An Australian friend of eucalyptus

An Australian sheep farmer (we would say “rancher”), Jane Pye, spotted our defense of eucalyptus and got in touch:

“G’day, I stumbled across the SFFA website researching ‘allelopathy’ in eucalypts and was amazed to find so much antipathy towards gum trees over there – like an arboreal cane toad! What I really wanted to know is do you have any evidence of ‘positive allelopathy’ re eucalypts?  I live in the Australian outback with areas of dry sclerophyll forest. The commonest eucalypt here (Eucalyptus populnea) is often surrounded by native scrub trees & bushes. Strangely some of these box trees also have other trees growing out of their trunks which I think were planted there by the traditional owners (Aboriginal). These Tree in Trees are found in clusters around the old indigenous campsites, which are in turn found around good natural water catchments or native wells.”

My article debunking the popular myth that eucalyptus is allelopathic was republished by the San Francisco Forest Alliance (SFFA).  The myth of allelopathy is that eucalyptus emits a chemical that prevents the germination of other species, eliminating competition with eucalyptus for resources such as water.  But Jane’s experience with eucalyptus goes beyond debunking allelopathy in eucalyptus.  She has documented many examples of different tree species that have seemingly been planted inside the cavities of eucalyptus trees.  The eucalypts are a sheltering host to the guest tree species.  Clearly, eucalyptus is not inhospitable to other plant species.

Wilga (Geijera parviflora) guest in Bimblebox (Eucalyptus populnea) host.
Apophyllum anomalum guest growing from Bimblebox (Eucalyptus populnea) host

Jane believes that these “trees in trees” were intentionally planted by indigenous Aboriginal “farmers.”  She believes that this is one of many techniques that were developed by Aboriginal people to manage the land and vegetation to provide food and cultural implements.  She explains on her website:

“After years of admiring and speculating about these scar trees, I have finally gotten around to photographing most of them. I spend many days in the paddocks mustering sheep and some of these trees are like old friends. They are an important link to our Aboriginal past and a reflection on how innovative and resilient these people were. Surviving out here west of Walgett with our unpredictable climate of harsh droughts and random floods is still tough but these people managed their environment and thrived.

“So I dedicate this website to Freddie Walford, an Aboriginal stockman we had who taught us some bush lore and like many of his people, died too young. I will always remember his natural affinity with livestock, his love of polocrosse and his quiet humour and grace. He never spoke much about the scar trees but did say if I was ever to see bones inside an old coolabah [Eucalyptus coolabah], I should go as fast as possible in the opposite direction! This website aims to increase knowledge and record these trees but not to display any pictures or information that is culturally secret or sacred.”

Australian aborigines. Photo by Thomas Dick, 1920

The land management practices of Australian Aborigines were very similar to those of indigenous Californians for much the same reason.  These were hunter-gatherer cultures living in similar climates with seasonal drought.  They moved around as seasons changed and their diets changed accordingly.  Both cultures used fire as one of their primary tools.  Periodic fires refreshed the grasses that fed the grazing animals they hunted.  The primary grazers in Australia were kangaroos and other marsupials; deer and other ungulates are the original grazers in California.

Native Americans setting grass fire, painting by Frederic Remington, 1908

The land management practices of indigenous people are enjoying a renaissance.  A recent study of Aboriginal land management in Australia said, “Indigenous agency and governance is driving innovations in land management worldwide that provide more equitable solutions and strategic approaches to looking after the lands, waters and all living things, particularly in the face of climate change.”

Such intentional burns are now seen as a way to keep the brushy fuels that carry fire to a minimum, reducing wildfire hazards.  Cal Fire’s new “Wildfire and Forest Resilience Plan” and the Governor’s recent annual budget proposal tell us that more prescribed burns are planned in California to reduce fuel loads and fire hazards:  “CAL FIRE will expand its fuels reduction and prescribed fire programs to treat up to 100,000 acres by 2025, and the California Department of Parks and Recreation and other state agencies will also increase the use of prescribed fire on high risk state lands.”

It has taken hundreds of years to appreciate the value of indigenous land management and its context in their culture.  When Europeans arrived in both America and Australia, settlers assumed that their culture was superior to indigenous culture.  Early settlers made no attempt to learn from indigenous people, which was the settler’s loss.  Indigenous people had learned to live off the land, in most cases without cultivating crops and without domesticated animals.  Rather, indigenous people learned what was edible and what had medicinal value.  The first European settlement in America, Jamestown, ended quickly with starvation, because the settlers weren’t able to understand what the land offered them.  In Australia, knowledge of indigenous land management was also delayed by the cultural taboos of the indigenous people that prohibited the revelation of many of their cultural practices outside their ancestral clans.

 Wildfire in Australia

Jane also had some interesting observations about wildfires in Australia that are consistent with our experience in California:

I’m sorry to hear eucalypt forests are being destroyed over there as they are wonderfully useful trees. We don’t have many fires in inland Australia. It’s more of a coastal / high rainfall problem. We just don’t get the fuel build up as it’s a semi-arid region and we have thousands of merino sheep eating the grass and shrubs. There have been no fires on this property in over a century and probably much longer. We also have efficient native grazers – kangaroos and wallabies and now also goats that are increasingly common as an alternative to sheep.”

Jane Pye’s home. Gingie Station, Walgett, NSW, Australia. There are many places in California’s Central Valley that look much the same.

This is the strategy that I promote on this website.  If we reduce ground fuels that ignite easily, we can prevent most fires from igniting tree canopies that are harder to ignite.  Fire travels fast on the ground if given a continuous field of dry grass during the dry season.  Grazing animals are a far safer way of reducing these grassy ground fuels than the herbicides that are often used.  Herbicides leave a dead, dry thatch on the ground that is very flammable and grazing does not.

Walgett, Australia. Average Hi Temp 80 degrees. Average Lo Temp 54 degrees. Average rainfall 19 inches.

“Also we are very used to fire over here and many people regard those foolish enough to build in fire prone areas have only themselves to blame. Some of our small coastal towns are totally surrounded by National park and State forest and only have one road in – that’s why there were so many images of people sheltering on beaches last summer. It’s a hard issue but better hazard reduction burns and more fire fighting aircraft seem to be the way forward here. Also better fire retardant building materials.”

These observations are also consistent with the strategy that makes sense to me.  We must learn to live with fire because it is an essential element in Mediterranean ecosystems.  We can’t prevent it, but we can work around it with zoning that prevents building in extremely hazardous areas, using fire retardant building materials, and creating safe evacuation routes.

Thanks, mate!

 Alerted by Jane, I noticed this woody shrub sprouting from a eucalyptus stump in Stern Grove a few days after I heard from Jane.  Clearly, eucalyptus does not retard the growth of other species.

Stern Grove, San Francisco

Many thanks to Jane for getting in touch with us.  Thanks for her admiration of eucalyptus and Aboriginal culture.  I’ve had some lovely email chats with Jane.  Perhaps you would like to drop her a line to thank her as well:

Highs and Lows of the 2018 Conference of the California Native Plant Society

I am pleased to publish the following report of one of our readers who attended the conference of the California Native Plant Society in Los Angeles at the beginning of February 2018. 

Million Trees

I attended the last conference of the California Native Plant Society in San Jose in January 2015.  It was interesting to note a few significant new themes in the recent conference in 2018.  Both fire and climate change were much more prominent themes in the recent conference.  While both are relevant to the future of native plants, neither seemed to have any effect on the “restoration” goals of the native plant movement.  For example, there were several presentations about massive die offs of native oak trees, resulting from higher temperatures, drought, and disease.  These presentations ended with urgent pleas to plant more oaks.  That seemed a fundamental contradiction and a denial of the reality of climate change.  When the climate changes, the landscape changes, but native plant advocates are not willing to acknowledge that.  In fact, the greater the threats to native plants, the greater the commitment to their preservation and “restoration.”

Beautiful pictures support nativist ideology

The conference began on a low point for me, but a high point for most attendees of the conference.  The keynote speaker was Doug Tallamy.  He was introduced as a “rock star” of the native plant movement, and indeed he is.  His presentation was very effective in delivering his message, which is that most insects are “specialists” with mutually exclusive relationships with native plants that evolved over “tens of thousands of years.”  If you believe that claim, you also believe that the absence of native plants will result in the absence of insects and ultimately the collapse of the entire food web.

Doug Tallamy’s closing photo, CNPS Conference 2018

Most native plant advocates believe that gloomy scenario, but few scientists still do, which creates a tension within this community of native plant advocates composed predominantly of amateur “botanists” and a smattering of academic ecologists.  For example, one of the first presentations after Tallamy’s keynote was an academic ecologist from UC Berkeley who advocated for accommodating the movement of plants outside of historical native ranges to accommodate climate change. (1) He said that restoring only with local natives is “maladaptive” and that a bioregional perspective is needed to create sustainable landscapes.  Allowing Monterey pines to grow in the San Francisco Bay Area, where they have grown in the past and are presently deemed “native” just 150 miles away, seems a good example of such a broader definition of “native.”  An amateur nativist, parroting Tallamy, asked this hostile question: “But if we move the plants how will wildlife survive?”  The academic delivered this tart dose of reality: “There are few mutually exclusive relationships in nature.  Wildlife will also move and will adapt to changes in vegetation.”

Science debunks a myth about eucalyptus

The high point of the conference for me was a presentation by Jennifer Yost, Assistant Professor at Cal Poly, San Luis Obispo.  She and her graduate student studied the claim that nothing grows under blue gum eucalyptus trees because of allelopathic chemicals emitted by eucalyptus that suppress the germination of other species of plants.  Two studies of this hypothesis were done in the 1960s, but the analytical methods used by those studies were misleading.

CNPS Conference 2018

Rigorous methods used by Yost’s team included planting seeds of 5 native plant species in the soil of eucalyptus forests and comparing germination rates of seeds planted in the soil of oak woodlands.  They also tested the effect of blue gum volatile leaf extracts, and water-soluble leaf extracts on germination and early seedling growth.

They concluded, “In these experiments, we found that germination and seedling growth of the species tested were not inhibited by chemical extracts of blue gum foliage, either at naturally-occurring or artificially concentrated levels.” (2)

CNPS Conference 2018

Yost observed that the lack of allelopathic effects of blue gum on the soil implies that blue gum forests theoretically can be successfully planted with native plants after removal of the trees.  However, she cautioned that those who destroy the blue gums should carefully consider what will replace them.  Will an aggressive non-native weed quickly colonize the bare ground?  If so, what is the benefit of destroying the blue gums? 

I had a conversation with one of the most influential nativists in the San Francisco Bay Area after Yost’s presentation.  This new scientific information does not alter his commitment to destroying blue gum eucalyptus in the Bay Area.  After all, there are many more negative claims that remain unchallenged by scientific studies.  For example, there are no studies that prove that blue gums use more water than native trees, as nativists claim.  Nor are there any studies that prove that eucalyptus leaves contain less moisture than the leaves of native oak or bay laurel trees, which theoretically makes eucalyptus more flammable, as nativists claim.  The lack of scientific evidence enables the persistence of speculation justifying irrational fear of blue gum eucalyptus.

Nativism dies hard because of lack of scientific studies

There appeared to be three distinct groups of people in the crowd of about 900 conferees.  There was a large contingent of grey-haired volunteers who are the backbone of every native plant “restoration.”  They are the dedicated weed pullers.  There is an equally large contingent of young people who are making their living writing the “restoration” plans and directing the activities of the volunteers.  The smallest contingent is a few academic scientists who study the underlying issues in their ivory tower.  The goals and conclusions of these three groups are increasingly divergent as scientific studies disprove the assumptions of the citizen “scientists.”

The tension between science and the citizenry is as evident within the native plant movement as it is in American politics at the present time. The general public rejects scientific evidence at its peril.  The rejection of science will not end well.  In the case of uninformed nativism in the natural world, the result will be a barren, poisoned landscape.

  1. “Climate change and open space conservation: Lessons from TBC3’s researcher-land manager partnerships in the San Francisco Bay Area,” David Ackerly1, Naia Morueta-Holme5, Sam Veloz3, Lisa Micheli2, Nicole Heller4 1University of California, Berkeley, Berkeley, CA, USA, 2Pepperwood, Santa Rosa, CA, USA, 3Point Blue Conservation Science, Petaluma, CA, USA, 4Peninsula Open Space Trust, Palo Alto, CA, USA, 5University of Copenhagen, Copenhagen, Denmark
  2. Abstracts of CNPS conference presentations are available here:  CNPS Conference abstracts

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