How Many Trees in Sutro Forest? And What Will be Left?

Mount Sutro is a very difficult site. Its soils are shallow, its rocks unstable. It’s very windy. Few trees survive those conditions. Nevertheless, the eucalyptus forest has naturalized there for over a century – nearly 125 years now – and taken on the characteristics of an old-growth forest. This success has only been possible because of the interdependent ecology of the forest. The roots are intergrafted, which both helps distribute nutrients and moisture, and to provide physical support to trees. As Peter Wohlleben points out in his book, the Hidden Life of Trees, trees in a forest are different from individual, standalone trees.


The tree density changes the conditions in the forest, reducing wind speeds and providing shelter not just for other eucalyptus, but also for other trees species found in the forest: Monterey cypress, Monterey pine, coast redwood, plum, cherry, California bay, coast live oak, willow among others. It creates a tiny microclimate inside it.

pix9 072 forest

This is why the new Plan for the forest – removing most of the healthy living trees and nearly all the dead ones – will destroy the forest’s ecosystem and very likely the forest itself.

UCSF, which has declared the forest to be in poor condition, will doubtless blame the drought and pathogens. In fact, it’s the thinning – including removing understory – and the tree removal that rob the forest of its resilience. A forest that’s thrived for 125 years may be destroyed in a decade.


Here’s how the forest works as a Cloud Forest:

Sutro Cloud Forest is in the fog belt, and all summer long, its gets fog nearly every day. It’s never dry — and here’s why.

The trees grab the moisture from the fog and clouds (1 in the picture); it rains down onto the forest floor (2). There, it soaks into the duff — the  crumbly layer of dead and decaying leaves, twigs and other plant material accumulating on the ground beneath the trees (3). This material holds it like a sponge.

Above the duff, there’s a dense layer of understory plants that stop the water from evaporating – blackberry, ivy, ferns, poison oak, and 90 or so other plant species (4).  And above it all, there’s the tree canopy, which not only captures the moisture in the fog, but by shading  the forest floor, further helps to slow evaporation (5). (In some areas, there’s a mid-canopy of acacia and plum, which further helps.)

The result is that not only is the forest damp all the time, it also would take a long time to dry out – especially since the longest period it goes without rain or fog  is about 7-10 days in a year.


flowering_gum with bees susan walter 3Here’s how it functions as an ecosystem and wildlife habitat:  Sutro Forest Ecosystem and Wildlife Habitat. Besides sheltering other plants from the wind and providing them with moisture, the eucalyptus – as the world’s tallest flowering plant – is also an excellent habitat for insects, birds, and animals. The plants it shelters – blackberry, ivy and acacia – also form part of the ecosystem and habitat.

bee on blackberry flowers sutro forest



Here’s how its connections and network preserve the forest: Something Like Avatar: Mt Sutro’s Networked Forest.

Something like AvatarMany kinds of trees, including eucalyptus, when planted close to others of the same species, will intergraft their roots to form an underground network.  This helps all the trees in the group to survive. It’s one reason why even in a mixed forest, you tend to find trees in clusters by species, rather than evenly spread through the area. What this means for the forest is that, rather than being 40-50,000 (or 13,500 according to the new estimate) individual trees, it’s an entity that functions as an interconnected forest. This benefits the trees by providing support to each, stabilizing the slope they’re planted on like a living geotextile, and share nutrients.

These interconnections will be destroyed by the Plan to cut down most of the trees in the forest.


UCSF in its new Plan estimates that its 61 acres of Sutro Forest has only 10,000 live trees and 3,500 dead and dying ones. This estimate is down drastically from 45,000 trees it published in its 2014 Draft Environmental Impact Report. We’re pretty sure we would notice if three-fourths of the trees in Sutro Forest disappeared, so there’s an estimation error somewhere.

Anyway, for argument’s sake let’s accept the 10,000 + 3,500 number. In the original Plan, UCSF planned to cut down some 27,000 trees (according to their numbers), leaving some 18,000 trees in the Reserve. (See the article:  Message to UCSF: Do the Math!  about the 2014 Plan.) Now that it’s only got 13,500 trees, it would make sense to leave it alone, right?

That’s not what the new Plan recommends. Instead, they plan to cut down around 6-7,000 healthy trees and nearly all the snags.


First, let’s talk about reducing the acreage.

  • The “defensible space” around buildings will cover 14 acres, and all large trees will be removed. These areas will be used for Native Plants.
  • Separately, the Native Plant garden will be increased from 2 acres to 5 acres.
  • The “inspection areas” along trails – where any tree that’s leaning or otherwise considered problematic will be expanded and cover 18 acres. The Plan also says “Where appropriate, combine tree risk assessment and abatement with other forest management activities.” This is obviously intended to mean, cut down trees and add native plants
  • Trees may be removed to make access roads for machinery. Not clear whether this is included in the 18 acres.

So that’s 19 acres removed from the forest altogether, and some percentage of the 18 acres will be treated so aggressively they will no longer be forest.

Let’s say – generously – that about 40 acres of forest remain instead of the current 59 acres (excluding the Native Garden).

The objective of the Plan is to have 100 trees per acre, but they recognize they may have to keep replanting – and so plan to maintain a minimum of 75 trees per acre. This yield 3,000 to 4,000 healthy trees in the forest – against (their estimate) of 10,000 trees now.

They plan to cut down over 6,000 healthy trees (many more, because they are going to replant saplings, which will be included in the per-acre count).

In addition, they plan to leave only 2-3 snags per acre standing, despite the huge ecological value of the deadwood. This means about 80-120 snags of the 3,500 will be left.

With 10,000 stems gone and all the ivy and blackberry removed, the forest will be a shadow of its current self. And that’s without reckoning for accelerated tree death, windthrow, and loss of resilience. The native plants introduced will be primarily shrubs and herbs, with a few trees in some areas.

Instead of a healthier forest, it’s going to be  weaker and more vulnerable.


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1 Response to How Many Trees in Sutro Forest? And What Will be Left?

  1. takebackthegreen says:

    The sad, almost insane irony about this plan is that San Francisco is already almost completely devoid of densely forested areas, or even areas that feel in the slightest like natural, “not-part-of-a-city” places to enjoy yourself.

    And yet people who fancy themselves “stewards” of our urban environment want to (in Northern California, in 2017) cut down thousands of healthy trees and destroy a century old forest that is most likely unique in North America.

    A forest that will NEVER recover in our lifetimes. Destroyed. In San Francisco. By alleged environmentalists.

    The madness of true believers has no limit.

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