On a Saturday morning in early September, the Sweet Springs Nature Preserve in Los Osos, Calif., bustles with activity. To the east, a dozen or so volunteers struggle to stem the tide of veldt grass that laps at the feet of waist-high lupines. To the west, a great egret stalks the shallows of a small pond, and a few dog-walkers meander along a trail shaded by a lacy canopy of towering Tasmanian blue gums, a type of eucalyptus tree.

Invisible but no less real is the cultural fault line that runs through this preserve, which is managed by the Morro Coast Audubon Society. On one side stand those who would remove some of the blue gums to restore an embattled native ecosystem; on the other are those who believe these tall exotic trees have as much right to exist as the indigenous redwoods, California's official state tree.

As environmental clashes go, this one seems fairly minor, even inconsequential. At issue is the fate of perhaps a hundred blue gums out of the more than 450 that presently grow on the 32-acre preserve, whose springs feed the Morro Bay estuary. Yet the dispute affords insight into a much wider conflict that affects the entire California coast, one that springs from different visions of the natural world and the place of non-native species in it.

Strikingly, the combatants on both sides consider themselves environmentalists, so that beneath the fraught rhetoric lie concerns that are not easy to dismiss. Both seem to share a passionate love for nature, but disagree about which organisms deserve priority. The raptors that sometimes nest in the blue gums and the monarch butterflies that use them for winter shelter? Or the Morro blue butterfly and the myriad less charismatic creatures that thrive amid native grasses and wildflowers?

The trees are problematic for a number of reasons. They grow so fast that their branch structure often gets out of whack; left unmanaged, they tend to lose large limbs, posing a danger to people and property. They drop so much bark and leaf litter and commandeer so much water and light that only the most robust, shade-tolerant natives can grow alongside them. Blue gums can also be invasive. Given access to year-round moisture, they readily spread beyond where they were originally planted. At Sweet Springs, preserve manager Holly Sletteland regularly pulls out the blue gum "volunteers" that sprout amid native shrubs and perennials.

And that's not all. Eucalyptus stands can pose a serious fire danger, as became apparent in 1991 when blue gums burst into flames that lit up the Oakland Hills. While these non-native trees hardly bear all the blame – tinder-dry grasslands, native scrub and Monterey pines also contributed to the firestorm – the fuel load provided by eucalyptus debris unquestionably intensified the blaze. In the span of 72 hours, 25 people died, and well over 3,000 houses and apartments were destroyed.

Yet these same trees, with their shaggy bark and spicy fragrance, are beloved by many. In the blue gum's immigrant history, people see an experience that parallels their own, observes environmental historian Jared Farmer, author of Trees in Paradise: A California History. "There are still a significant number of Californians who are the first generation in the state. They have come to love California. They have come to think of it as home. And they feel this kinship with the eucalyptus. They say to themselves, 'This tree is not from here, but it's beautiful, and it seems to fit.' "