A few days ago, under the auspices of Shaping San Francisco, ‘Nature in the City’ (NC) offered a public talk on the “urban forest” (their quote-marks, they don’t appear to believe in urban forests). NC is the parent organization of the Mt Sutro Stewards, which backs the destructive plan to gut Mt Sutro’s Cloud Forest.
The thrust of the talk, by Josiah Clark, was that non-native trees in San Francisco — particularly eucalyptus, Monterey cypress, and Monterey pine — should all be replaced by native ones that probably existed here in pre-European times. He believes that native trees actually would grow here, (contrary to the understanding that most of them cannot stand the wind and difficult growing conditions in San Francisco) — if we expand our definition of native trees.
Someone sent us this response, noting a number of unsupported or erroneous points made in the talk.
1. NC claims that our palette of “native” trees is too narrow. They advocate for destroying Monterey pine and cypress, and for planting trees like Douglas fir. (Impractical)
Fact: Douglas fir is no more native to SF than Monterey pine and cypress, but it is less well adapted to the San Francisco climate. Douglas fir needs more moisture than San Francisco gets: “Douglas fir is grown on the sites where the rainfall is higher than 800 mm [32 inches] per year…” (Wind and Trees, Cambridge University Press, 1995, page 474.) San Francisco’s annual rainfall averages 22 inches.
2. Non-native trees are invasive. (No)
Fact: Using historical aerial photographs of six Bay Area parks in the East Bay and Marin County, McBride and Russell found that from 1939 to 1997, areas of eucalypts and Monterey pines actually decreased while areas of Manzanita and coyote brush increased. (McBride, Joe and Russell, William, “Vegetation Change and Fire Hazard in the San Francisco Bay Area Open Spaces” )
3. Insects are dependent upon native plants. (Not necessarily)
Fact: As we might predict based on simple evolutionary principles, this is a myth. Insects reproduce abundantly and have short lives, which means that they can adapt and evolve more quickly than longer-lived mammals. .
Art Shapiro at UC Davis reports that California native butterflies have adapted to non-native vegetation and some now depend upon those non-natives for survival. He explains we would expect this, because most native plants are dormant during the dry season and therefore not a year-around food source. The native butterflies have adapted to the continuous food source provided by many non-natives. (Shapiro, Arthur M., “The Californian urban butterfly fauna is dependent on alien plants”, Diversity and Distributions, 2002, 8, 31-40. )
The writer goes on to add: “I have met Art Shapiro a couple of times and had the opportunity to ask if his findings about the adaptation of native butterflies could be generalized to other insects. He said, without a doubt other insects were equally capable of making such transitions to non-native plants, but that it would take research dedicated to each individual insect to prove that.”
4. A lot of trees are “native” to San Francisco. (Untrue)
NC claims there were many trees in San Francisco before the arrival of Europeans. They believe that the conventional wisdom (that there were few trees) is based on the lack of an historical record predating the arrival of Europeans, and that early settlers removed trees for firewood and to convert forest to grassland for grazing.
Fact: There is an historical record that predates the Europeans, proving there were few trees:
- A UCB thesis document contains many quotes from the first Europeans explorers, all indicating that there were few trees. (Clark, William Carey, “Vegetation Cover of the San Francisco Bay Region in the Early Spanish Period”, Geography Master’s thesis (UCB), 1952.)
- The book The Making of Golden Gate Park has evidence that virtually all of Golden Gate Park was sand dunes. It also describes how difficult it was to grow trees in San Francisco because of the wind. (Clary, Raymond H., The Making of Golden Gate Park, The Early Years: 1865-1906, San Francisco: California Living Books, 1980.)
- Richard Henry Dana visited California prior to the Gold Rush. At the time of his visit there were a handful of Spanish living in what is now San Francisco. Dana describes the desolate, treeless landscape in his book Two Years Before the Mast. (Dana Jr., Richard Henry, Dodd, Mead & Co, 1840.)
5. Native Americans may have destroyed the trees. (Unlikely)
Fact: There were very few native Americans living in San Francisco because of the climate and the lack of food sources.
6. Native trees grow in neighboring locations such as Muir Woods and Crystal Springs, proving that they also grew in San Francisco. (Not really)
Fact: The microclimates are different. A tree that will grow in Muir Woods will not necessarily grow in San Francisco. Most of San Francisco’s so-called “natural area” are on the tops of hills (Tank Hill, Kite Hill, Golden Gate Heights, Mt. Davidson, Twin Peaks, Edgewood, Buena Vista, Billy Goat Hill, etc) because these are the few undeveloped areas in the city. Because they are hills, we know that they are windy. They were barren historically and that suggests that the trees that are native to SF did not grow on them. Both Muir Woods and Crystal Springs are sheltered valleys, with different climates and therefore growing conditions.
Josiah Clark was followed by Doug Wildman from “Friends of the Urban Forest.” Doug considered the idea of replacing San Francisco’s tree inventory with “native” trees interesting.
But he also raised the practical issues of growing them in street conditions (tolerance of traffic and pollution, fruit drop and leaf drop, invasive root systems, low-growing or spreading habit), and pointed out that sidewalk trees needed owner acceptance.