Our subscribers have probably noticed that we are studying the case the California Invasive Plant Council (Cal-IPC) has made to classify Blue Gum eucalyptus (Eucalyptus globulus) as “invasive.” We have reported to our readers that Cal-IPC has made speculative claims about harm to wildlife that are unsupported by scientific evidence:
- Monarch butterflies overwintering in California are using predominantly tall non-native trees, especially eucalyptus. There is no evidence that the monarchs overwintered in California prior to the planting of these non-native trees, contrary to the claim of Cal-IPC.
- Contrary to the claim of Cal-IPC, there is no evidence that eucalyptus trees kill birds.
Is Blue Gum eucalyptus invasive?
In this post, we will look at the “evidence” provided by Cal-IPC that Blue Gum eucalyptus is invasive in California. Here is how Cal-IPC described the “local rate of spread with no management” of Blue Gum eucalyptus:
“Once a tree matures and produces seed, it can produce a profusion of progeny within a few years; doubling of stand area within 10 years possible but not well documented Without quantitative data, this response is conservative; stands have certainly expanded far beyond initial plantings in many locations, based on unpublished photodocumentation (1, 2) and personal observations (3)” [numbers refer to cited “references”]
And here is the “evidence” Cal-IPC provides in support of this rather dire prediction of the invasiveness of Blue Gum in California:
“Potts, Michael. 2003. About this edition. Caspar News. Online @ http://casparcommons.org/Press/News0305.htm. 2. Site Stewardship Program, Parks Conservancy. Unpublished photographs of Oakwood Valley, Marin Headlands, Golden Gate National Recreation Area. 3. Warner, PJ. 2004. Personal observations from 1980-2004 working in and adjacent to Eucalyptus stands in Marin, Sonoma, and Mendocino Counties, CA. 707/937-9172; firstname.lastname@example.org.”
With the exception of an article in “Caspar News,” all evidence provided by Cal-IPC is unpublished. Although the one written source is described as “Caspar News,” in fact its title is “Caspar Newsletter.” The edition of this newsletter that is cited is the first unprinted edition of the “Caspar Newsletter.” Some of the unpublished “evidence” cited by Cal-IPC is described as “personal observations” of Peter Warner, who is the author of the Cal-IPC assessment for Blue Gum eucalyptus.
Therefore, the only source of information about the invasiveness of Blue Gum that we can evaluate is the one that is available on the internet HERE.
First a word about the town of Caspar, which is located 4 miles north of Mendocino on the coast of California. According to the 2010 census, it has a population of 509 souls. We celebrated New Years Eve there many years ago in a rocking bar, so we have fond memories of it. It is a lovely little town. We mention its small size to put its newsletter into perspective. It’s hardly mainstream journalism.
The article in the “Caspar Newsletter” starts with the recommendation of Peter Warner to eradicate all eucalyptus in Caspar:
“In this newsletter you find several articles written by strong advocates of dire means, including the authoritative Eucalyptus indictment written by State Parks’ expert on managing exotics Peter Warner, who advocates a draconian solution: cutting and then careful application of a dire chemical to eliminate every tree.”
In other words, the “Caspar Newsletter” is merely a repetition of Peter Warner’s agenda to eradicate eucalyptus and poison them with herbicides to prevent them from resprouting. It’s not an independent source of information.
Photographic evidence of invasiveness?
The only photographic evidence of the invasiveness of Blue Gum eucalyptus provided by Cal-IPC’s assessment is in the article in “Caspar News:”
There are three problems with this photograph with respect to the claim that it is evidence of the invasiveness of eucalypts:
- We are asked to trust the memory of the photographer about the history of this eucalyptus grove. Credible evidence of spread of the eucalyptus grove would provide dated photographs taken at each period of time represented in this photo, i.e., 1989, 1994, 1999, and 2003.
- We see the ocean in the far distance, west of this grove of trees. As the forest approaches the ocean, we see that the trees are smaller. This is as we would expect, because the wind from the ocean has suppressed the growth of the trees on the windward side of the grove. The fact that wind suppresses the growth of trees was established by Joe R. McBride in his study of trees in the San Francisco Presidio which the Presidio contracted with him to conduct: “Wind at the Presidio affects tree growth, form, and mortality. Exposure to winds in excess of 5 mph usually results in the closure of the stomata to prevent the desiccation of the foliage (Kozlowski and Palhardy, 1997) Photosynthesis is thereby stopped during periods of moderate to high wind exposure resulting in a reduction in tree growth…Eucalyptus showed the greatest reduction in growth with trees at the windward edge being only 46 percent as tall as trees on the leeward side.” (1) (emphasis added)
- The photographer asks us to believe that the eucalyptus forest is spreading towards the ocean. Given that the seeds of eucalyptus are dispersed by gravity and wind and that the wind is coming from the ocean, we would not expect the eucalypts to spread towards the ocean, but rather on the leeward side of the forest.
In other words the “evidence” provided by the Cal-IPC assessment that E. globulus is very invasive is not supported by the evidence that is provided.
It is possible to document invasiveness with photographic evidence. We have provided our readers with two such examples that indicate that Blue Gum eucalyptus is not invasive in the San Francisco Bay Area:
- In “Vegetation Change and Fire Hazard in the San Francisco Bay Area Open Spaces,” William Russell (USGS) and Joe McBride (UC Berkeley) used aerial photos of Bay Area parks taken over a 60 year period from 1939 to 1997, to study changes in vegetation types. They studied photos of 3 parks in the East Bay (Chabot, Tilden, Redwood), 2 parks in the North Bay (Pt Reyes, Bolinas Ridge), and one on the Peninsula (Skyline). These photos revealed that grasslands are succeeding to shrubland, dominated by native coyote brush and manzanita. Eucalyptus and Monterey pine forests actually decreased during the period of study. In those cases in which forests increased in size, they were native forests of oaks or Douglas fir. In other words, they found no evidence that non-native trees are invading native trees or shrubs.
- Another example of photographic evidence that E. globulus is not invasive is from Mount Davidson in San Francisco. Adolph Sutro purchased Mount Davidson in 1881. He planted it—and other properties he owned in San Francisco—with eucalyptus because he preferred a forest to the grassland that is native to the hills of San Francisco. Here are historical photos of what Mt. Davidson looked like in 1885, 1927 and 2010:
Since Sutro didn’t own all of Mt. Davidson, there was a sharp line between the forest and the grassland when this photo was taken in 1927.
Over 80 years later, in a photo taken in 2010, there is still a sharp line between the forest and the grassland. We see more trees in the foreground where residential areas have been developed and home owners have planted more trees, but the dividing line on the mountain is nearly unchanged.
There is one well-documented case of significant expansion of planted E. globulus on Angel Island. Using historical records of planting of E. globulus on 23.6 acres as well as observations of uniform spacing of those plantings, McBride et. al., determined that E. globulus spread to 86.1 acres. The trees were planted starting in the mid-1870s to 1933 and their spread was measured in 1988. The authors of the study reported that most spreading occurred in areas of high soil moisture, such as swales, and in disturbed areas such as road cuts. This is also the only documented case of significant expansion of planted E. globulus mentioned in the US Forest Service plant data base. (2)
The one exception to the general rule that Blue Gum eucalyptus has not been invasive in California is consistent with what we know about Angel Island and about the limitations of seed dispersal and germination rates of Blue Gum eucalyptus:
- Angel Island is an extremely windy and foggy place because it is located in the San Francisco Bay, close to the Golden Gate to the Pacific Ocean, where wind and fog enter the bay.
- Eucalyptus seeds are dispersed by gravity and wind. Therefore we can expect seeds to travel further in a very windy place.
- Optimal soil moisture increases the success of seed germination. Fog drip increases soil moisture and spreading of the eucalyptus forest on Angel Island occurred in drainage swales, where moisture would be greatest.
How invasive is Blue Gum eucalyptus?
Blue Gum eucalyptus is rarely invasive. The only documented case of significant spread of eucalyptus forest occurred in ideal conditions for seed dispersal and germination. Therefore, Cal-IPC’s claim that Blue Gum eucalyptus is extremely invasive is exaggerated at best and fabricated at worst.
If our readers are aware of any other documented cases of spreading of eucalyptus, we invite them to inform us. We are committed to accurately informing ourselves and our readers of the reality of invasiveness of Blue Gum eucalyptus.
(1) “Presidio of San Francisco, Wind Study, First Phase,” Joe R. McBride, circa 2002, page 6. (unpublished, contracted study) (2) “Focused Environmental Study, Restoration of Angel Island Natural Areas Affected by Eucalyptus,” California State Parks and Recreation, July 1988, pg 47 & 51.