Urban forest diversity matters more than ever
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Anne-Marie Walker
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Views of San Francisco Bay through majestic trees like this Monterey cypress are undeniably iconic. Who planted this tree and when?
Monterey cypress trees, native to the Central Coast around Monterey are an introduced species in Marin. Monterey pines, cypress and non-native eucalyptus were first planted in the Presidio during the last decades of the 19th century for wind protection and to stabilize sand dunes and slopes. Today, they constitute a visible segment of our urban forest and demonstrate planting a tree is both a multi-year decision and a long-term investment. The benefits of urban trees are numerous, including improved air quality, storm water catchment, increased property value, carbon dioxide sequestration, savings in electricity and natural gas, providing habitat and — not to be overlooked — improving health and happiness of humans.
(Go to treesaregood.com and enter the tree species in your backyard and let the tree calculator show you the value of your tree and the urban forest surrounding us.)
About the time scientist Alexander von Humboldt was observing and drawing the distribution of vegetation in relation to altitude, citizens were participating in planting trees in large public open spaces in Paris and London as well as New York’s Central Park and our Golden Gate Park. By the 20th century, scientists pretty much understood that vegetation was primarily influenced by moisture and temperature factors. This is important for us in Marin as we live in a recognizably unique biome; the Mediterranean woodland/scrub characterized by winter rains and dry summers. Professor Joe McBride, of the University of California at Berkeley, cites in “The World’s Urban Forests,” the most commonly used Mediterranean tree species include the Canary Island date palm, Italian cypress and olive tree. As tree species become successful in urban environments, they are selected and planted repeatedly resulting in worldwide urban forest composition of seven most frequently encountered street trees: London plane, Canary Island date palm, Norway maple, jacaranda, rowan, black locust and common lime trees. Monocultures of trees have left us vulnerable to pests spreading rapidly and often with devastating results — Dutch elm disease, emerald ash borer and the Asian long-horned beetle among others. Urban tree diversity has never been more important!
To protect and enhance urban forests for the future, we must select and plant trees after assessing climate, pests and cultural requirements. We can also encourage tree planting projects, known as releaf projects, and stay attentive to the importance of maintaining tree diversity. The importance of urban trees is underscored by population shifts; one half of the world’s population now lives in cities. In the 1950s, only a third lived in cities and it is projected that by 2050, the number rises to two thirds. Our neighborhoods should be planted with diverse species best suited to climate and site.
Natural disasters such as hurricanes, typhoons and earthquakes can provide opportunities for re-leaf projects that practice planting diverse species with particular characteristics like high wind resistance afforded by oaks, hickory, holly, podocarpus, cypress and palms.
So too, climate change will impact the future composition of urban forests. One recent UC study found that a number of common street trees are unlikely to perform well in California as temperatures increase and water supplies remain limited. While this study suggests that coastal areas will experience fewer problems with their street trees in a warming climate than the inland areas of California, it is nevertheless useful to consider a tree’s native range, and perhaps reconsider the species for which the climate of Marin represents the “edge” of their heat and drought tolerance (e.g., birch).
Genetic engineering of new varieties of trees may help fend off disease; for example, the University of California at Davis helped develop a new Monterey pine that is resistant to pitch canker; you can see trial plantings around the World War II war memorial in the Presidio.
Education of the public about urban forestry issues, encouragement of volunteer organizations like Tree People in LA and Friends of the Urban Forest in SF, and continued research in the genetic nature of trees that do best in urban forests will build soil quality, help control invasive pests and ensure diverse urban forests.