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This unpublished master’s degree thesis does not prove that eucalyptus leaf extracts inhibit growth of native plants.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

Comparing the Sutro Forest with Albany Hill

In a recent edition of Jake Sigg’s Nature News, one of his readers compared the Sutro forest unfavorably to the eucalyptus forest on Albany Hill:

“The [Sutro] ‘ forest ‘ is unmanaged, many of the trees are deteriorated and unhealthy, spacing is unusual and given the super invasive ivies and H. blackberry, overall habitat diversity is degraded.

For example , I look at El Cerrito Hill [officially Albany Hill] (near Central Av exit of east bay I-80 at Golden Gate Fields) and see a ‘ eucalyptus’ forest with fewer and larger trees , featuring an adjacent and penetrating coast live oak woodland and a ground cover that features wide species diversity.

Granted El Cerrito does not have the fog intensity of Mt. Sutro, BUT it also shows signs of management

Anyway, all of this is my un-scientific observation.  I think, with some thinning of eucs, control of ivy and blackberry spread, the establishment of habitat corridors and discreet regions and maintenance of trails, that Mt. Sutro could continue to provide a ‘ forest’ aesthetic and include much improved habitat diversity and fire potential reduction.”

We are always trying to understand the perceptions of native plant advocates in the Bay Area, so we went to take a look at Albany Hill.  We are happy to report that we found much to like.  However, the comparison with the Sutro forest is mistaken in many ways, which we will explain in this post. 

Creekside Park

Cerritos Creek
Cerritos Creek

We started our walk at the Creekside Park on the north side of Albany Hill.  It is a riparian corridor created by the confluence of two creeks, Cerritos and Middle creeks.  The vegetation is almost exclusively native.  The Creekside Park is carefully tended by native plant advocates who have planted many of the natives, but the Coast Live Oaks that are the predominant tree species are said to have been here prior to settlement. (1)

The oaks cover the northern slope of Albany Hill, which was typical of native oak woodland in the East Bay .  The oaks benefit from the water provided by the creek and they also occupied north-facing slopes where there is more moisture in the soil than on the south facing slopes that are exposed to the sun.  The prevailing wind in the Bay Area is usually from the southwest, so there is also some protection from the wind on the north side of the hill.  There were never any oaks on Mount Sutro, and the assumption that there will be in the future seems delusional, given their horticultural requirements.

Creekside Park
Creekside Park

Looking up from the Creekside Park, we can see the eucalyptus forest on the top of Albany Hill which has not “invaded” the oak woodland.  The fact that the eucalyptus forest has not encroached on the oak woodland is documented by two planning documents.  The first “Albany Hill Master Plan” was written in 1991.  It included vegetation maps that can be compared to the second “Albany Hill Creekside Master Plan” which was written in 2011.  The second plan states that the eucalyptus forest had not expanded during that 20 year period.  It also states that the eucalyptus forest isn’t regenerating.  That is, it is not replacing itself, let alone expanding.  Yet, it is consistently called an “invasive species” in the master plan approved in 2012. (2)  “Invasive” seems to be the pejorative adjective used by native plant advocates to describe all non-native plants and trees, whether actually invasive or not.

During that 20-year period from 1991 to 2011, the eucalyptus forest was not “managed” as Sigg’s correspondent believes, because the plan was not funded. (1)  Hence, a second plan was written in 2011.  This is a scenario we often see played out on our public lands.  Elaborate plans are written.  Often they are not implemented.  Sometimes we are just as happy they aren’t.

Let’s enjoy a few of the beautiful native plants and trees before we leave Creekside Park to visit the eucalyptus forest on the top of the hill.


This is a lovely little Madrone in full bloom.  We recall wanting to plant a Madrone in our backyard over 30 years ago.  They weren’t available in nurseries then.  We assume they are now, which is great.  But we digress.

ceanothusThis is one of the few California lilacs (ceanothus) we saw.  Bumble bees were busy in the lilacs.  They are nesting close by in the ground beneath an oak.  If the bared ground on Mount Sutro is covered in the wood chips of the trees that are destroyed, as planned, native bees will not be able to penetrate that deep wood mulch.

Coast Live OakThis is one of many lovely oak trees, surrounded by ivy, which doesn’t seem to be doing it any harm.  In fact, the master plan for Albany Hill says there are no plans to eradicate ivy in the riparian corridor because it “…would require considerable cost and labor to fully eradicate and whose spread is often limited to areas in the immediate vicinity.”

The Eucalyptus Forest on Albany Hill

The eucalyptus forest on Albany Hill was planted by a dynamite company in the late 19th century.  The trees were planted to muffle the sound from frequent explosions, as well as provide some protection to neighboring residents. (1)   There were many reasons why early settlers to the Bay Area planted eucalypts.  Some were practical reasons, such as this, and some were aesthetic.

Albany Hill is much drier than Mount Sutro. It is further away from the ocean where fogs form and often hover for weeks on end during the summer.  In general, the East Bay is considerably less foggy than San Francisco.


Because it is a much drier environment, the eucalyptus forest on Albany Hill is less dense than the Sutro forest and it has considerably less understory.  The understory on Albany Hill is non-native annual grass and native toyon shrubs, which are said to have been planted either by man or by birds “carrying” seeds from other locations.  (2)   Hmmm….let’s stop to think about that.  The toyon is a native shrub that was “introduced” to Albany Hill and is thriving there under the canopy of the eucalyptus forest.  The 2012 master plan for Albany Hill says that the toyon understory has expanded since the 1991 master plan.

The future of the eucalyptus forest on Albany Hill

As we have said, the eucalyptus forest on Albany Hill has not been “managed” to date.  Since the master plan was approved in 2012, only $50,000 has been allocated for vegetation management on Albany Hill. So, what is planned for the eucalyptus forest in the future?  Here are the plans for the future, according to the master plan that was approved in 2012:

  • The eucalyptus forest will be “phased out” slowly over time by removing hazardous trees as necessary to ensure public safety, removing new seedlings where the forest interfaces with native oak woodland, and not replacing trees that die of old age.  Since the predominant species of eucalyptus in the Bay Area (Blue Gum) lives in Australia from 200-500 years, and it has been here for only about 100 years, it is reasonable to assume that this is a very long-term plan.
  • This plan should require a lot less herbicide than destroying 90% of the trees on 75% of Mt. Sutro, as planned there, because we assume a dead tree will not resprout.   A lot of herbicide is often needed to prevent resprouts after healthy eucalypts are cut down.
  • The forest will be managed for fire safety by mowing the annual grasses, where ignition is most likely to occur; by limbing trees up to separate the understory shrubs from tree limbs to remove “fire ladders” to the tree canopy; and by cleaning woody debris from under the trees on an annual cycle.
  • The plan acknowledges the benefit of maintaining a closed canopy, both in the eucalyptus forest and the oak woodland.  The closed canopy shades the forest floor and suppresses the growth of non-native weeds.

This all sounds eminently reasonable to us:  much less destructive than plans for the Sutro forest, yet addressing fire hazard and safety issues in a responsible way.

Neighbors on the leeward side of the hill will eventually lose their windbreak as the tall trees disappear, but that will happen so slowly that they are unlikely to react.  On the other hand, plans for Albany Hill could change many times in the next 100 years.  We hope the current preoccupation with the nativity of plants and trees will have faded long before the eucalyptus forest dies of old age.

And so, ironically, we agree with Jake Sigg’s correspondent that plans to manage Albany Hill are better than the plans to destroy the Sutro forest.  We just don’t agree about why.



(2)    “Albany Hill Creekside Master Plan,” City of Albany, 2012