Mount Sutro Forest Ecosystem and Wildlife Habitat

We’ve talked earlier about how Sutro Forest works as a Cloud Forest, and why it’s always damp. Today, it’s Sutro Forest as an ecosystem and a habitat.  UCSF recently had a scoping meeting for its Environmental Impact Review for their plan to convert the dense, naturalized forest into an “urban forest” with only 7% of the existing trees, no understory, and no dead trees. Over 90% of the trees will be destroyed.

This is the forest that will be lost in the process.


On the website of Muir Woods, there’s a discussion about forests. Here’s part of what it says:

An old-growth forest commonly has three distinct layers: Herbaceous; Understory; Canopy.

To better understand this, think of an old-growth forest as a human home: the herbaceous layer is similar to the carpet, the understory like the furniture, and the leafy canopy similar to the roof. Each of these three layers support a different community of plants and each one is well adapted to its position in the forest.

In Sutro Forest, the herbaceous layer consists of a multitude of small plants (including forget-me-nots and grass and fern) and vines; the understory is primarily blackberry and acacia, and the canopy is eucalyptus.

The Muir Woods article also stresses the importance of dead trees:

It was not long ago that the National Park Service used to remove all dead material from this forest to keep it clean and reduce fire threat. However, dead trees are vital for the forest and take many different forms. They can fall and become part of the forest floor and aquatic community or can remain standing, becoming what is known as a snag.

The plan includes removing dead and dying trees, converting a wild naturalized forest into a tree-park.


The forest, though it includes around 100 species of plants, is dominated by a few trees and bushes that form its habitat. The trees are eucalyptus in the canopy, with acacia in the sub-canopy in many places. The understory is blackberry bramble (both California and Himalayan) with a mixture of smaller plants. (Many natural forests are based on one or two dominant species. Redwood forests. Oak-bay forests. Pine forests. ) Vines – primarily ivies – climb the tree trunks, but seldom reach the canopy of the trees.

Over forty species of birds use the forest; thirty species of birds were seen in one birding session last year. The forest provided an attractive dense forest habitat to birds ranging from the tiny winter wren to the huge great horned owl. No one’s made an inventory of the insect life or the reptiles that live here, but pumpkin spiders, and several butterflies have been seen in the forest; so have ensatina salamanders that like the cool damp world that exists here.


These beautiful 125-year-old trees, between 100-200 feet in height, form the canopy of the forest — the forest cover you see when you look at the satellite pictures.

  • They provide birds with cover and perches — both the host of songbirds that prefer forest environments and tall trees, as well as for the Great Horned Owls, Red-Tailed Hawks,  and other raptors.
  • The mosses, fungi and lichens on the trees are hiding places for various insects — the base of the food chain. [ETA3: Also, intriguingly,  it’s a habitat for nitrogen-fixing cyanobacteria that help fertilize the soil. See our new post here.]
  • Eucalyptus flower in winter, providing food for bees, and for nectar-feeding birds. The nectar attracts other insects, too, and the birds that feed on them.
  • Eucalyptus fruit are hard-shelled “gum-nuts” that hide nutritious seeds. The gum-nuts, both on the tree and on the ground, are also food for birds and animals – including snacks for coyotes, as Janet Kessler notes in her observational blog.
  • The roots of the trees intergraft, creating a networked interdependent forest that stabilizes the soil. This forms the matrix on which the whole forest grows.
  • The tall eucalyptus also harvests moisture from the fog, watering all  the plants below it.
    Photo credit: Janet Kessler
  • The fallen leaves become part of the duff, the layer of organic litter that holds moisture and encourages plant growth. It’s also an insect haven, and birds from wrens to sapsuckers hunt there.
  • [Edited to Add: Hollows and cavities in older trees provide homes for animals, nesting spaces for birds — and, occasionally, a hive for wild bees like this one in the picture.]


This tree occurs naturally as an understorey tree in the wet eucalyptus forests of Australia, and so it does here, too, in Sutro Forest, where it forms the sub-canopy in some areas.  It tolerates a wide range of conditions, including fog and wind.

  • This is a leguminous tree, and fixes nitrogen — thus providing food to surrounding plants and making the thin mountain soil more fertile. In an experiment in Hawaii, researchers found eucalyptus planted with acacia grew 25-28% larger than plantings that were only eucalyptus. (The link is to a PDF describing the experiments.)
  • Blackwood acacia blooms with pale yellow flowers in the spring, attracting insects of all kinds and the birds that feed on them. (It’s relatively non-allergenic because of its heavy pollen though of course some people do react to it.) Bees like acacia flowers, and acacia honey is valued.
  • Its dense foliage provides cover to nesting and foraging birds, which eat insects that live in its leaves and densely-scored bark.
  • The seeds, which form in pods like twisted peas, have a reddish “eril” or stalk, which contains energy-rich lipids that attract and feed both insects — especially ants — and birds.
  • Unlike the eucalyptus, the acacia is relatively short-lived (though some specimens have lived hundreds of years). Dead and dying trees provide important habitat for insects that feed on decaying wood, and birds and animals that prey on those insects: woodpeckers; sapsuckers; raccoons; skunks. The logs provide shelter for insects and reptiles including skinks.


Thriving in the damp shade under the eucalyptus, the dense thorny thickets of blackberry are bird and animal heaven.

photo credit:

It’s difficult for predators to follow them in there. These thickets are full of insects that provide food, as do the flowers in spring, and the occasional berries. (The blackberries in the forest do not fruit profusely for lack of sunlight, but where they still exist along the road and wider paths, they do carry berries in season.) They’re great for birds nests, hidden from predators.

A single blackberry thicket can provide a great deal of habitat. Consider one that’s say 8 feet by 10 feet and 6-7 feet tall  (about as high as a person — the normal height for blackberry to grow unless it’s mown down). That’s around 500 cubic feet of habitat for insects and birds. The same thicket, mown down to 1 foot high, will only provide 80 cubic feet of habitat. And it’s much inferior habitat because it offers a lot less protection — it’s flatter and more visible.


The vines (mainly English and Algerian ivy) growing up the trunks of the trees are another rich habitat for insects and birds. They not only provide cover, nesting and foraging areas, some of them bear fruit that is attractive to wildlife such as nuthatches and wrens. The vines also trap moisture, creating miniature ecosystems on the trunks of the trees. Other plants can gain a foothold, adding to the typical cloud forest layering of vegetation. This is turn provides a place for insects and birds

There is some question as to whether they are killing the trees. If they are, it’s taking them many years. We have seen hardly any instances where the vines actually reach the canopies of the trees, and vine-covered trees don’t appear to fall more easily than clean-trunked ones. In fact, most of last year’s fallen trees had no vines.

We’ve been wondering: Why don’t the vines reach the canopies and kill the trees? It appears there may be two reasons:

  • These trees are too tall. They’re between 100-200 feet high. Most ivy can only climb 50-90 feet (Michael Dirr, 1998). This allows the vines to get near the tops of these trees, but not into the canopy where they could kill the trees.
  • The eucalyptus is flexible at the top. We’ve all seen how the tops of eucalyptus wave in the wind like great green fans. University of Florida’s Francis Putz notes that in a study he did in Panama, “Trees of predominantly liana-free species were more flexible and had longer leaves…”  Is a similar effect at work here, with the  movement making it more difficult for the vines to grab hold?

Anyway, the vines reach the tops only of smaller trees that are below the canopy. When these remain standing, they provide a leafy green column that is splendid habitat for insects and for birds such as nuthatches, woodpeckers, and sapsuckers.

[ETA 2: We’ve heard said that eucalyptus woods in California provide a “simplified” eco-system compared with the native oak-bay woodlands established over thousands of years. Not so, apparently. A recent article on the Death of a Million Trees website looks at the findings of a scientific comparison. The eucalyptus forest had as much biodiversity as the oak-bay woodland. The only significant difference was that the eucalyptus forest had fewer rodents.]

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19 Responses to Mount Sutro Forest Ecosystem and Wildlife Habitat

  1. M Gardner says:

    Lovely article: presenting all the complexity so simply! From out here in Australia , I appreciate your determination to spread appreciation of local ecology. I like getting these insights into your urban forest.

    • webmaster says:

      Thanks! On a foggy day, I think this may be one of the most beautiful places in San Francisco, and it’s one of the few remaining pieces of dense forest habitat. Whether or not it’s destroyed in the end, I feel privileged to have seen its beauty and to research and understand it better.

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