Understanding Eucalyptus in the Bay Area – Dr Joe R. McBride

This article has been republished from the  website of the San Francisco Forest Alliance, with permission. (Edited to add: A somewhat different selection of illustrations has been used.) Dr. Joe McBride of UC Berkeley spoke at the Commonwealth Club in April 2014 as part of the series “The Science of Conservation and Biodiversity in the 21st Century.” His main message:

  • Eucalyptus groves in California provide habitat for as many native species as do most ‘native’ habitats.
  • They grow well at high densities and an average spacing of 8 feet between trees is quite typical.
  • They have relatively high fuel loads, but the cool and damp dense eucalyptus forests reduce the risk of fire.
  • Eucalyptus is subject to few diseases or pests, and parasitic wasps provide pest control.
  • It provides a host of ecosystem services including carbon sequestration, pollution reduction, slope stabilization, windbreaks, wildlife habitat, and recreational value.

Dr. Joe R. McBride was Professor of Landscape Architecture and Environmental Planning, Department of Environmental Science, Policy, and Management, UC Berkeley. (He has since retired.) Read on for notes from Dr. McBride’s talk. (There are also links to his Powerpoint presentation.)

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sutro forest with dappled sunlight

THE HISTORY, ECOLOGY AND FUTURE OF EUCALYPTUS PLANTATIONS IN THE BAY AREA

Notes From a Talk By Dr. Joe R. McBride

Dr McBride’s wide-ranging talk covered a lot of ground. He talked about the ecology of the eucalyptus forest in the Bay Area: its structure, the variety of plants and animals that live within it, its health and the ecological functions it performs; the dynamics within these forest stands; and their invasive potential.

WHY EUCALYPTUS CAME TO CALIFORNIA

Eucalyptus was first planted in California during the Gold Rush, possibly for oil to use in eucalyptus king of the forest smgold-mining and in medicine. In the 1870s, eucalyptus planting was encouraged for many objectives: to beautify cities, to improve farmland, as windbreaks, and to dry out swamps to combat malaria. It was grown in woodlots for firewood, but as people switched to natural gas and other fossil fuels this became rare. Later, it was planted for timber – which didn’t work out because the trees were harvested too young; and still later for bio-fuel, which did not become commercially attractive.

By the 1950s, it had become an integral part of the California landscape. Six species were planted, primarily blue gum in Northern California, and red gum and river gum in Southern California. (Worldwide, there are perhaps 640 species.) Eucalyptus beautifies our cities, and helps stabilize soil on steep hills. The surface area of the leaves, broader than those of conifers, help trap particulate pollution. Unlike deciduous trees, the evergreen foliage of eucalyptus removes pollutants all year long.

HOW DENSE MAKES SENSE?

The density of eucalyptus plantations in Bay Area ranges from 150-160 trees per acre to about 1700 trees per acre. mt davidson understory(The highest density resulted from a freeze in 1970s: trees were cut down because of the perceived fire hazard, but the trees presumed dead later resprouted.) On Angel Island, the normal density was 8ft spacing (about 680 trees per acre) but it ranged to 30 feet between trees. In the East Bay, 8 ft. x 8 ft. is quite typical eucalyptus plantation density. Left to grow naturally, stands become denser through in-growth, mainly by sprouting and also by sibling establishment.

Is management by thinning necessary for the health of the forest, someone asked, and what density is ideal? Dr McBride had seen no examples of stands that could be improved by thinning. Eucalyptus grows well with a high density at an average of 8 ft x 8ft spacing between trees. In Australia and New Zealand no one thins; they just harvest the trees and let them regrow from sprouts. In the US, eucalyptus was not marketable, so there’s no history of managing eucalyptus plantations. Also, until recently there were no diseases or insects. The long-horned borer and the psyllid have now appeared in some places, and thinning is not seen as a solution to these insect problems; they are better controlled by certain predatory wasps. Logging eucalyptus would mean a lot of ground disturbance and erosion. If the logs are removed, the skid trails can destabilize the soil.

MUCH GROWS UNDER EUCALYPTUS – NATIVE AND NOT

Contrary to popular belief, eucalyptus forests have as many species (or more!) growing in their understory as do oak woodlands. A 1990 survey in Tilden Park found 38 species in the understory of eucalyptus forests (24 native plants; 14 introduced plants), while the oak woodland had 18 species in understory (14 native plants; 5 introduced plants). Only the riparian woodlands in Murray Park are somewhat richer in species than riparian eucalyptus forest (58 species vs 34).  In East Bay eucalyptus forests, California Bay, Coast live oak, poison oak, bedstraw, California blackberry, and chickweed were ubiquitous. The amount of light reaching the ground influences which species can be found in the understory.ferns and blackberry and poison oak What about allelopathy? Under experimental conditions, eucalyptus litter inhibited germination and growth of cucumber seeds, so eucalyptus litter may be somewhat allelopathic to some plants. But a study from UC Santa Barbara indicates that if eucalyptus litter is removed, within 2 years there’s no inhibitory effect on other plants germinating. And clearly, it isn’t allelopathic to all the species mentioned earlier.

Someone asked whether it would be advisable to “manage eucalyptus stands that have an invasive understory.” Dr McBride responded: “I have no prejudice against invasive plants. I am an invasive Californian myself.” (There was amused applause.) He continued that each eucalyptus grove is different, so it’s important to look at it on a stand-by-stand basis and measure the fire hazard of eucalyptus plantations against the value of each stand for wildlife habitation, recreation, and wind break functions. eucalyptus trees in Sutro Forest In response to a question about whether ivy kills eucalyptus trees, Dr McBride said he has not seen evidence the ivy shades the foliage of eucalyptus trees. He’s seen no evidence of ivy killing eucalyptus, although on Mt. Davidson, he did see ivy growing over trees that had been killed by girdling with an axe or chainsaw.

INSIDE A EUCALYPTUS FOREST

Shading and leaf litter changes the microclimate of a eucalyptus grove. As you move in from the edge to the interior of the forest, conditions change. The species change from the edge to the interior of the forest as the amount of light decreases, so there are different species at the edge of the forest and inside it. A 1980s study in the Presidio compared conditions outside a eucalyptus forest and inside it. It showed:

  • Temperature moderation: Daytime temperature fell an average of 10%, and night-time temperature rose an average of 5%
  • Windbreak: Wind velocity dropped 40%
  • Relative humidity was 5% higher (from the edge to the interior).
  • Shade: Light intensity was 90% lower.
  • Moisture: Precipitation (rain) decreased 12%; but fog-drip (i.e., moisture precipitated from the fog) increased 300%

EUCALYPTUS STORES CARBON

Eucalyptus increases the carbon content in the soil compared to grasslands (Zinke et al, 1988). Its fast growth and large size means it sequesters a lot of carbon in its trunk and root systems.

EUCALYPUS SUPPORTS WILDLIFE

Owlets in an eucalyptus tree nest

Owlets in an eucalyptus tree nest

Again, contrary to belief, eucalyptus provides a good environment for a wide variety of wildlife. A number of studies demonstrate this.

  • A 1970 study showed many birds make “moderate use” of eucs as habitat and a few birds make “great use” of eucs. (Almost all these species are native.) Birds that make most use: mourning doves; Great Horned Owls, whose range has been extended by CA eucalyptus; Stellar’s jays; yellow-bellied sapsuckers; Allen’s hummingbirds; olive-sided flycatchers; brown creepers; dark-eyed juncos; Audubon warblers.
  • Some reptiles make great use of eucalyptus groves: Southern Alligator lizard and the slender salamander. Among mammals, deer mice make “heavy” use of eucalyptus.
  • photo credit: Janet Kessler

    photo credit: Janet Kessler

    Robert Stebbins’ monumental 1978 study on the attractiveness of eucalyptus for habitat in the East Bay found that all species making use of eucalyptus for habitat found eucalyptus about the same as grasslands in attractiveness, but oak/bay woodlands were even more attractive.

  • Monarch butterflies most commonly use eucalyptus trees in state parks. But some of the insects in eucalyptus hurt the trees. One is the eucalyptus long horned borer – but can be controlled by a parasitic wasp. The red gum lerp psyllid is more of a problem in Southern California, which has more red gum. However, it’s part of the food chain: woodpeckers and other bird species feed on their larvae.
  • A study showed that eucalyptus in a riverside environment doesn’t impact species diversity of stream insects or pollution tolerance compared with native riparian environments.

NATURAL SUCCESSION IN EUCALYPTUS?

Over the next 200-300 years, the eucalyptus forests in the East Bay could gradually – and naturally – shift to oak-bay woodlands. In the East Bay (though not at Mt. Davidson or Mt. Sutro), the eucalyptus plantations have California Coast live oaks and California bay trees in the understory, and they are doing well. The live oaks are “tolerant” of shade and the bays are “very tolerant” of shade. If they aren’t disturbed, the oaks and bays regenerate well in the understory, and being even longer-lived than the eucalyptus trees, they will eventually naturally succeed the eucalyptus. The bay tree is higher in regeneration than the Coast live oak in Tilden Park (McBride, 1990).

WHAT ABOUT FIRE HAZARD? fog in mt sutro cloud forest sept 2013

Eucs support considerable fuel load on the ground because of rapid decay of foliage and shredding of its bark. They have a higher fuel load than California bays or Coast live oaks. They release an aromatic compound that can ignite with sparks, and they burn hot. However, while the tree density of eucalyptus plantations can mean a greater accumulation of fuel in the understory, the higher density means a cooler, wetter understory that might not dry out as fast. Three risk factors in fire risks of any tree: amount of fuel it produces; tissue moisture content; fuel ladder based on presence of other plants in its understory.

IS EUCALYPTUS INVASIVE?

Under certain circumstances, eucalyptus can spread – for instance, on Angel Island, some stands spread through road cuts and prescribed burns (which destroyed competing vegetation). However, in most cases they don’t: aerial photographs show that boundaries are stable. The eucalyptus forests on Mount Davidson and in Tilden Park show stable boundaries. mt D comparison 1927 -2010In the Bay Area, Dr McBride found eucalyptus forest area declined between 1939 and 1997. The natural spread hasn’t increased the area of eucalyptus groves.

DO TREES DEPLETE AQUIFERS?

Someone mentioned attending a talk where the speakers said that tree removal would help to replenish aquifers. Was that true? Dr McBride thought it very unlikely; most aquifers are much deeper than tree roots.

WHAT ABOUT PESTICIDES?

Someone speaking for people with disabilities owing to chemicals said herbicide use in these areas violated their right to access, and wondered how “environmental” organizations – like the Sierra Club – could support this. Dr McBride sympathized, said he was also concerned about toxic herbicide use. He mentioned that the East Bay tree-felling project is on hold owing to a number of unanswered questions that would need further research.

McBride Presentation in ppt format: mcbride-presentation-eucalyptus

McBride Presentation in pptx format: mcbride-presentation-eucalyptus (1)

McBride Presentation as PDF :McBride Presentation – Eucalyptus

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6 Responses to Understanding Eucalyptus in the Bay Area – Dr Joe R. McBride

  1. Richard Croois. says:

    Why in hell would anyone want to destroy this eco system. The benefits are so great for we as humans and all of our animal friends.

  2. Pingback: Cal-IPC Eucalyptus Reassessment: Same Result (for Different Reasons) | Save Mount Sutro Forest

  3. Pingback: Understanding the eucalyptus forest – Professor Joe R. McBride | Death of a Million Trees

  4. miss415 says:

    As someone who hikes daily in places like Stern Grove, Sutro forest & Mt Davidson, and also took several courses in Horticulture, I am stunned by the misinformation and extremist view stated here.
    [Webmaster: Thanks for stopping by to comment, Miss415. Considering that Dr McBride is perhaps the leading authority on eucalyptus in the Bay Area (at least) with years of experience, we’re surprised that you regard his talk as misinformation. Please read it in detail?]
    It baffles me how someone could say that Eucalyptus stabilizes the slopes. These trees have shallow roots and become a dangerous threat when after long periods of drought we have a heavy storm. At least 5 trees fell in 1 day at Stern Grove after the recent storm in Jan 2015 and that does not include the hundreds of tree limbs that snapped off from high winds.
    [Webmaster: Eucalyptus does not necessarily have shallow roots; its root system varies depending on where it is. The ones in Stern Grove may well have been destabilized by the tree-cutting that has been happening there in the last few year. This exposes trees that are not wind-hardened to wind throw, and knocks them down.]
    And your explanation about fire hazard is completely false- Eucalyptus consume & store the moisture leaving the forest floor dry. Responsibly managing a forest that poses a dangerous threat to human safety is not destroying an eco system as you want people to believe.
    [Webmaster: Sorry, this is just not true. Sutro Forest is a damp environment nearly year-round. The forest floor is dry for very short periods of time. It’s a de facto cloud forest, harvesting moisture from the fog. https://sutroforest.com/2013/09/10/mt-sutro-cloud-forest-evening-puddles-not-fire/]
    Throughout all 3 of the sites I mentioned Eucalyptus & invasive ivy dominate the landscape. Many of the Eucs are close to 100 years old and very likely approaching the end of their life span.
    [Webmaster: Many of the eucs are over 100 years old, but the life-span is 300-500 years. These trees are young relative to their total life-span. https://sutroforest.com/eucalyptus-myths/ Oh, and ivy is also fire-resistant – more so than “native” shrubs and grasses.]
    Doing nothing is such an extreme and irresponsible approach and if that is your preference, we will begin to see not just Eucs but Monterey Cypress & Monterey Pine coming down on their own at alarming rates.
    [Webmaster: There is indeed an issue with (some) Monterey pines (not Monterey Cypress or eucalyptus). They have a shorter life-span, and some of the trees are infected with a canker. We don’t oppose removing hazardous trees. We oppose making trees more hazardous by drying out the forests, and exposing them to windthrow.]
    I love the trees and the habitat they provide for all the birds and other creatures- but we need to find a common ground that protects public safety AND minimizes impact on wildlife by restricting pruning & removal during breeding & nesting and surveying trees prior to removal- before a catastophic event.
    [Webmaster: We share common ground there. We too would like to see hazardous tree work focus directly on public safety, and include pruning rather than removal – and not during the nesting season.]

    • milliontrees says:

      Native plant advocates would be wise to get together and agree on one consistent story to support their demand to destroy non-native trees. Some of them say the roots of eucalyptus are shallow and others say they are very deep. When they want to claim that eucalypts are hazardous, they say the roots are shallow. When they want to claim that eucalyptus is draining the water table during the drought, they say the roots are very deep. They also claim that the roots of eucalypts are so strong that they are breaking apart the rock outcrops on Mount Sutro and Mount Davidson. Such contradictions are amusing to those who don’t love to hate eucalyptus, but they aren’t helping native plant advocates make their case.

      Here’s what the California Invasive Plant Council said about the roots of blue gum eucalyptus in support of their draft assessment of “ecological damage” caused by eucalypts: “Eucalyptus globulus is adept at tapping into deep groundwater, even under drought conditions (DiTomaso & Healy 2007), altering water availability to depths of 45 feet and distances of 100 feet from the trunk.” The US Forest Service tree database suggests that this is an exaggeration, but we offer it as an example of how native plant advocates would be wise to agree on some consistent strategy.

      Likewise, native plant advocates claim there are no insects in the eucalyptus forest when they want to support their claim that there are no birds in the eucalyptus forest. When they want to claim that the eucalyptus forest is diseased and dying, they claim that the forest is infested with insects.

      There are many other examples of the muddled message of those who demand the destruction of the eucalyptus forest.

      Many trees fell in San Francisco during the last wind storm. If you watch or read the news, you saw photos of several ficus street trees that fell on cars. You also saw a few pines and oaks that fell around the Bay Area. One of the ways in which you can reassure yourself that the wind is an equal opportunity feller of trees is to visit the “California Tree Failure Report Program” available on the website of UC Berkeley: http://ucanr.edu/sites/treefail/CTFRP_Statistics/ You will learn that the tree species that falls over the most often in California is oak. It falls over roughly twice as often as eucalyptus trees. Pines also fall over more frequently than eucalypts.

      Stern Grove is a special case. Hundreds of trees have been destroyed there in the past few years. Many were hazardous, but some were not. They were removed to bring more light to the native plants that have been repeatedly planted there. When you remove trees, those that remain are more vulnerable to wind throw. Ironically, if you are concerned about your safety in Stern Grove, you should be against removing any more trees than are absolutely necessary. Also keep in mind that the last “tree death” in Stern Grove was by a huge limb of a redwood tree in 2008.

      It’s also amusing to witness the presumption of someone whose credentials are daily walks in the park and taking a few horticultural classes questioning the expertise of a professional academic forester who taught at UC Berkeley for 40+ years. You might choose your target a bit more carefully if you wish to establish your credibility.

    • Keith McAllister says:

      Dr. Joe McBride is a distinguished professor at UC Berkeley who has studied, and published on, eucalyptus forest in the Bay Area for 30+ years. He has consulted on eucalyptus forest for the National Park Service, the East Bay Regional Park District, the California State Parks system, and the Presidio, among others. When discussing eucalyptus, the word “expert” really does apply to Dr. McBride. To miss415, not so much.

      Factual errors in miss415’s comments are dealt with by the webmaster; I won’t repeat that material. Just one more item: When UC, SFRPD, EBRPD, and others promise to destroy their trees outside of nesting and breeding season, they miss the point. When habitat is destroyed, it’s destroyed, gone. The birds can’t return for the following nesting season, nor the one after that, ad infinitum. (This leaves aside the fact that they sometimes don’t abide by that promise, and simply cut the trees down during nesting season.)

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