Eucalyptus is an invasive plant in California; it was introduced in the nineteenth century to provide timber for railroad use. Although ecologically damaging, the trees look and smell pleasant – the scent is recognizably similar to, but more complex than, that of eucalyptus oil.

I recently learned that some species of eucalyptus may have the potential to grow taller than sequoias. It is speculated that some trees in Australia at the time of European colonization may have been the tallest on Earth at the time, but they were cut down before measurements were taken, so it’s impossible to confirm this. How tall these particular trees may eventually be, I don’t know. (There are about seven hundred species.)

The area from the San Pedro Valley and Montara Mountain north to the Golden Gate Bridge has few native tall trees. Madrones do exist here – you can see one to the right of the photograph – but they don’t grow as tall as they do elsewhere. Before the introduction of eucalyptus, there were no forests:

As chronicled by Spanish explorers as early as 1769, the slopes and ridges surrounding Montara Mountain appear smooth and undulate from a distance. Once, much of the San Francisco Peninsula—from Montara Mountain north to the Golden Gate—looked this way, the landscape shaped by frequent fog, salt spray, and cold winds. Only six miles across, this island-like San Francisco Peninsula is a “hot spot” for endemics (species that are found only in one area) and unusual natural communities, and has thus been deemed a distinctive bio-geographic unit, the “Franciscan Landscape.” Urbanization of the peninsula has left little of this remarkable biota intact, but here, in the valleys and ridges that surround Montara Mountain, it is alive and humming.

San Bruno Mountain is apparently also such a place. Michael Vasey goes on to offer some historical speculation:

When the Spaniards arrived in San Pedro Valley in 1769 and noted the absence of tall trees, a village of Ohlone people was well-established; Native American artifacts from nearby coastal sites demonstrate that people had occupied this area for over 5,000 years. During that time, sea level rose over 300 feet and moved in steadily from several miles offshore, the Bay formed, and the climate of the northern San Francisco Peninsula became progressively more maritime. My guess is that San Pedro Valley once hosted a conifer forest, but the conifers could not tolerate the increasingly maritime conditions (especially the salt spray). The introduction of a frequent burning regime by local Native Americans (seeking to encourage the growth of desirable plants) may have added stress to the conifers. Although Douglas fir eventually disappeared, some of the understory species—such as hazelnut, western burning bush, trilliums, and fetid adder’s tongue—were left behind. These more salt spray- and fire-tolerant species managed to persist until the present as our special hazelnut-cream bush scrub.

How could one try to confirm this hypothesis? I’m no expert, unfortunately.

The eastern slopes of the mountain – mostly inaccessible to the public, I think – also contain unusual serpentine soil and therefore other endemic plants.