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This unpublished master’s degree thesis does not prove that eucalyptus leaf extracts inhibit growth of native plants.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

Professor Joe McBride defends the forest on Mount Davidson

With great pleasure we share with our readers the following letter from Professor Joe R. McBride to Phil Ginsburg, the General Manager of San Francisco’s Recreation and Park Department, expressing his criticism of the plans of the Natural Areas Program to destroy 1,600 trees on Mount Davidson.

 Joe McBride is Professor of Environmental Science in the College of Natural Resources at University of California, Berkeley and an expert on urban forestry in the San Francisco Bay Area and around the world.  He is the author of many studies of urban forests, several of which he cites in his letter to the General Manager.  He is particularly expert on the failure of trees caused by extreme wind conditions. 

Professor McBride kindly accepted the invitation of several neighbors of Mount Davidson to read the plans of the Natural Areas Program (SNRAMP) for Mount Davidson and tour the mountain with them to evaluate those plans within the context of the actual conditions there.  The neighbors and all lovers of the urban forest are extremely grateful to him for his time and willingness to share his expertise and decades of experience with us to help us save this beautiful forest from being needlessly destroyed.


June 29, 2013

Mr. Phil Ginsburg
General Manager
San Francisco Recreation and Park Dept.
San Francisco, CA 94117

Re: Significant Natural Resource Areas Management Plan for Mt. Davidson Park

Dear Mr. Ginsburg,

I am writing to express my concern over the plan for removal of trees on Mt. Davidson.  This concern is based on the historical importance of the trees, their contribution to San Francisco landscape, and several specific aspects of the Significant Natural Resource Areas Management Plan for San Francisco. As a Professor of Urban Forestry at the University of California I have for many years studied plantations of trees in the city and compiled several reports for the U.S. Army, National Park Service, Presidio Trusts, and the Golden Gate Conservancy concerning the condition and management of eucalyptus, Monterey pine, and Monterey cypress stands.  My concern over the proposed management plan for Mt. Davidson is based both on my experience in urban forestry and on my experience as a citizen of the Bay Area who has enjoyed the urban forests of San Francisco for many years.  These concerns are elaborated in the following paragraphs.

The eucalyptus and Monterey cypress on Mt. Davidson were planted under the direction of the former Mayor of San Francisco, Adolph Sutro.  He was also responsible for planting other areas in the city that have subsequently become city parks.  The plantations he established have served to protect park users from the wind, provide wildlife habitat, and in some cases define the visual character of the San Francisco landscape.  They present an important historical heritage that I think should not be discarded lightly.  I found no mention of the historical significance of the Mt. Davidson forest in justification for the proposed management in the Natural Areas Resource Management Plan for San Francisco.  San Francisco might review the vegetation management plan developed by the Presidio Trust for the National Park Service to see the approach taken at the Presidio to maintain and manage historically significant forest plantations.

From a number of vantage points in San Francisco one can see several of the city’s hilltops covered in plantations of eucalyptus and Monterey cypress.  These plantations stand in contrast to the architecture that surrounds them.  They have been part of the San Francisco landscape for over one hundred years.  Eucalyptus plantations are as much a part of the California landscape as the coastal grassland, chaparral, and oak woodland plant communities for many people growing up in the Bay Area.  I did not find the visual value of the eucalyptus and Monterey cypress plantations on Mt. Davidson addressed in the plan.  I was, however, alarmed by the use of the term ”invasive forest” in reference to eucalyptus plantations.  This is a pejorative term that should not be applied to eucalyptus plantations.  I have found little evidence of eucalyptus invading adjacent areas of grassland or other native vegetation types in the San Francisco Bay areas in studies I conducted in open space areas (McBride, Sugihara, and Amme, 1987; McBride, Cheng, and Chorover, 1989; Cheng and McBride, 1992; Russell and McBride, 2003).  Comparison of photographs of Mt Davidson taken in the 1920s and 1950s show no evidence of the eucalyptus invading the adjacent grassland area (Proctor, 2006).  These photographs indicate that a stable boundary exists between the eucalyptus plantation and the adjacent grassland.  I see no justification for the establishment of a stable boundary between the eucalyptus and grassland habitats as called for in the “Site Improvements” section of the Significant Natural Resource Areas Management Plan. Mt Davidson 1885

MD 1927 RPD presentation        MD 2010 RPD

My concerns over the management plan for the eucalyptus and Monterey cypress plantation on Mt. Davidson are based on portions of the Significant Natural Resources Areas Management Plan : 5. General Recommendations, 6.2 Mount Davidson, Appendix F Urban Forestry Statements.  I am concerned with the justification for tree removal and the proposed levels (%) of trees to be removed.

Justification for Tree Removal

The primary justification for tree removal in the documents is the restoration of native habitat.  Various statements are made concerning the minimal amount of habitat within the eucalyptus urban forest.  This assumption is not supported by any data or reference to publications on this topic.  Stebbins (1976) concluded that eucalyptus plantations in the East Bay were far richer habitats for vertebrates than either redwood or Monterey forest and that they vie with ‘dry’ chaparral and grasslands in species diversity and ’attractiveness’ to vertebrate species.

The general recommendation to maintain a basal area between 200 and 600 square feet per acre is appropriate.  However, a conflict exists at Mt. Davidson where some stands (MA-1c) within the plantation currently have basal areas less than 200 square feet yet the plan proposes the removal of 82% of the trees.  I think there is a problem with the use of tree density measured in eucalyptus stands in Glen Canyon Park in developing the proposed cutting of trees at Mt. Davidson.  The point-quarter survey mentioned in Appendix F (Urban Forest Statements) of the Significant Natural Resources Areas Management Plan indicates a tree density of 353 trees per acre.  Three eucalyptus plantations measured in the Golden Gate National Recreation Area had tree densities of 50, 98, and 726 trees per acre (McBride, Cheng, and Clausen, 2004).  These numbers demonstrate the wide range of tree densities found within eucalyptus plantations in San Francisco.  I estimated the tree density in stand MA-1c from Google Earth images of Mt. Davidson to range from 24 to 33 trees per acre.  Trees in this stand average about 24 inches in diameter.  Trees of this size with a density of 33 trees per acre would have a basal area of a little over 100 square feet per acre (103.6 square feet).  No trees from the area designated MA-1c could be removed if the basal area recommendation was followed.  The same would apply to stands MA-2e and MA-2c where recommendations are for removal of 23% and 31% respectively.  I think a major shortcoming of the Plan is that lack of stand-specific tree density data.

Various sections of the Significant Natural Resource Areas Management Plan justify tree removal as a means of allowing re-vegetation with native understory vegetation.  Species commonly found in the understories of native forests and woodlands of the Bay Area are adapted to the low light intensity of these forests and woodlands.  Removing the eucalyptus overstory up to 82% as proposed for area MA-1c will expose the ground surface to light levels that most native understory plants will not be able to tolerate.  The management plan also points out that removal of eucalyptus will result in the promotion of growth of existing exotic understory species.  These will no doubt, compete with any native species for the site.  The suggestion that these exotic species will be controlled by manual removal and the use of herbicides indicates the City is prepared for a large investment of time and labor to combat these plants.  Projects to eliminate exotic understory plants at the Presidio after overstories of Eucalyptus and Monterey cypress have been removed have proved to be very expensive and only partially successful.

The Significant Natural Resource Areas Management Plan states that the proposed forest management will not result in long-term changes in recreational use of the natural areas.  I cannot agree with this conclusion.  The proposed cutting of trees will increase the windthrow and wind breakage of the remaining trees.  Trees that have grown up together in a plantation have buffered each other from the wind.  When individuals are exposed by the removal of surrounding trees they are very vulnerable to the wind.  This is well documented in studies of native forests and forests which have been thinned or opened for subdivision development (Franklin and Forman, 1987; McBride, 1999, 2002, 2003; Sinton et al, 2000).  The tree fall and wind breakage hazard to walkers using the Mt. Davidson area after the proposed tree removal and thinning would, I believe, seriously compromise the use of the area for recreational purposes.  The existing forest plantations currently contribute to the use of Mt. Davidson by walkers because of the reduction in wind velocity by the trees.  Forest plantations studied at the Presidio and at Lands End significantly reduce wind velocity and protect people walking from uncomfortable wind chill effects (McBride, 2002; McBride and Leffingwell, 2003).  Choice of coastal bluff trails at the Sea Ranch made by walkers is often dependent on the amount of protective cover from the wind provided by areas planted with Monterey cypress.  The exposure of Mt. Davidson to winds from the ocean will result in a less pleasant recreational experience if trees are removed.

There is an assumption in the Significant Natural Resource Areas Management Plan that minimal impact will occur to species such as hawks and owls as a result of tree removal because the overall acreage of the forest will remain high.  This is not a valid assumption for two reasons.  First, hawks and owls choose specific trees for nesting and perching.  These trees are chosen on the basis of their position in a forest stand and the structure of the tree.  Nests are used by some species year after year so that the removal of a nesting tree can present a major problem for the specific bird using the tree.  Avoiding the cutting of nest trees during the nesting season, but felling of these trees after the nesting season is a major impact that should not be part of the management plan.  It is also important to not remove trees surrounding nesting trees.  Most recovery plans for rare and threatened tree nesting birds require a protected area with a minimum radius of 300’ around a nesting tree.  No trees can be removed within this zone.

In the “Site Improvements” section of the Significant Natural Resource Areas Management Plan it is suggested that the management proposals will improve the health of the eucalyptus forest.  It is suggested that tree thinning will promote a more healthy forest.  This certainly is true in densely stocked forest stands, but I did not observe conditions in the eucalyptus plantations where tree density required thinning.  Several standing dead eucalyptus trees are present at Mt. Davidson, but the standing dead trees I examined had all been girdled.  It was evident that some individual or individuals have had a vendetta against eucalyptus trees and had girdled trees in the past.  I did not see any indication of natural mortality in the overstory of the plantations.  Concern has been raised over the potential for ivy to grow up the trunks and eventually smother the eucalyptus trees.  I have not observed this taking place in eucalyptus plantations in the East Bay.  Ivy (English and Algerian) may climb the trunks of trees, but in my experience it does not have the capacity to grow over the smaller limbs and branches.  There were a couple of eucalyptus snags completely covered by ivy at Mt. Davidson, but these snags were the result of girdling of the trees snags, not the growth of ivy.  The ivy, Cape ivy, and the Himalayan blackberry in the under story of the eucalyptus plantation are restricting establishment of eucalyptus seedlings.  I do not see this as a problem at the current age of the plantation.  Perhaps in another hundred years an examination of the plantation could establish the need for regeneration.  At this time in the life of the Mt. Davidson plantation I do not consider the lack of regeneration a problem.  Removal of the exotic understory species at this time would reduce the habitat quality of the plantation, especially the removal of Himalayan blackberry that provides a valuable food source for many species.

I conclude that the Significant Natural Resource Areas Management Plan for the removal and thinning of different portions of the eucalyptus plantation on Mt. Davidson is not justified.  The plantation serves an important role in the history and visual characteristics of the city.  Trees and the existing understory provide habitat for wildlife and wind protection for walkers.  The justifications for the management prescriptions have not been properly developed.  Furthermore, the cost of removal of the trees seems unjustified in view of other priorities in the San Francisco budget.

Joe R. McBride

CC:  Mayor Edwin M. Lee
City and County of San Francisco Board of Supervisors
San Francisco Recreation and Park Commission
San Francisco Planning Commission
San Francisco Urban Forestry Council
Park, Recreation and Open Space Advisory Committee
Bill Wycko, Environmental Review Officer (Case No. 005.1912E)

Literature Cited

Cheng, S. and J.R. McBride. 1992. Biological Assessment of Mills Creek Riparian Corridor. Report to the California Department of Parks and Recreation. Monterey Co., CA 89p.

Franklin, J. and R.T.T. Forman. 1987. Creating landscape patterns by forest cutting: ecological consequences and principles. Landscape ecology 1:5-18.

McBride, J. R. 1999. Identification of areas of high windthrow potential at the Sea Ranch. McBride and McBride. Consulting Landscape Ecologists. Berkeley, CA.

McBride, J. R. 2002. Presidio of San Francisco Wind Study, First Phase.  Report to the Presidio Trust. San Francisco, CA. 35 p.

McBride, J. R. 2003. Re-evaluation of the windthrow problem at The Sea Ranch. Report to the Planning Department. The Sea Ranch, CA. 6 p.

McBride, J. R. and J. Leffingwell. 2003. Effects of Forest Stands on the Microclimates of the Presidio. Report to the Presidio Trust. San Francisco, CA. 27 p.

McBride, J.R., N. Sugihara and D. Amme. 1987. Vegetation Assessment. In: D. Boyd (Ed.)  Environmental assessment for Eucalyptus Removal on Angel Island. California Dept. Parks and Recreation, Sacramento, CA. pp 23-45

McBride, J.R., S. Cheng and J. Chorover. 1989. Natural Resources Assessment – Jack London State    Park. Calif. Dept. Parks and Recreation. Sacramento, CA. 432 p.

Proctor, J. 2006. San Francisco’s West of Twin Peaks. Charleston, SC: Arcadia Publishing.

Russell, W. H. and J. R. McBride. 2003. Landscape scale vegetation-type conversion and fire hazard in the San Francisco Bay Area open spaces. Landscape and Urban Planning 64:201-208.

McBride, J. R. , S. Cheng, and J. Clausen. 2004. Vegetation management Strategy for Lands End, GGNRA. Report to the Golden Gate National Park Conservancy. San Francisco, CA

Sinton, D. S. et al.  2000. Windthrow disturbance, forest composition, and structure in the Bull Run Basin, Oregon. Ecology 81(9): 2539-2556.

Stebbins, R. 1976. Use of habitats in the East Bay Regional Parks by free-living vertebrate animals. August 1975. In “Vegetation Management Principles and Policies for the East Bay Regional Park District”.  East Bay Regional Parks District. Oakland, CA.