But just where in 21st century California do blue gums belong, I wonder as I stroll through the Sweet Springs Preserve. How should the managers of private and public lands balance their positives and negatives? How, for that matter, can Californians weigh the value of shade and scent and high-rise drama against that of reassembling ecological communities that could serve as Noah's arks for increasingly rare native species?

The eucalyptus genus embraces upwards of 700 species, which range in size from multi-stemmed bushes to trees that approach 300 feet, making them the tallest flowering plants in the world. Eucalyptus globulus, the Tasmanian blue gum, along with Eucalyptus diversicolor, the karri tree, and Eucalyptus regnans, the mountain ash, rank among the latter. In the world of trees, only California's coastal redwoods grow taller.

European explorers first took note of these singular trees, with sticky resin or "gum" oozing from their bark, in the 17th century. Then, in 1792, when a French expedition landed in Tasmania, botanist Jacques-Julien Labillardière noticed a tall blue gum in bloom and had it felled in order to collect its flowers. Little more than a half-century later, eucalyptus seedlings, Tasmanian blue gums among them, were being offered for sale by nurseries in San Francisco and Oakland and planted, often en masse, as windbreaks, as wood sources, as aesthetic enhancements.

Among those who displayed an early interest in the trees was wilderness advocate John Muir, cofounder of the Sierra Club. In 1889, he planted a gum tree on a friend's estate in South Pasadena. Some 15 years later, Muir spent four months traveling through Australia and New Zealand, reveling in his walks through "the heart of the forest primeval, where trees are tallest and least changed by man." Today, a statuesque manna gum, one of the blue gum's many relatives, stands watch over Muir's gravesite in Martinez, Calif.

Not until the 1980s did the generally positive attitude towards eucalyptus trees begin to erode. It was then, says historian Farmer, that conservation biologists began voicing alarm about the impacts of invasive species on native ecosystems. (In California, at least, "native" has a strict definition: It means the trees, shrubs and flowering plants that existed here before 1769, the year Father Junipero Serra crossed the Tijuana River.) From the scientific literature, the concern spread, and soon terms like "non-native" and "invasive" entered the rough-and-tumble of public debate.

Starting in the 1990s, shortly after the Oakland Hills fire, prominent parks in the coastal region began removing significant numbers of blue gums, as did other public and private institutions. The process has continued, often driven by risk assessment – the danger posed by fire and falling limbs from unmanaged eucalyptus stands. Other times, it's inspired by the encroachment of the trees on habitat deemed critical to native plants. At Sweet Springs, for example, the area at the center of the controversy retains small populations of the rare Blochman's leafy daisy and the federally endangered Morro shoulderband snail.

In response, pro-eucalyptus activists have packed public meetings and established websites to protest what they view as a broad, at times conspiratorial, attack on beloved trees by those they disparage as "nativists." Last year, the Morro Coast Audubon Society was so stunned by the outcry when it filed for a county permit to remove some of the Sweet Springs blue gums that it put the project on hold. In San Francisco, some 240 miles north of Los Osos, eucalyptus fans are currently battling the city's Department of Recreation and Parks over plans to reduce the number of blue gums in areas it manages. They have also protested the University of California, San Francisco, which in August began thinning blue gum stands on 900-plus-foot-tall Mount Sutro, one of the city's highest points.