On a drizzly winter day in San Francisco, a pickup truck loaded with eucalyptus seedlings pulls up to a bare hillside in the Presidio, a former U.S. Army base turned national park. A crew of shovel-wielding men starts moving across the slope, planting knee-high trees in tight formation. Dressed in a bright red rain suit, Presidio Trust forester Peter Ehrlich waves in welcome.
"Look over there," Ehrlich says, walking towards the truck. "The seedling with the blue-green leaves is Eucalyptus pauciflora, or cabbage gum. It'll probably top out at 40 feet. Here, with the narrow leaves, is Eucalyptus dalrympleana, mountain gum. It'll grow 80 to 100 feet tall. And there's Eucalyptus diversicolor. Now, that's a spectacular tree. In Australia, diversicolors get to be almost 300 feet tall. They look like giant missiles, like they're going to grow right through the clouds!"
I feel like I'm witnessing a slightly subversive act. A number of parks have removed eucalyptus, notably blue gums, but I'm aware of relatively few that are replanting them. Among the exceptions is the Presidio, whose mission includes preserving and restoring both natural and cultural landscapes. Early on, the park's managers recognized the historic significance of the forest that more than a century ago rose above coastal dunes, marshes and scrublands.
The first trees were planted shortly after 1883, when Army engineer Major William A. Jones drew up a grand plan: "The main idea is to crown the ridges, border the boundary fences and cover the areas of sand and marsh waste with a forest that will generally seem continuous and thus appear immensely larger than it really is." Soon, blue gums, Monterey pines and cypresses occupied more than a quarter of the 1,480-acre base. Today, this 300-acre treescape – the historic forest, it's called – coexists with areas set aside for native plants and ornamental landscaping.
Managing the trees is complicated; Ehrlich oversees some 60,000 big trees throughout the Presidio. Those that overhang places where people walk, work and live are monitored for structural weaknesses and, when necessary, pruned or taken down. Those that encroach on native plant communities are gradually being removed. And those in the historic forest are preserved for as long as possible, then replanted. Already, Ehrlich is replacing aging pine and cypress stands with vigorous young specimens.
The spot where Ehrlich and I are standing overlooks a former landfill on its way to becoming a grassy ball field. As part of the cleanup, the tree crew cut down a group of blue gums and blackwood acacias. The plan now, Ehrlich explains, is to array the smaller cabbage gums along the field's perimeters, with the larger mountain gums and diversicolors towering in the background. It's an experiment, he says, similar to an earlier eucalyptus planting that followed a blue gum removal at the Rob Hill Campground. By monitoring the trees as they grow, he hopes to identify species of eucalyptus that are better behaved than blue gums.
At the moment, Ehrlich is trying to reduce his management headaches in areas near populated places. To prevent structural problems, blue gums require a lot of maintenance. Ehrlich is also trying to identify less invasive eucalyptus that can eventually replace blue gums on a broader scale. So far, mountain gums appear the most promising. But blue gums can live for around three centuries, so the need to rejuvenate deep-forest stands is unlikely to become urgent for decades.