The Global War on Non-Native Trees

“In some cases, removal of urban trees because they are non-native may represent an ‘over-shoot’ where the removal of non-natives becomes an end unto itself… the non-native trees being removed are not necessarily highly invasive, and removal is more driven by a desire for native species rather than any real or perceived problems caused by the non-native species.”

An international team of academic scientists studied the many conflicts around the world between those who find value in introduced trees and those who demand their destruction. (1) Team members were from Australia, France, New Zealand, and South Africa.  Professor Marcel Rejmanek at UC Davis was the only American on the team.

Professor Rejmanek is well known to us as the author of the chapter about eucalyptus in Daniel Simberloff’s encyclopedic tome about biological “invasions.”  Rejmanek said, “…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.  He noted that eucalyptus is useful to bees and hummingbirds and I add here that it blooms throughout winter months when little else is blooming in California.  He said,  “Conclusions about positive or negative environmental and economic impacts of eucalypts are often anecdotal, highly controversial and context dependent.”    Professor Rejmanek’s assessment was instrumental in my effort to convince the California Invasive Plant Council to remove blue gum eucalyptus from its list of invasive species.  Cal-IPC downgraded its assessment of invasiveness of blue gum eucalyptus from “medium” to “limited” in response to my request. 

Professor Rejmanek is also the author of the most recent report on plant extinctions in California, published in 2017.  At that time there were 13 plant species and 17 sub-species native to California known to be globally extinct and another 30 species and sub-species extirpated in California but still found in other states.  Over half the globally extinct taxa were reported as extinct over 100 years ago.  Although grassland in California had been converted to Mediterranean annual grasses by grazing domesticated animals nearly 100 years before then, most of the plants now designated as “invasive” in California were not widespread over 100 years ago.  Only one extinction mentions “invasive species” as one of the factors in its disappearance.  Rejmanek speculates that livestock grazing is the probable cause.  He said, “Indeed, one needs quite a bit of imagination to predict that any native plant species may be driven to extinction by invasive plants per se.”

This recap of Rejmanek’s expertise about so-called “invasive” trees and plants establishes his credentials as a reliable witness as the co-author of “Conflicting values:  ecosystem services and invasive tree management,” which I will summarize for readers today.

Setting the stage for conflict

As Europeans colonized the new world in the 18th and 19th centuries, they often brought trees from home with them, motivated primarily by an aesthetic preference. When the colonial era came to an end, nationalism during the 19th century encouraged a new appreciation of indigenous flora.  When planting their own gardens and farms, America’s founding fathers had a strong preference for planting native trees.  While fighting the Revolutionary War, George Washington wrote to the caretakers of his farm at Mount Vernon instructing them to plant NO English trees, but rather to transplant trees from the surrounding forests.

Sources of conflict

By mid-20th century, this preference for indigenous trees escalated to the current belief that non-native trees are threatening indigenous ecosystems.  Conflict arises when there is a “failure to account for, assess, and balance trade-offs between the eco-system services or, at times, a failure to agree on the relative value of particular services.” (1) The study identifies the tree species that are the focus of such conflicts around the world and the ecosystem services those species provide:

Conservation Sense and Nonsense has reported on many of these conflicts around the world:

  • The stated purpose of the destruction of forests in Chicago was the “restoration” of grassland that existed prior to the arrival of Europeans.  Conservation Sense and Nonsense described the conflict regarding that destruction in one of my first articles in 2011 because the issues were similar to those in the San Francisco Bay Area. The debate raged in Chicago for over 15 years, but the destruction of the forest was finally accomplished, despite opposition.  Likewise, in San Francisco after 20 years of conflict, the eradication of eucalyptus forests is being achieved.
  • In 2012, we republished an article by Christian Kull about the practical value of acacia trees to Vietnamese farmers and their opposition to the attempt to destroy them.
  •  We republished an article in 2014 about opposition to the destruction of willow trees in Australia that were planted to control erosion.  Willows are one of many examples of a tree that is considered valuable in North America where it is native and hated in Australia where it is not. The authors of the article described the arguments used to justify the project, ‘Sure, it makes a big mess and causes erosion, and nutrient release, and carbon emissions, and local temperature increases, and loss of habitat, but it’s necessary because we’re going to make Australia a place for natives-only again.’
  • Conservation Sense and Nonsense published a guest post by Matt Chew in 2017 about the eradication of tamarisk trees that were introduced for erosion control in southwestern US.  In that case, the survival of an endangered bird is threatened by this misguided attempt to eradicate tamarisk by introducing a non-native insect.
  • Conservation Sense and Nonsense published a guest post in 2015 by a South African who objected to the destruction of jacaranda trees.  In that case, the beauty of these iconic trees was the primary objection to their destruction.
Jacaranda trees in Pretoria, South Africa

Many similar conflicts around the world are described by the study, which categorizes the conflicts as focused in three areas:  urban and near-urban trees; trees that provide direct economic benefits; and invasive trees that are used by native species for habitat or food.  I will focus on conflicts in urban and suburban areas because they are close to home.

Where is conflict greatest?

The study searched for examples of such conflicts around the world and found that most were in developed countries where ecological knowledge has suggested that eradication is necessary and democracy is strong enough to enable dissent.  Such conflicts are well documented in urban areas where many non-native trees have been introduced. Based on my experience with many of these urban conflicts, I can agree with the authors of the study that they are “frequently vitriolic, as seen in letters to editors, public protests, websites, and blogs.” (1)

How NOT to reduce conflict

The authors of this study dismiss suggestions that “educating” those who object to eradication projects can reduce conflict.  Their assessment of why that approach intensifies conflict is consistent with my own reaction to being lectured about the claimed benefits of eradication projects:

“However, the concept of ‘education’ implies that opponents of tree removal are inherently ignorant or unaware and discounts the importance of their views and values.  Sceptics of environmental issues are frequently highly educated and scientifically literate, with conflict driven by fundamental values, not lack of knowledge.  Further, what one party in a conflict views as education can be viewed as propaganda by those with opposing priorities.” (1)

The authors suggest that the planning process for such projects must be a two-way dialogue that recognizes shared values, such as a strong commitment to conservation of the environment.  The authors describe some of my own reservations about eradication projects:

“In some cases, removal of urban trees because they are non-native may represent an ‘over-shoot’ where the removal of non-natives becomes an end unto itself…Objective evaluation of the ecological services affected may not result in the removal of non-native trees being justified.  Indeed, in some cases the non-native trees being removed are not necessarily highly invasive, and removal is more driven by a desire for native species rather than any real or perceived problems caused by the non-native species.” (1)

There is no doubt that the demand to destroy eucalyptus in California is a case in which removal has become an end in itself that is not justified.  These are some of the accusations used to justify the destruction of eucalyptus that have been disproven by academic scientists without getting eucalyptus off nativists’ hit list.

Source: Conference of California Native Plant Society, 2018

Pessimistic conclusion

The study concludes that we should expect plant invasions around the world to increase and that increased wealth and democracy will make conflicts about tree eradications more widespread.  The authors “suggest that conflict should be seen as a normal occurrence in invasive species removal…Avoiding conflict entirely may be impossible…”

I can’t disagree with the authors of this study about the poor prospects of resolving conflict regarding the destruction of non-native trees that are the heart of our urban forest in California.  However, I am grateful to the authors for their understanding of the issues and their respect for introduced trees as well as those who advocate for their preservation. They understand that lectures by those who demand that trees be destroyed despite the functions they perform are condescending and exacerbate conflict rather than resolving it. 

A Postscript

Jake Sigg has been the leader of the crusade to destroy eucalyptus forests in the San Francisco Bay Area for over 30 years.  He and I have debated this issue many times, without resolution.  In his newsletter of January 20th, Jake seems to acknowledge the futility of our debate as well as his motivation to create a native landscape. It seems he has reached the same conclusion as the authors of the international study of the inevitability of conflict about the destruction of non-native trees, although he concedes that he won’t quit trying…and neither will I. 

“For years I’ve been fighting tree huggers, who understandably don’t want to cut healthy trees down.  The blue gums are handsome brutes.  In my eye I see the rich diverse native biological communities that they displaced; those I fight with don’t see that and don’t value that.  So you can see the communication problem at the beginning.  The same consideration plagues many contentious issues in the world.

How do you explain this to them?  Mostly, you can’t; you do what you are able to do.  This is not an age for listening to fellow beings.  I find it hard to do.  David Brooks, a favorite, wrote a fraught opinion piece in today’s 
NYT.  He has just about thrown up his hands, as have I—except that I can’t—and neither can Brooks.”

Eucalyptus canopy on east side of Glen Canyon Park, taken from Turquoise Way December 2012, before tree destruction escalated beyond riparian areas. Glen Cayon Park is one of 33 parks in San Francisco where most eucalyptus trees are being destroyed because they are not native. Courtesy San Francisco Forest Alliance

  1. Ian Dickie, et. al., “Conflicting values:  ecosystem services and invasive tree management,” Biological Invasions, 2014.

2 thoughts on “The Global War on Non-Native Trees”

  1. Thank you SO much for this important article. You do so much to disprove the dangerous myths destroying so many trees, animals, and the environment.

    Those who hate Eucalyptus often don’t even recognize the iconic Blue Gum where they are growing in places where they were not planted en masse as plantation trees, but are growing amidst Oaks, Bays, etc. And most people also don’t recognize some of the other beautiful Eucalyptus here, like Ironbark, grown as street trees.

    Those who love birds and Monarch Butterflies should know how important Eucalyptus are to native animals. Anyone doubting how needed Eucalyptus are for large raptors should look at photos from local bird groups to see the magnificent hawks, owls, and eagles in so many of the most exquisite photos. I’ve speculated that these wise raptors prefer Eucs because they are high enough to be safe from nest predators, but also that their open branches make it much safer for fledglings to not die while learning to fly.

  2. A big problem–that goes unrecognized by nativists–with the removal of “invasive” plants is that it often simply destroys habitat by leaving the ground devoid of plants. Even if people plant anything, it’ll be years before mature specimens are available to host wildlife as well as the mature plants that were removed were able to do. Nativists tend to not bother observing nonnative-plant habitat to see how much it’s used by wildlife. In so doing, they remain ignorant of the value of these plants and can continue to fool themselves into thinking they are doing something positive for the environment, when they actually are not. It’s very sad. Sincerely, Marlene

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