The Trees of San Francisco. Michael Sullivan

      The Catalina ironwood is one of the few California native trees planted on San Francisco streets. Although fossil records prove that this tree was once widespread throughout California, it is now limited in the wild to the Channel Islands of Santa Catalina, and to Santa Cruz, Santa Rosa, and San Clemente off the Southern California coast. This tree demonstrates how islands often develop unique flora as environmental threats from the mainland (in this case, perhaps plant-eating animals) are stopped at water’s edge.

      Catalina ironwood was first discovered in 1884 by William Lyon and named in his honor, as you can see from its scientific name, but it was pioneering nurseryman Francisco Franceschi who introduced this tree into the nursery trade. Upon finding that the tree was difficult to germinate from seed or branch cuttings, he set out to the Channel Islands to collect a full-grown specimen with large enough roots to permit root cuttings. Suspected as an outlaw by the Coast Guard, he was fired upon until his vessel began to leak. Furiously bailing water, he managed to reach Santa Barbara Harbor with his prize in hand. Young plants from this specimen were introduced to nurseries a few years later. Ironically, today this tree is planted worldwide, but is threatened in its native habitat of the Channel Islands as a result of grazing by introduced feral goats.

      In addition to its interesting history, this tree has a distinctive appearance, with deep green, glossy leaves divided into three to seven leaflets. Each leaflet is deeply notched, giving a fernlike grace to the entire tree. The bark is reddish brown and peels into narrow vertical strips. In May and June, the tree produces small white blossoms that occur in large flat clusters 3–6 inches wide.

      Ulmus parvifolia

      CHINESE ELM

      LOCATION: 245–255 Hartford St./19th St. in the Castro; many examples on both sides of Folsom St. between 24th and 26th Sts. in the Mission

      The Chinese elm is by far the most common elm on San Francisco streets. A medium- to large-sized tree (growing to 30–35 feet), it keeps its leaves year-round in San Francisco’s mild climate and develops an attractive weeping form as it matures. The 1- to 2-inch leaves are smaller than those on other urban elms; this elm is also distinguished by bark that sheds in patches, creating beautiful two-toned (gray and brown) mottled patterns. Chinese elms are resistant to Dutch elm disease, which has decimated the American elm in most of the United States. San Francisco elms have not yet been affected by Dutch elm disease, and you can still find American elms here and there in the city. The Chinese elm is native to northern China, Japan, and Korea.

      Sequoia sempervirens

      COAST REDWOOD

      LOCATION: Redwood Park on the east side of the Transamerica Pyramid in the Financial District. Muir Woods in Marin County is the best spot nearby to see coast redwoods in their native glory.

      The coast redwood is the tallest tree on Earth, growing easily to more than 300 feet. (The tallest individual tree on Earth—discovered in 2006 and measured at 379 feet—is a coast redwood in Redwood National Park, north of Eureka, California.) These evergreen conifers are densely branched and gracefully pyramidal in shape, with flat, feathery needles and reddish-brown, fibrous bark. The coast redwood’s native range is limited to a thin coastal zone, rarely extending more than 20 miles inland, from Monterey County in the south to extreme southwestern Oregon in the north. Fossils of the coast redwood date back 65 million years to the Cretaceous period, when the tree coexisted with the dinosaurs in North America, Europe, and Asia. As Earth’s climate became cooler and drier, the tree gradually retreated to its current range, where the mild Mediterranean climate and coastal fogs provide the humid environment that redwoods prefer. Although not as long-lived as their relatives the giant sequoias, coast redwoods live a long time: many attain 800 years of age, and some last a millennium. Redwood lumber has always been in great demand for timber, as the wood is durable, decay resistant, and easy to work. Most of San Francisco’s Victorian homes were constructed from redwood. The wood has proven so popular over the years that more than 95% of the old-growth redwood forests have been wiped out, with conservationists scrambling to preserve the rest.

      Erythrina crista-galli

      COCKSPUR CORAL TREE

      LOCATION: 366 Cumberland St./Sanchez St. in Dolores Heights

      The cockspur coral tree is likely to be better known to readers from Southern California, where it is common. San Francisco is outside the normal range of this warmth-loving tree. In fact, I am aware of only one sizable example of this tree in San Francisco: a large and healthy specimen in a wind-protected spot in Dolores Heights. (The best time to visit this tree is in late June and July, when it is in full bloom.) Cockspur coral trees are native to southern Brazil, Uruguay, and northern Argentina. Crista-galli is Latin for “cock’s comb,” which the distinctive showy, pink flowers resemble, and they are like nothing else you will find in a Bay Area tree. In Argentina, children call the flowers patitos (ducklings), because they float like little ducks when dropped in water.

      Quercus suber

      CORK OAK

      This native of the western Mediterranean is the tree from which natural cork is made, and the species name suber is Latin for “cork.” The tree’s bark is its distinctive feature; if you stick your finger into the fissures on the trunk of a cork oak, you’ll be able to feel the spongy new bark from which cork is made. Cork is formed from multiple layers of strong cell walls that are both airtight and waterproof. This kind of bark protects the tree from two common environmental threats in the Mediterranean region: drought (watertight bark keeps moisture in) and fire (airtight bark acts as a fire retardant, preventing damage to the living layers inside and allowing the tree to rebound quickly after fires).

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