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Tracking down the truth about blue gum eucalyptus

July 21, 2014

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 info@cal-ipc.org).  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, et.al., “The influence of urban park characteristics on ant communities,” Urban Ecosyst., 11:317-334, 2008

 

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