypts to spread towards the ocean, but rather on the leeward side of the forest.

In other words the “evidence” provided by the Cal-IPC assessment that E. globulus is very invasive is not supported by the evidence that is provided.

It is possible to document invasiveness with photographic evidence.  We have provided our readers with two such examples that indicate that Blue Gum eucalyptus is not invasive in the San Francisco Bay Area:

  • In “Vegetation Change and Fire Hazard in the San Francisco Bay Area Open Spaces,” William Russell (USGS) and Joe McBride (UC Berkeley) used aerial photos of Bay Area parks taken over a 60 year period from 1939 to 1997, to study changes in vegetation types.  They studied photos of 3 parks in the East Bay (Chabot, Tilden, Redwood), 2 parks in the North Bay (Pt Reyes, Bolinas Ridge), and one on the Peninsula (Skyline).  These photos revealed that grasslands are succeeding to shrubland, dominated by native coyote brush and manzanita.  Eucalyptus and Monterey pine forests actually decreased during the period of study.  In those cases in which forests increased in size, they were native forests of oaks or Douglas fir.  In other words, they found no evidence that non-native trees are invading native trees or shrubs.
  • Another example of photographic evidence that E. globulus is not invasive is from Mount Davidson in San Francisco.  Adolph Sutro purchased Mount Davidson in 1881.  He planted it—and other properties he owned in San Francisco—with eucalyptus because he preferred a forest to the grassland that is native to the hills of San Francisco.  Here are historical photos of what Mt. Davidson looked like in 1885, 1927 and 2010:

Mt Davidson 1885

Since Sutro didn’t own all of Mt. Davidson, there was a sharp line between the forest and the grassland when this photo was taken in 1927.

MD 1927 RPD presentation

Over 80 years later, in a photo taken in 2010, there is still a sharp line between the forest and the grassland.  We see more trees in the foreground where residential areas have been developed and home owners have planted more trees, but the dividing line on the mountain is nearly unchanged.

MD 2010 RPD

There is one well-documented case of significant expansion of planted E. globulus on Angel Island.  Using historical records of planting of E. globulus on 23.6 acres as well as observations of uniform spacing of those plantings, McBride et. al., determined that E. globulus spread to 86.1 acres.  The trees were planted starting in the mid-1870s to 1933 and their spread was measured in 1988.  The authors of the study reported that most spreading occurred in areas of high soil moisture, such as swales, and in disturbed areas such as road cuts.  This is also the only documented case of significant expansion of planted E. globulus mentioned in the US Forest Service plant data base. (2)

The one exception to the general rule that Blue Gum eucalyptus has not been invasive in California is consistent with what we know about Angel Island and about the limitations of seed dispersal and germination rates of Blue Gum eucalyptus:

  • Angel Island is an extremely windy and foggy place because it is located in the San Francisco Bay, close to the Golden Gate to the Pacific Ocean, where wind and fog enter the bay.
  • Eucalyptus seeds are dispersed by gravity and wind.  Therefore we can expect seeds to travel further in a very windy place.
  • Optimal soil moisture increases the success of seed germination.  Fog drip increases soil moisture and spreading of the eucalyptus forest on Angel Island occurred in drainage swales, where moisture would be greatest.

How invasive is Blue Gum eucalyptus?

Blue Gum eucalyptus is rarely invasive.  The only documented case of significant spread of eucalyptus forest occurred in ideal conditions for seed dispersal and germination.  Therefore, Cal-IPC’s claim that Blue Gum eucalyptus is extremely invasive is exaggerated at best and fabricated at worst. 

If our readers are aware of any other documented cases of spreading of eucalyptus, we invite them to inform us.  We are committed to accurately informing ourselves and our readers of the reality of invasiveness of Blue Gum eucalyptus.


(1)	“Presidio of San Francisco, Wind Study, First Phase,” Joe R. McBride, circa 2002, page 6.  (unpublished, contracted study) 
(2)	“Focused Environmental Study, Restoration of Angel Island Natural Areas Affected by Eucalyptus,” California State Parks and Recreation, July 1988, pg 47 & 51.

Eucalyptus trees do NOT kill birds!

The claim that eucalyptus trees kill birds originates with an article in the publication of the Point Reyes Bird Observatory, written by Rich Stallcup.  (1)  Mr. Stallcup was well known as a knowledgeable birder, but he was not a scientist.  He based his claim that eucalyptus trees kill birds on his observation of two dead birds in two different eucalyptus forests over a period of many years.   

Bird mortality rates

Given that Mr. Stallcup was a serious birder who spent much of his time in the field observing birds, we begin our critique of his hypothesis by pointing out that a sample of two is absurdly small from which to extrapolate to a general rule about bird mortality in eucalyptus forests.

Furthermore, the scientific literature about bird mortality informs us that a sample of two does not justify the assumption that these deaths were caused by eucalyptus trees.  For example, the annual survival rate of adult song sparrows has been reported as only 30%.  That is, in a single year, 70% of adult song sparrows will die of predation, disease, starvation, or other factors including old-age. Mortality rates are greater for small birds.  (2)

Mortality rates for young birds are substantially greater than for adult birds.  For example, a longitudinal study of Eurasian kestrels found that 51% of 245 kestrels were dead within their first year.  All of the 245 kestrels in this study were dead by the end of 10 years.  (2)

We have a beautiful Coast Live Oak tree on our property, under which we have found dead birds. Yet we have not concluded from those observations that the birds were killed by the tree.  Of course, they were not.  Although there were probably many different causes of death of these birds, we don’t feel the need to speculate about those causes because bird death is not a rare or unusual event unless, of course, you are looking for an excuse to blame the tree.

Flowers of Blue Gum eucalyptus
Flowers of Blue Gum eucalyptus. Photo by John Hovland

Stallcup’s Theory

Spotted pardalote is an Australian bird with a short beak that feeds in eucalyptus forests in Australia.  Creative Commons
Spotted pardalote is an Australian bird with a short beak that feeds in eucalyptus forests in Australia. Creative Commons

Stallcup speculated that eucalyptus trees kill birds by “gumming” up their beaks or nostrils with the nectar of the eucalyptus flower which blooms from about December to May in California.  He supports this theory by claiming that the birds that are found in Australia, where eucalyptus is native, have long, curved beaks which enable them to eat nectar from the flower without gumming up their beaks or nostrils. 

This theory is not consistent with the “evidence” that Mr. Stallcup uses to support his theory:

  • He found a dead ruby-crowned kinglet.  The kinglet is an insect-eating bird, not a nectar eating bird.
  • Years before that sighting, he found a dead hummingbird.  Hummingbirds eat nectar, but they have long beaks.
  • Many Australian birds that feed in eucalyptus forests do not have long, curved beaks; e.g., spotted pardalote, striated thornbill, and white-naped honeyeater. (3)
  • One study in Santa Cruz, California found many small birds feeding on insects in red gum eucalyptus, including yellow-rumped warblers, Townsend’s warblers, ruby-crowned kinglets, and bushtits.  The beaks of these birds are not shorter than those found in eucalyptus trees in Australia mentioned by the same study.  The study reported no dead birds or gummed beaks. (3)
  • The nectar of eucalyptus flowers is not “gummy.”  It feels watery to the touch.  Eucalypts are not called “gum” trees because of the nectar in their flowers.  They are named for the sap under their bark.
Ruby-crowned kinglet is a North American bird that feeds in the eucalyptus forest in California.  USFWS
Ruby-crowned kinglet is a North American bird that feeds in the eucalyptus forest in California. USFWS

We can compare Mr. Stallcup’s “data” of two dead birds with a database of a local wildlife hospital.  The report of this database was posted to SFBIRD (San Francisco Bird is an email listserve that anyone can subscribe to and report the birds they see in San Francisco) by Richard Drechsler on March 12, 2012 and is quoted here with his permission:

“I have access to a database containing 56,960 records of birds brought to a local wildlife hospital between 1992 and 2010.  During this period this facility also accepted 2500 birds who were classified as DOA (dead on arrival).  I volunteered in that facility for seven months during 2010 where I worked with ‘small birds’ in order to get a better idea of how birds are being injured.  I was aware then of the belief that resin, gum, nectar, etc. from Eucalyptus trees might harm feeding birds.  During my time there I did not encounter any birds whose passages were blocked with any natural resins that would have prevented them from eating or breathing.  This evening I compared my observations with the 19 years of data by searching on the following words:  ‘euc,’ ‘euk,’ ‘bill,’ ‘beak,’ ‘tree,’ ‘asphy,; ‘breath,’ ‘mouth,; ‘nar,’ ‘gum,’ ‘resin,’ ‘nectar,’ ‘nostril,’,’ starv,’ ‘horn,’ ‘stick,’ ‘stuck,’ ‘glue.’ 

Preliminary Data Findings:

(1)    There is no reference to Eucalyptus or any of its byproducts.

(2)    There is one reference to a bill or mouth being restrained…by a synthetic adhesive.

(3)    There is one vague reference to an Anna’s Hummingbird that was “Stuck in Resin”…treated and released one day later.

(4)    Querying on ‘sticky,’ ‘stuck,’ or ‘glue’ yields many records and a wide variety of species being trapped by synthetic adhesives such as rodent traps and building adhesives.”

 Given the ubiquitous presence of Eucalyptus in this area and the birds craving for its nectar one would expect more incidents of starving birds or ones surrendered DOA.  Also, if birds did not have the capacity to preen or molt away this nectar, wouldn’t every (indulging) bird ultimately display this residue?”

 Bird anatomy trumps the absence of data

There is no empirical evidence to support the claim that eucalyptus trees kill birds, but the most compelling evidence that this claim is not factually correct is that it contradicts the basic facts of bird physiology and anatomy.  That is, birds can and do clear their beaks and nostrils with their feet or by rubbing their beaks on branches when necessary.  If their nostrils are obstructed, they can breathe through their mouth and vice versa.

Ask yourself this question to appreciate the absurdity of the claim that birds would passively suffocate rather than using the tools they have at hand:  If you were suffocating because there was something stuck in your mouth or nose, wouldn’t you raise your hands to your face and clear the offending obstacle?  Is there any reason to assume that birds are not physically or mentally capable of the same defensive behavior?  If you have a cold and your nose is stuffed up, don’t you breathe through your mouth?

In 2010, a student taking the Cornell Lab of Ornithology correspondence course on bird biology asked his instructor this question:  Do eucalyptus trees kill North American birds?”  This is the email reply from his instructor:

“I have no firsthand knowledge of the effect of eucalyptus gum suffocating birds (or not), but I share your skepticism for the reasons you mention. The story has birds feeding in flowers and getting gum on their faces. The first bird mentioned in this saga appears to have been a Ruby-crowned Kinglet. They don’t feed on or in flowers much; they’re leaf gleaners and flycatchers. Is there even “gum” to be picked up on the flowers? Again, that seems unlikely. Birds can breathe through their mouths, so just plugging up the nostrils won’t kill them. (Nestling crows often get the feathers covering their nostrils so encrusted with food matter that there is no way they could breathe through them, but the young are fine. I chip it off when I band them.) It seems like this story could be investigated rather easily, but I see nothing about it in the scientific literature. I would need to see evidence before I believed it.”  

Those who love to hate eucalyptus

The Cornell ornithologist would need to see evidence before he believed the claim that eucalyptus trees kill birds.  Native plant advocates apparently don’t need any evidence.  They have been repeating this absurd, baseless claim since 1996, when it was originally fabricated.  One subscriber to the SFBIRD email listserve mentions this claim often, although he never offers any new actual dead birds to add to the two that were used to fabricate the story. 

 So, why do we try, once again, to set the record straight despite the stone wall built around this fable by native plant advocates?  Because we find this claim in the “assessment form” used by the California Invasive Plant Council to justify its classification of eucalyptus as “invasive:” “purported to cause mortality in native bird species.”  The California Invasive Plant Council classifies eucalyptus as “invasive” based partly on the existence of two dead birds. 

This story has been repeated by native plant advocates for nearly 20 years without any supporting evidence.  It has taken on a life of its own until those who repeat it apparently are unaware that there is no evidence to support the myth.

The US Forest Service social scientist, Dr. Paul Gobster, interviewed native plant advocates while a visiting professor at UC Berkeley about ten years ago.  At the end of his visit, he delivered a lecture at the Randall Museum about his observations of the local native plant movement.  He said they were victims of “incestuous amplification,” the trading of misinformation in a vacuum caused by their isolation.  The ridiculous story about eucalyptus trees killing birds is surely an example of incestuous amplification.   


(1)    Rich Stallcup, “Deadly Eucalyptus,”  Point Reyes Bird Observatory, Fall 1996

(2)    Handbook of Bird Biology, Cornell Lab of Ornithology, Princeton University Press, 2004

(3)    Julie Lockhart and James Gilroy, “The portability of food-web dynamics:  reassembling an Australian psyllid-eucalypt-bird association within California,” Global Ecology and Biogeography, 2004, 13, 445-450

Monarch butterflies in California need eucalyptus trees for their winter roost

Monarchs are probably the best-known butterfly in North America, partially because they are distinctively beautiful, but also because of their epic migration.  East of the Rocky Mountains, monarchs migrate from the Canadian border and the Atlantic Coast to spend the winter months in fir trees in Michoacan State in Mexico.  West of the Rockies, monarchs migrate from the Canadian border and the Pacific Coast to overwinter along the coast of California from Mendocino County to San Diego County, near the Mexican border.

Monarch Butterfly.  Creative Commons
Monarch Butterfly. Creative Commons

No single monarch makes the entire journey.  It takes two to three generations of monarchs to make the entire round trip.  How each successive generation knows the route remains largely a mystery, although theories exist.  There are a couple of fascinating books about the migration that we recommend to our readers.  Four Wings and a Prayer is a book about the 38-year effort of Canadian entomologists, Fred and Norma Urquhart, to understand the migration.  It reads more like a suspenseful mystery than the non-fiction book that it is.  Flight Behavior is by Barbara Kingsolver, one of our favorite novelists because nature is often the subject of her work.  Although it is fiction, it has been carefully researched by Kingsolver who studied biology before becoming a writer.  It is engaging both as a cautionary tale for environmentalists and as a personal redemption story.

The western migration of the monarch

Monarch butterflies roosting in eucalyptus tree.
Monarch butterflies roosting in eucalyptus tree.

We will focus on the western migration of the monarch because that’s our neck of the woods, but also because this migration is one of the reasons why many people who care about nature and wildlife object to the destruction of eucalyptus trees.  Eucalypts are the preferred trees for over-wintering monarchs“Three types of trees were used most frequently by roosting monarchs:  eucalyptus (75% of the habitats primarily Eucalyptus globulus), pine (20% of the habitats primarily Pinus radiata), and cypress (16% of the habitats Cupressus macrocarpa).  Twelve other tree species were identified…with a combined prevalence of only 10%.” (1)

For those who may not know the botanical names, that’s Monterey pine and Monterey cypress that are the runners-up to eucalypts as the most popular trees for over-wintering monarchs.  Although monarchs roost in those trees in their native range on the Monterey peninsula, they also use those species outside their native range.  Unfortunately, just as the eucalyptus is a target of native plant advocates who demand their destruction because they are not native, both Monterey pine and Monterey cypress are targeted for destruction outside their native range.  For example, both Monterey pine and Monterey cypress will be eradicated from hundreds of acres of public land if the FEMA grants are funded in the East Bay.  This is just 150 miles away from where those trees are native and there is fossil evidence that they existed in the East Bay in the distant past.  In other words, most of the trees used by monarchs for their winter homes are in jeopardy of being destroyed by the native plant movement.

Another nativist myth BUSTED!

One of the reasons why we are telling this story is that it is a tidy little example of the justifications fabricated by native plant advocates to support their destructive agenda.  In the case of the monarch, native plant advocates claim that prior to the arrival of Europeans, before eucalypts were planted and Monterey pines and cypresses were planted outside their native range, the monarch used native trees for their over-wintering habitat.  The “assessment form” used by the California Invasive Plant Council to classify Blue Gum eucalyptus as invasive says,  “[The Blue Gum] provides roost sites for migratory monarch butterflies…ecological niches for butterflies and raptors probably formerly filled by native plant species.”  No evidence is provided in support of that statement.  We have also read that claim in comments of native plant advocates on internet articles in response to those who defend eucalypts because they are needed by monarchs.

Like many of the “cover stories” of native plant advocates, this is just not true.  A search of the scientific literature about monarchs enables us to bust this particular myth to smithereens.  It would be simple enough for native plant advocates to look at the evidence before spinning their tales, but it is apparently easier to make it up, especially when they are rarely questioned.  Million Trees exists to fill this informational void.

The historical record of the western migration of monarchs

The earliest record of over-wintering monarchs in California is from 1864, when monarchs were observed over-wintering in Monterey pines in their native range.  Richard Vane-Wright, the scientist who reports this record, explains why he believes it is probably the first incidence of over-wintering monarchs in California:

“’Previous to that, no mention has been found of this interesting phenomenon…The early Spanish chronicles and traditions make no mention of it, although Monterey, a scant three miles distant, was gay with life when the last century came in…even David Douglas, the world famed botanist, and the keenest-eyes of all the strangers who came [to California] is silent regarding it.’…Douglas, the indefatigable fir tree collector, appears to have made no mention of the phenomenon in 1830-1832, despite spending two winters at Monterey.” (2)

Vane-Wright believes the eastern monarch migration to Mexico also began around the same time.  His theory is that the agricultural practices of early settlers, which cleared trees, created a population explosion of the milkweed that is the host plant of monarchs.  More milkweeds resulted in more monarchs and monarchs began to migrate in response to population pressure, he believes.  He calls this the “Columbus Hypothesis.”  (2)

Biological facts explain why monarchs choose these species of trees

Aside from the historical record, the biology of monarchs and the physical characteristics of the trees in which they over-winter explain why these species of trees are required by the over-wintering monarch.  During the late fall and winter, monarchs enter a dormant phase called diapause.  They continue to need nectar and moisture during that period, but they are not very active, so these resources must be close by.  Although they migrate to the coast from Mendocino to Mexico, they are most abundant around the mid-point of that range, where temperatures and rainfall are moderate.  Most of the approximately 250 roosting sites are within 2.4 kilometers of the ocean, so wind protection is important for them while they are roosting.  All of these factors predict the ideal conditions provided by eucalyptus trees:

  • Monarchs need tall trees (of at least 60 feet) because they roost in the intermediate level of the canopy where wind protection is greatest (3)
  • The forest must be dense enough to provide wind protection,
  • The tree canopy must be open so that the roosting monarchs receive filtered sunlight to keep their bodies warm enough.
  • The monarchs need enough moisture for hydration, but not so much that they are soaked and lose their body heat.  So, dew and/or fog provide the ideal amount of moisture.   (1 & 4)

All of these requirements for the monarch’s winter roost point to their dependence on eucalyptus, pines and cypress.  The trees that are native to the narrow strip of the coast of California do not meet these criteria.  They are not tall enough and they do not grow that close to the ocean because they do not tolerate wind.  The native vegetation of that narrow strip of California coast is predominately dune scrub and coastal grassland prairie.  And these are the vegetation types that the ecological “restorations” in the Bay Area are trying to re-create.  These vegetation types will not be suitable habitat for over-wintering monarchs.  Furthermore, plans to drastically thin eucalyptus forests on hundreds of acres of the East Bay Regional Park District will render those habitats useless for over-wintering monarchs.

In addition to the physical properties of eucalyptus, the monarch benefits from the fact that it is flowering from about December to May, while the monarch is roosting in the tree.  The flowers of eucalyptus contain a copious amount of nectar which is also important to the honeybee because it is flowering at a time when there are few other sources of nectar.  One study reported observing monarchs feeding on the flowers of Eucalyptus globulus. (5)

Risky Business

We have mixed feelings about reporting this research about monarchs to our readers because there is some risk to the monarchs in doing so.  The evidence suggests that monarchs did not over-winter in California prior to 1864, after the magical date that nativists have selected to freeze-frame California’s landscape to their nativist ideal.  Here in the San Francisco Bay Area, this magical date is 1769, when the expedition of Portola laid eyes on the San Francisco Bay.  Will nativists declare the monarch an alien invader to be eradicated along with the hundreds of plants and animals they claim “don’t belong here?”  This may seem a far-fetched conjecture, but keep in mind that the European honeybee is being eradicated in some “restorations” because it is not native.  The honeybee is essential to the survival of American agriculture, yet its existence is threatened by the radical agenda of the native plant movement.

That’s the risk we take in reporting this evidence because we hope that it helps our readers to understand the absurdity of the nativist agenda.

Update:  Monarchs have returned to Natural Arches State Beach in Santa Cruz in big numbers.  Here is a link to a report that includes a lovely video of the roosting Monarchs.  


(1)    Dennis Frey and Andrew Schaffner, “Spatial and Temporal Pattern of Monarch Overwintering Abundance in Western North America,” in The Monarch Butterfly Biology and Conservation, Cornell University Press, 2004.

(2)    Richard Vane-Wright, “The Columbus Hypothesis:  An Explanation for the Dramatic 19th Century Range Expansion of the Monarch Butterfly,” in Biology and Conservation of the Monarch Butterfly, Natural History Museum of Los Angeles County, 1993.

(3)    Andres Kleiman and Miguel Franco, “Don’t See the Forest for the Butterflies:  The Need for Understanding Forest Dynamics at Monarch Overwintering Sites,” in The Monarch Butterfly Biology and Conservation, Cornell University Press, 2004.

(4)    Kingston Leong, et. al., “Analysis of the Pattern of Distribution and Abundance of Monarch Overwintering Sites along the California Coastline.” in The Monarch Butterfly Biology and Conservation, Cornell University Press, 2004.

(5)    Susan Chaplin and Patrick Wells, “Energy reserves and metabolic expenditures of monarch butterflies overwintering in southern California,” Ecological Entomology, 7:249-256, 1982