The Importance of Eucalyptus

It’s Earth Day today, and we’d like to talk about the importance of Eucalyptus trees. Surprisingly, even some people who identify as environmentalists have no idea of the far-reaching value of these trees in our environment.

eucalyptus in san francisco

San Francisco has a lot of eucalyptus trees, mainly in its parks. According to a 2007 USDA report on San Francisco’s urban forest, this species accounts for over 16% of the the total of 669,000 trees in the city. This more than any other kind of tree. It’s a sturdy, fast-growing, graceful tree that grows where others will not – in areas of high wind.

(Monterey pine and Monterey Cypress are next, together accounting for about 12%. But unlike the eucalyptus trees, which are healthy and long-lived, many of these trees are nearing the end of their lives and have infections of bark beetles and pitch pine canker.)

CARBON STORAGE

looking up a treeAs we enter a world of increasing climate change, we’re starting to understand the value of carbon storage. Trees capture carbon dioxide from the atmosphere, and store it in their wood, roots and leaves – and in the soil beneath them. The actual amount stored is proportional to the dry weight of the wood and organic matter. Eucalyptus is particularly good at sequestering carbon.

  • Its wood is very dense, which means it holds more carbon for each cubic inch of wood;
  • It grows fast, which means it take up carbon quickly;
  • It grows large, which again means it stores more,  in the trunk and branches and its root system.
  • It lives a long time, around 400-500 years in the kind of wet conditions we have in San Francisco, which means it will not be giving back the carbon any time soon.

According to the Report, “Of all the species sampled, blue gum eucalyptus stores
and sequesters the most carbon (approximately 24.4% of the total carbon stored and
16.3% of all sequestered carbon).”

In places like Sutro Forest, where it grows in association with acacia trees – a nitrogen-fixing species – it’s even better at storing carbon. The carbon gets stored in the trees and the soil.  Sutro Forest is an excellent carbon sink.

HABITAT FOR WILDLIFE

flowering_gum with bees susan walter 3Eucalyptus is also excellent as wildlife habitat, despite contrary claims.

  • Summer Tanager eating bee, San Francisco. Copyright Mark Rauzon

    Summer Tanager eating bee, San Francisco. Copyright Mark Rauzon

    Eucalyptus has nectar-rich winter-blooming flowers. It’s the largest flowering plant in the world. In the Bay Area, it flowers when few other trees or plants are in bloom, and its flowers are rich in nectar. It provides a food source to insects and the birds that feed on them, as well as directly to nectar-feeding birds.

(And if you’ve heard the urban legend about eucalyptus resin killing birds – it’s not true.)

  • hawk nest by Janet KesslerEucalyptus trees provide nest sites for a whole variety of birds. As the trees mature, cavities form or are excavated by woodpeckers. These form nest sites not just for woodpeckers, but Western Bluebirds and other cavity-nesters. The crooks of the branches provide nesting spaces for Great Horned Owls and for Red-tailed Hawks.
  • Bewick's wren in San Francisco photo from SFForest.netWhere ivy or other vines are allowed to grow on eucalyptus trunks, they provide additional nest sites – as well as excellent cover for little birds, bats, and small animals to hide from predators like hawks.

In areas like parks – or Sutro Forest – where the leaf litter is allowed to accumulate under the trees, it provides an excellent habitat for small reptiles like salamanders. The damp environment is good for animals like banana slugs, which have been seen in the forest.

POLLUTION AND SOUND CONTROL

Like all trees and bushes, eucalyptus helps control pollution, especially particulate pollution. The particles are trapped on the leaves, and washed to the ground when it rains – thus staying out of our lungs. Barriers of trees and dense vegetation also help absorb the sounds of urban life.

The Report calculates an “Importance Value” based on the percentage of trees plus the percentage leaf area. For eucalyptus, that is calculated at around 27%. (Monterey Pine is higher, at 31%, but as we said – there are issues with the health of that species.)

SLOWING WATER RUNOFF

San Francisco is working hard to slow water run-off, to keep rain water out of our sewer system and let it soak into the ground. They’re encouraging people to use build sidewalk gardens and use permeable paving so water can get through to the ground rather than run off.

Eucalyptus is excellent at slowing water. It moves a lot of water, sucking it in through the roots, and transpiring it through its leaves.   The leaf litter and plants that grow below it also help in trapping wand slowing water. In San Francisco, they help enormously in slowing run-off; you can see the obvious difference after a rain-storm between the water running off Twin Peaks Boulevard in a river, and the relatively small amounts running off Mount Sutro.

NATIVISTS DESPISE EUCALYPTUS

As we’ve been reaching out to let people know about the threat to Sutro Forest, we’ve been encountering some people who despise eucalyptus. We’re not talking of developers, who don’t actually hate trees, only obstacles of whatever species or nature. We mean people who identify as environmentalists – as we do, too. But they have focused on Native Plants, and fail to recognize the importance of eucalyptus in our environment.

We urge UCSF, the Sutro Stewards, and SFRPD to recognize the urban environment we live in, and the unique value of eucalyptus in our city. No other tree is as useful from an environmental standpoint. It’s time to stop attacking these trees and to start recognizing how much they’re doing for us.

San Francisco had, in 2007, only 12% canopy cover – less than almost every city in the report except Jersey City, NJ. Our city cannot spare its majestic and valuable eucalyptus trees.

Two girdled trees

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9 Responses to The Importance of Eucalyptus

  1. Tom says:

    Though well-meaning, you are not environmentalists. You are conservationists, as your central aim is to conserve eucalyptus that, as it turns out, is a non-native species to which you have a romantic attachment. You use environmentally-related arguments to defend this central conservationist aim.

    [Webmaster: Thanks for stopping by to comment, Tom. We won’t get into semantic arguments. Some of our supporters hesitate to call nativists environmentalists, too, because many of the plans offered are anti-environmental. We do like eucalyptus. We’ll admit to an attachment to trees.]

    Your argument that Eucalyptus is “excellent” wildlife habitat sidesteps the fact that the native habitat is crowded out by Eucalyptus, in turn reducing or eliminating habitat for native plant, bird, insect, butterfly, and animal species. Are you arguing that Eucalyptus provides more habitat for native species than native habitat? Surely not. Perhaps you should acknowledge as much, though to do so would unravel your central defense.

    [First, eucalyptus is “crowding out” native habitat only in the sense that any park or building is doing so. The eucalyptus in San Francisco was planted there, whether in parks or on streets. And yes, the theory that native animals need native habitat is not accurate. Anise swallowtails use non-native fennel. The Western Tiger swallowtail uses London Plane Trees. Monarch butterflies need tall trees like eucalyptus. And eucs are good for all nectar-loving and tree-loving species of insects, birds and animals. Nearly all the species that use eucalyptus trees for habitat are actually native; honeybees are among the few that aren’t. But even they are part of the food chain – they get eaten by native birds. HERE’s an example. (Obviously, it is not so good for grassland creatures, and we’re all for preserving the existing grasslands on Twin Peaks, for instance, for larks and gophers and other wildlife.) We think we can have both the dense forests and the species that need it, and the grass and scrublands and the species that need those.]

    Prevents water run-off? In a city that should conserve more water, this is water down the tubes, or up the (eucalyptus’s) tubes, as it were.

    [The water we’re talking of is from rain – or fog moisture. No one irrigates eucalyptus. It’s the water that would flow down the roads and into the sewer system, and San Fran is working to slow down so rainstorms don’t overwhelm the sewers.]

    And comparing it to Twin Peaks? Surely that’s related to Twin Peaks being more paved, no?

    [Twin Peaks has roads running in a figure of eight around it, but the peaks themselves are covered in grass and shrubs. Mount Sutro is surrounded by paved roads, and Medical Center Way and the Nike Rd are paved surfaces. Most of the rest (except for the Native Garden n the summit) is forest.]

    Anyway, what’s the comparison to water absorption of native habitat on Mt. Sutro? That’s the relevant comparison.

    [If Mount Sutro were converted to native shrubs and grass, it would probably behave very like Twin Peaks does.]

    And as a key part in the profound fight against climate change? Even if your numbers are correct, its contribution this fight is incredibly minute. If we take care of the other thousand more things that must be done to fight climate change, and the problem still isn’t solved, then let’s talk about eucalyptus in SF.

    [Obviously, the eucalyptus of San Francisco can’t stop global warming. But we’re not comfortable with your argument – it applies to everything. Let’s not bother with saving a few acres of rainforest. Let’s not install energy-saving fixtures. Let’s not reduce our fuel consumption. Let’s not plant trees. There are a thousand other things to do… Once the trees are gone, it’s not like you can go Oops and get them back.]

    To be clear, I’m also influenced by romance. In my case, it’s a romantic attachment to lifeforms that thrived before humans messed with their very existence, in this case by introducing a species that failed to accomplish what its introducers said it would.

    [Where you’re coming from is understandable, and we’re in favor of preserving actual areas of native plants – like those above Laguna Honda reservoir, or on Mt Davidson’s east side. We’re not in favor of destroying existing thriving and functioning ecosystems like Sutro Forest. As for eucalyptus – in the city it was introduced primarily as a decorative tree. In fact, it’s been much more valuable than Adolph Sutro had envisaged.

  2. philran@comcast.net says:

    FYI – Please see the Bay Guardian 4/17-23, 2013 , page 10, Sh!T Happened: The Battle of Mt. Sutro – good article

    Phillip

  3. Barry Roth says:

    This article does not mention the metabolic trick of eucalypts or resorbing nitrogen and phosphorus from their leaves before leaf fall. This means that the leaf litter under eucalyptus is deficient in these nutrients, compared to the litter under forests with more mixed canopies. My personal observations (WAY short of a random quadrat sampling study, but still suggestive) find low molluscan diversity under eucs. Ariolimax (banana slugs) do occur, but the native snail genera Helminthoglypta, Vespericola, Trilobopsis, and Haplotrema are rare (three of them seen by me in one spot in the Presidio, and Vespericola not at all).

    [Webmaster: Apologies for the delay in approving/ responding, and thanks for stopping by. We weren’t aware of the “metabolic trick” so we went and researched it. It seems almost all trees do this to some extent, and it varies by soil conditions, and species. We couldn’t find evidence that eucs do more or less than other species, though we’re interested to know. (If we find more, we’ll edit to add.)

    We’re wondering if the understory vegetation – blackberry, acacia, ferns, grasses – of Sutro Forest wouldn’t provide good habitat; it’s not just eucs. But we haven’t (yet!) researched snails and don’t know whether this is the right habitat for them. Unfortunately, the ‘Biological Resources’ part of the Draft Environmental Impact Report is basically guess work based on two daytime visits, so it’s no help at all.]

    • Keith McAllister says:

      Several “random quadrat sampling” studies have been done. A very relevant local one, comparing eucalyptus forest to coast live oak forest in the East Bay, was done by Dov Sax. His conclusion:

      “Species richness was nearly identical for understory plants, leaf-litter invertebrates, amphibians and birds; only rodents had significantly fewer species in eucalypt sites. Species diversity patterns…were qualitatively identical to those for species richness, except for leaf-litter invertebrates, which were significantly more diverse in eucalypt sites during the spring.”

      The whole study is: Dov Sax, “Equal diversity in disparate species assemblages: a comparison of native and exotic woodlands in California,” Global Ecology and Biogeography, 11, 49-52, 2002.

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