As we have said before in “FIRE!!! The Cover Story,” fear is a powerful motivator of public policy. The fear of fire is not the only tool in the toolbox of native plant advocates. They would also like the public to believe that non-native plants are invasive, that they will overwhelm the environment if they are not promptly eradicated. So, we will take a closer look at this claim and show that non-native trees are not invading Bay Area open spaces.
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.
They also studied the implications of these changes in vegetation types for fire hazard by measuring surface biomass for each vegetation type as an indicator of fuel load and by using a computer model (FARSITE) to simulate the speed of spread of a fire. They concluded,
“A significant increase in the cover of shrublands was apparent in the general analysis…The results from the fuel and fire hazard analysis suggest that the succession from grasslands to Baccharis [coyote brush] shrublands indicates dramatic increase in fire hazard for those areas. Fire line intensity, flame length, and total biomass were found to be significantly higher with the shrub dominated areas. In the context of the landscape matrix as a whole this increased hazard indicates a greater possibility of fire being spread into adjacent forested areas and residential communities.”
This is a view of one of the "recommended treatment areas" in the East Bay Regional Park District's "Wildfire Plan" in Anthony Chabot Park. In the foreground are many acres of coyote brush that are about six feet tall. The plan does not propose any "treatments" in these acres of highly flammable coyote brush. In the background is the eucalyptus forest that will be thinned in some places and removed in others. This is not a plan that will reduce fire risk.
This study, based on actual aerial photos, tells us that native shrublands are increasing in size while non-native forests are actually decreasing in size. It also tells us that this succession of vegetation types from grassland to native shrubland is increasing fire hazard. There is no evidence that non-native forest is invading the open spaces of the San Francisco Bay Area. The public has no reason to fear that non-native trees will overwhelm our environment.
Native plant advocates attempt to support their claim about the flammability of eucalypts by citing specific characteristics such as shreddy bark and volatile oils. Shreddy bark and volatile oils are characteristics of many plants, both native and non-native. They are not characteristics exclusive to eucalypts:
“The [chaparral] community has evolved over millions of years in association with fires, and in fact requires fire for proper health and vigor…Not only do chaparral plants feature adaptations that help them recover after a fire, but some characteristics of these plants, such as fibrous or ribbonlike shreds on the bark, seem to encourage fire. Other species contain volatile oils.” (page 341, A Natural History of California, Schoenherr, UC Press, 1992)
Madrone and Manzanita are examples of native plants with “ribbonlike shreds on the bark” that are highly flammable. Coyote brush and bay laurels are examples of native species which contain highly flammable oils.
Shreddy bark of Madrone
Shreddy bark of manzanita
Anyone with knowledge of the natural history of California could provide any number of such invidious comparisons between native and non-native plants with respect to their flammability. We hope the examples we have provided illustrate that flammability characteristics of plants are unrelated to whether the plants are native or non-native. The claim that non-native plants are more prone to fire than native plants is fallacious.
Conservation Sense and Nonsense 