Presidio’s forest resists the sands of time

On a sunny day in San Francisco, you might find yourself walking through the Presidio – taking in some fresh air, finding respite among the trees, or enjoying views of the San Francisco Bay.

What you may not know, though, is that if you had been walking through that same area 130 years ago, you’d be trudging through sand dunes. 

Sand, and lots of it – that’s what makes up the natural habitat of the Presidio. Salt, wind, and sand-loving plants like the blue-violet dune gilia, thrived in this terrain alongside the grizzly bears, wolves, and coyotes that frequented the area before the arrival of Europeans.

Once they did arrive on the sandy mounds, soldiers soon realized that despite the sweeping views, the Presidio was not a charming seaside fort. The endless dunes and bluffs seemed to be a wasteland – the few naturally surviving trees were cut down and used for fuel. Bay winds barraged the soldiers with sand and turned their homes into deserts. In 1883, one man had a plan to transform the Presidio into something more palatable to European tastes.

A man with a plan

Major William Albert Jones, an engineer, drew up the preparations for developing the Presidio. He believed that planting a forest would not only create a much-needed windbreak, but that the transformation of sand into woodland would dazzle San Franciscans.

More than 130 years ago, the Presidio was coastal sand dunes, a landscape the U.S. Army successfully transformed as an engineering feat to dazzle San Francisco. 

“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,” Jones wrote in an 1883 planning document for the Presidio. 

He continued: “In order to make the contrast from the city seem as great as possible, and indirectly accentuate the idea of the power of the Government, I have surrounded all the entrances with dense masses of wood.”

The area’s soil was a mixture of sand and serpentinite, the blue-green California state rock. This made for a soil that was poor at retaining water and nutrients, and too high in toxic metals for anything to thrive that hadn’t adapted there over the millennia. Taking notes from the newly created Golden Gate Park, the U.S. Army brought in horse manure collected  from the city’s main commercial thoroughfare on Market Street and mixed it into the existing soil. This addition, combined with initial experiments planting various sand-trapping grasses, created an environment where trees could grow.

The first official planting took place on the first celebration of Arbor Day in California. By 1907, thousands of Monterey cypress and Monterey pine (both California natives but nonnative to the area) and imported Tasmanian blue gum eucalyptus densely crowded the Presidio.

Preserving the Forest

In 1962, 300 acres of the historic forest became a protected landmark, with many of the same tree species remaining today. Because of this status, if any trees are removed, they have to be replaced with another of their kind.

That may sound restrictive, but Peter Ehrlich, the Presidio Trust forester, has found creative ways to improve the historic forest.

“Since 1886 to 1910 is a very short amount of time to plant 100,000 trees, they basically created an even-aged forest,” he said. “Because of that, they’re all declining at the same time – especially the cypress and pines.”

Since 2003, the Presidio Trust’s reforestation team has removed and replaced two and a half acres of trees each year, to create a healthier, uneven aged thicket. Ehrlich believes it will take 65 years to get through the conifers planted by the Army.  He said he has also had success with disease-resistant Monterey Pines, and is seeking out a similar replacement for the Monterey cypress. 

Meanwhile, Tasmanian blue gum – a highly invasive, rapidly reproducing, Australian eucalyptus, makes up nearly half of the historic forest. If any of these trees are removed, they still have to be replaced with eucalyptus because of the landmark status. But Ehrlich has a solution for that too, and has been experimenting with less invasive eucalypts, like the slow-flowering mountain gum, to preserve the original design intent of the plantation but without the harmful consequences.

Sands of time

Managing the forest is complicated. The challenge the Army faced with the soil still holds true; the persistent sandy soils have to be amended each time new trees are planted in the dunes. Three hundred yards of well-cured, organic compost is added to each half-acre plantation to increase the water holding capacity of the sand, which helps get the new trees started.

The Presidio Trust replaces 2.5 acres of trees each year from the historic forest in an effort to create unevenly aged growth. Photo by Wolfgang Schubert.

Getting new trees to grow isn’t the only challenge. The historic trees have been damaged by storms and are still recovering from the Army’s tree topping techniques. Debates rage on with native plant advocates over how the historic forest should be restored.

But the forest is more than just a leafy addition to the Presidio grounds. Around 300 bird species have been identified at the Presidio, many of which make their home in the forest. Great horned owls, red-tailed hawks, and downy woodpeckers, and others find habitat in the trees. 

Increasing biodiversity is one of Ehrlich’s major goals, and he is leading the effort to create height diversity in the canopy for birds that call the lower branches home.

“Birds like to nest in the holes in the dead limbs, so some of the older trees provide habitat for birds that wouldn’t otherwise be here,” Ehrlich said. “We also recently had bluebirds return.”