The opposition has become increasingly sophisticated, with eucalyptus fans scouring the scientific literature to find articles that bolster their position. Among the prime exhibits is a controversial essay by Macalester College ecologist Mark Davis and 18 other scientists, which calls into question the danger posed by many non-natives and the utility of fighting them on a broad landscape scale.

At its core, though, the pro-eucalyptus position draws its strength not from science but from a deep reverence for trees. "Eucalyptus trees, oak trees, redwood trees, they're all trees, and trees are good," says Los Osos resident Joey Racano, a musician, artist and environmental activist who lives less than two miles from the Sweet Springs Preserve.  "Trees make things cooler. They support life. They provide places where birds can hide." Why should it matter that blue gums trace their origins to distant shores? "They are now part of our ecosystem and causing us to redefine the word 'native.' "

Seeking clarity on the subject, I visit Matt Ritter at California Polytechnic State University in San Luis Obispo, some 12 miles east of the Sweet Springs Preserve. A professor of botany, Ritter is also the director of the Cal Poly Plant Conservatory, chair of the city's Tree Committee and author of A Californian's Guide to the Trees Among Us. Many of the state's eucalyptus stands, he says, were planted in the early part of the 20th century during a short-lived commercial craze. Fast-growing and drought-tolerant, the tall trees seemed like the solution to the "wood drought" then plaguing the West. If all went according to plan, blue gums and other species would provide lumber for houses, piers, railways and fences, not to mention tannin, fiber and fuel.

As building material, however, the young wood proved disappointing, and many plantations were abandoned. Near Los Osos, a major speculator was Alexander S. Hazard, who farmed and ran a dairy in San Luis Obispo County. In 1965, his former property, with its rows upon rows of eucalyptus, became part of Montaña de Oro State Park. The Sweet Springs Preserve was once part of a cypress plantation that evidently grew blue gums as well. "In many ways, what we're dealing with today is the legacy of an unharvested crop," Ritter says.

Ritter is one of the few scientists who have seriously studied eucalyptus invasiveness in California. Of more than 200 species of eucalyptus that presently grow in the state, he and a colleague reported in 2009, only 38 are widespread. Of those, only 18 species have reproduced enough to become naturalized, and, of those 18, only two –– blue gums and red gums –– can be considered even moderately invasive, and then only in places refreshed by perennial moisture in the form of streams, springs or summer fog.

On the computer screen in his office, Ritter pulls up a series of aerial photographs that illustrate the expansion of a former blue gum plantation, 150 miles north of us. "Here's a photo that was taken in 1931," he says. "Here's what it looked like in 2001." The outward march has occurred at the rate of about two feet a year, he calculates.  "And over the course of seven decades, that will take you somewhere." At the same time, two feet a year comes nowhere near the expansion rate of, say, veldt grass, an import from Southern Africa whose featherweight seeds ride the winds to distant locations.

Even more interesting to Ritter are the 16 eucalyptus species that, so far, have shown no inclination to spread. Among these is the karri tree, or Eucalyptus diversicolor, which rivals the blue gum in size. What if karris had been planted instead of blue gums? Would that have defused the eucalyptus problem? "It would remove invasiveness as an issue," Ritter replies, but the other complications, such as the hazards the big trees can pose and their competition with natives for light and water, would remain.