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This unpublished master’s degree thesis does not prove that eucalyptus leaf extracts inhibit growth of native plants.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