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More evidence that eucalypts are not invasive

November 6, 2012

Eucalyptus, Mountain View Cemetery, Oakland

We have provided our readers with photographic evidence that eucalypts are not invasive in the San Francisco Bay Area (click here and here).  Now we are going to tell you about more confirmation of this fact from a reputable source that will be difficult for native plant advocates to ignore:  Encyclopedia of Biological Invasions.

The Encyclopedia was edited by Daniel Simberloff (U of Tennessee) and Marcel Rejmanek (UC Davis) and published by UC Berkeley Press in 2011.  Many of our readers will recognize Simberloff as a prominent scientist in invasion biology.  He is responsible for the “invasional meltdown” hypothesis which is central to invasion biology.  A recent survey of empirical tests of the hypotheses of invasion biology found that there is considerable support for the “invasional meltdown” hypothesis, but that support is declining. 

Professor Simberloff has aggressively defended the assumptions of invasion biology against scientists who think that a revision of those assumptions is required by recent empirical evidence.  When Professor Mark Davis and 18 of his colleagues in ecology signed a comment in the Nature journal entitled, “Don’t judge species on their origins,” Professor Simberloff promptly recruited 140 of his colleagues to publish a rebuttal. 

We establish Professor Simberloff’s credentials for our readers as a scientist who firmly believes that non-native species are a serious threat to biodiversity so that native plant advocates will consider him a credible source of information regarding eucalyptus.   

“Eucalypts” according to the Encyclopedia of Biological Invasions

According to the Encyclopedia of Biological Invasions, eucalypts are “some of the most important solid timber and paper pulp forestry trees in the world.”  There are about 40 million acres of eucalypts planted in tropical, sub-tropical, and temperate countries.  The predominant species of eucalyptus in the Bay Area, Blue Gum (E. globulus), is grown in 13 countries in addition to the US and Australia.  About 70 species of eucalypts are naturalized outside their native ranges. “However, given the extent of cultivation, eucalypts are markedly less invasive than many other widely cultivated trees and shrubs…they have been orders of magnitude less successful as invaders than pines and several other widely planted trees…Where eucalypts have invaded, they have very seldom spread considerable distances from planting sites, and their regeneration is frequently sporadic. “ (1)

Although the Encyclopedia admits to being puzzled by why eucalypts aren’t invasive, it offers “three major reasons for the limited invasiveness of eucalypts:”

Reason One:  Seed dispersal of eucalypts is limited

The seeds of eucalypts have no natural means of dispersal, such as fleshy tissue which can function as wings on the wind.  Tests have shown that the seeds “are dispersed over quite short distances.”  (1) “Seed dispersal is mainly by wind or gravity and is virtually limited to twice the tree height.” (2) 

The seeds of the Blue Gum are encapsulated in a woody pod which makes them inedible to birds and mammals.  So, the seeds of the Blue Gum are not dispersed by animals.

Reason Two:  High mortality of eucalyptus seedlings

Eucalyptus seedlings die quickly if they don’t establish roots in moist soil quickly.  If the soil is too moist they are susceptible to destruction by fungus.  If there is too much leaf litter or there is an understory, they are unlikely to find the quick access to the soil they need to survive.  There is a narrow range of conditions needed to successfully establish eucalyptus seedlings.

Reason Three:  Lack of compatible mycorrhizal fungi

Mycorrhizal fungi exist in the soil and sometimes form a symbiotic relationship with the roots of plants. They are often essential to the health of the plant because they facilitate the absorption of water and nutrients by the plant.  Some biologists speculate that the specific species of mycorrhizal fungi needed for successful seedling development have not been exported with the eucalypts to foreign soils. 

A balanced discussion of the pros and cons of eucalypts

Given the strong commitment of the authors of the Encyclopedia of Biological Invasions to invasion biology, we are impressed with its even-handed discussion of the ecological pros and cons of eucalypts as well as its recognition of the lack of hard data to support a particular conclusion:  “Conclusions about positive or negative environmental and economic impacts of eucalypts are often anecdotal, highly controversial and context dependent.”   

The authors suggest that eucalypts not be planted near streams as the moving water is a means of seed dispersal.  On the other hand, when planted on degraded soil, the eucalypts have provided a fuel source which reduces pressure on remnants of native forests.  Eucalypts have been a valuable source of nectar for honey production all over the world.  More birds are said to be found in native forests than eucalyptus forests in California.  However, three times as many salamanders are found in the eucalyptus forests compared to native forests in California. 

Eucalyptus and honeybee. Painting by Brian Stewart

The Encyclopedia also addresses the controversial question of whether or not eucalypts are allelopathic, which means chemicals in their roots or leaves suppress the germination of the seeds of other plants.  It reports that there is no conclusive evidence on this question.  However, the accumulation of leaf litter is probably a physical barrier to the germination of seeds in its understory, which is not an allelopathic method of suppressing competition.  This is clearly true of other trees as well.  For example, the tannins present in both oak and eucalyptus leaves prevent the rapid break down of the leaf litter which accumulates and creates a physical barrier to competing vegetation.  This is one of many examples of the characteristics that both native and non-native plants have in common.

The Encyclopedia attributes the flammability of the eucalyptus forest to leaf litter which is exacerbated in California by rare deep freezes.  These deep freezes cause die-back of eucalypts, contributing to fuel loads.   It makes no mention of the oiliness of leaves as a factor in flammability. 

There has not been such a deep freeze in the East Bay in over 20 years and 20-year intervals of such weather events have been historically typical.  These deep freezes do not occur on the San Francisco peninsula because its climate is moderated by the ocean and bay surrounding it.  Its climate is therefore warmer in winter and cooler in summer.  Therefore, this caveat about the flammability of eucalypts does not apply in San Francisco.

The myth lives on…..

Despite the fact that there is no evidence—scientific or experiential—that eucalypts are invasive, the myth lives on amongst the community of native plants advocates.  We will continue to provide the evidence that eucalypts are not invasive.  We hope that eventually the public will be sufficiently informed that they will become resistant to this claim of native plant advocates which is one of many myths used to justify the needless destruction of eucalypts.

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(1)    Marcel Rejmanek and David Richardson, “Eucalypts,” in Encyclopedia of Biological Invasions, eds, Daniel Simberloff and Marcel Rejmanek, University of California Berkeley Press, 2011.

(2)    Craig Hardner, et. al., “The Relationship between Cross Success and Spatial Proximity of Eucalypts Globulus ssp. Globulus Parents,”  in Evolution, 212, 1998, 614-618.

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