Drought-Adapted Eucalyptus NOT Dying by the Thousand

This is a recent post from SutroForest.com, republished with permission and minor edits. We think it is important because the allegation that tens of thousands of eucalyptus trees are dying can be used as an excuse for forest destruction.

Jake Sigg, retired San Francisco Recreation and Parks Department (SFRPD) gardener who is considered the doyen of the Native Plant movement in San Francisco, has a widely circulated email newsletter. In it, he has been pushing the argument that thousands of eucalyptus trees in San Francisco are dying of drought, as evidenced by epicormic growth on these trees: “2015 is the year of decision, forced upon us by 20,000 to 30,000 dead trees.” He is suggesting they will be a fire hazard and that SFRPD act, presumably by cutting down the trees. In a recent post, he published a picture of a tree covered in young blue-green leaves, and predicted it would be dead within a year.

But he’s mistaken.

Eucalyptus trees are drought-adapted, and the shedding of mature leaves followed by sprouting of juvenile leaves (epicormic sprouting) is one of their defense mechanisms. These trees survive in areas far drier than San Francisco, where fog-drip provides an important source of summer moisture.

2015-05-27 ab eucalyptus with epicormic growth wordEUCALYPTUS RESPONSE TO DROUGHT

Eucalyptus trees are adapted to drought. They shed mature leaves and twigs so they don’t lose water through transpiration (the tree version of breathing, which takes place mainly in the leaves.) Later, they can replace the lost branches and leaves through “epicormic sprouting.”

Blue gum eucalyptus trees have buds buried deep under their bark. When the tree is stressed, they may shed adult leaves and later sprout new leaves along their branches. When you see a eucalyptus tree that seems to have shaggy light bluish-green new leaves along its branches or trunk – that’s epicormic sprouting.

Here’s what Jake Sigg said in a recent newsletter: “According to arborists, the trees produce these abnormal shoots from epicormic buds when their lives are seriously threatened. In this case, the tree is expected to be dead by the end of 2015. On Bayview Hill, barring heavy unseasonal rain, hundreds of the trees will be dead this year. Yet the City continues to not see a problem.”

We asked UC Berkeley Professor Emeritus Joe McBride and California’s leading expert on eucalyptus for his opinion. He’s observed this condition in trees along the edge of the Presidio forest and explains, “This response is common in blue gum as a mechanism to reduce transpiration rates in order to survive drought years.”

He continues: “I am not convinced that the trees will die in large numbers.

bayview-hill-2010 smTwo girdled trees

THE GIRDLED TREES OF BAYVIEW HILL

As an aside, we find it ironic that Mr Sigg should be so concerned with dead trees on Bayview Hill, given that’s where nativists girdled hundreds of healthy eucalyptus trees to kill them.

(This is done by cutting around the tree, thus starving it of nutrients that are carried only in the outer layers of the tree-trunk.) It’s clearly visible in the two photographs here, both taken on Bayview Hill.

EUCALYPTUS ADAPTS

Eucalyptus globulus thrives in Southern California, Spain, Portugal, India – all places hotter and drier than San Francisco. In fact, one of the reasons eucalyptus is so widely planted – including in climates both hotter and drier than in San Francisco – is that it adapts to a wide range of conditions. Here’s a quote from R.G. Florence’s textbook, Ecology and Silviculture of Eucalyptus Forests:

florence quote

From p.121 of the same book: “… they regulate their water usage in hot dry summers by closing their stomata [breathing pores in the leaves] during the day and lowering their rates of gaseous exchange. They adapt by their elastic cell structure to water stress.”

EPICORMIC SPROUTING IS IMPORTANT IN EUCALYPTUS

Mr Sigg describes “how to identify a dying blue gum” as follows: “Look for trees with thinning foliage and copious juvenile leaves (called coppice shoots) hugging the main stems. These coppice shoots are easy to see because of their blue color and tight clustering, as opposed to the adult leaves, which are 6-8 inches-long, dull-olive-colored and sickle-shaped and which hang from the ends of long branches. These coppice shoots are the give-away that the tree is in trouble and is destined to die soon…” (He later corrected “coppice shoots” to epicormic growth.)

But again, this is not actually true.

In fact, epicormic sprouting allows eucalyptus to survive not only drought, as described above, but even fire. The epicormic sprouting grows into new branches to replace the ones that have been damaged in the fire. This is from Wikipedia: “As one of their responses to frequent bushfires which would destroy most other plants, many Eucalypt trees found widely throughout Australia have extensive epicormic buds which sprout following a fire, allowing the vegetative regeneration of branches from their trunks.[4][5] These epicormic buds are highly protected, set deeper beneath the thick bark than in other tree species, allowing both the buds and vascular cambium to be insulated from the intense heat.[4]”

(The references are: [4] “Effects of fire on plants and animals: individual level”. Fire ecology and management in northern Australia. Tropical Savannas CRC & Bushfire CRC. 2010. Retrieved 27 December 2010. [5] “Learn about eucalypts”. EUCLID – Eucalypts of Australia. Centre for Plant Biodiversity Research. Retrieved 27 December 2010.)

And sometimes, dead branches and leaves and epicormic growth don’t even indicate stress – it’s part of the normal growth cycle. R.G. Florence’s book on eucalyptus says: the “mature crown of a eucalypt maintains itself by the continual production of new crown units, which die in turn. There will always be some dead branches in a healthy mature crown.” He goes on to say an “undue proportion of dead branches is an unhealthy sign” but a “reasonable proportion of death of crown units should be accepted as normal.” He also discusses the “epicormic shoots from dormant buds on the top and sides of the branch develop into leaf-bearing units of the mature crown.” (p.13) Eucalypts go through stages of development that include extensive self-thinning, particularly in younger trees. (p. 194)

Another reason for epicormic sprouts on eucalyptus is increased light. From Wikipedia, with references: “Epicormic buds lie beneath the bark, their growth suppressed by hormones from active shoots higher up the plant. Under certain conditions, they develop into active shoots, such as when damage occurs to higher parts of the plant or light levels are increased following removal of nearby plant. Epicormic buds and shoots occur in many woody species, but are absent from many others, such as most conifers.” [The Wikipedia article references the Encyclopedia Britannica.]

We have seen these epicormic sprouts in eucalyptus trees around the clubhouse in Glen Canyon after many trees were removed.

epicormic sprouts on eucalyptus when nearby trees removed

We also saw them on Mount Sutro near where 1,200 trees were removed for “fire safety.”

MISTAKING DEFENSES FOR DEATH THROES

In summary, then, epicormic sprouting does not indicate that the tree is near death. It may indicate that the tree is responding to drought (or even to other stresses like pesticide use or damage to its root systems) with defensive measures. It’s like declaring that everyone who has a fever is bound to die of it. The trees below are the same ones featured in the picture at the start of this article – one year later, they’re surviving, not dying.

Epicormic sprouting on eucapyptus 2014In some cases, epicormic sprouting may indicate nothing at all, except that the tree is going through a normal growth phase, or changed light conditions following removal of nearby trees.

LIVING WITH A FEW DEAD TREES

We asked Dr McBride if it made sense to cut down these trees. “I do not think the city would be justified in cutting trees down as a fire prevention action,” he says. “Cutting down drought-stressed trees at this point would be much more costly, sprouting would be difficult to control without herbicides, and the litter on the ground would have to be removed to decrease the fire hazard.”

“The problem as I see it is the accumulation of leaves, bark, and small branches on the ground. This material presents a serious fuel problem when it dries out sufficiently.” However, he points out that “In many eucalyptus stands in San Francisco the eucalyptus ground fuel (leaves, bark, and small branches) seldom dries to a point that it can be ignited because of summer fog and fog drip.” In dry areas, the best course is to “launch a program of ground fuel reduction by removing the litter from beneath eucalyptus stands.”

The eucalyptus-tree nest hole of the red-shafted flicker - San Francisco. Janet Kessler

Eucalyptus-tree nest hole of red-shafted flicker – San Francisco. Copyright Janet Kessler

A few trees may indeed die, with the drought or without it. If you think of a forest as a normal population, you expect to find some trees that are in thriving and some that are hanging on, and some that are dying – just like in any population. And dead and dying trees are very valuable to wildlife: They’re more likely to have cavities that are suitable for nesting (and are easier to excavate for woodpeckers and other cavity-building species). They also have bugs that come to feast on the decaying wood, and that’s bird-food.

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

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.)

mt-davidson-forest 1

 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 gold-mining and in medicine.

eucalyptus king of the forest smIn 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.

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.

ivy does not reach canopy of densely growing trees

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.

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

Young Great Horned Owls being raised in Eucalyptus tree 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.
  • Brown creeper forages on eucalyptusRobert 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?

EEucs 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 -2010

In 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.

Here are his Powerpoint presentation in ppt , pptx, and  PDF formats – and on Youtube with audio notes taken by audience members and including the Q & A.

PPT: McBride Presentation – Eucalyptus

PPTX: McBride Presentation – Eucalyptus

PDF: McBride Presentation – Eucalyptus

Here’s a link to Youtube which has the full Powerpoint presentation and audio notes of both the lecture and the Q&A afterward:

 

 

Why UC Berkeley’s Prof. McBride Wants to Save Mt Davidson

Mt Davidson-map 2

Click on map for larger version – 1600 trees will be cut

We’re very concerned about the SF Rec & Park plans, under the Natural Areas Program, to fell 1600 trees on Mount Davidson.

We were pleased to learn of a letter Professor Joe McBride wrote to Phil Ginsburg, General Manager of SF Rec & Parks after assessing the situation on Mount Davidson. He presented a strong case for leaving the forest alone.

(Professor McBride is Professor of Environmental Science in the College of Natural Resources at University of California, Berkeley. He is the author of many studies of urban forests, several of which he cited in his letter.  He is particularly expert on the failure of trees caused by extreme wind conditions.)

Mt Davidson

WHAT PLAN?

Mount Davidson falls under the Natural Areas Program (NAP) of the SF Rec & Parks Department. Some years ago, NAP issued its Significant Natural Resource Area Management Plan (SNRAMP, “Sin-Ramp”).  The NAP – and the Sin-Ramp – cover 32 parks. In total, the Sin-Ramp looks to cut down 18,500 trees; close ten miles of trails,  restrict access and remove up to 80% of dog-play areas, and use growing amounts of toxic pesticides.

The Sin-Ramp is currently on hold.  A Draft Environmental Impact Report (DEIR)  has been prepared. The City’s planning department, which is managing the process, received a large number of comments on the DEIR, and they are employing a consultant to respond to them. Once they respond, the DEIR will need to be certified, and then implementation of the Sin-Ramp is possible. However, some work is already being done as parts of other projects such as trails or construction.

Mt Davidson stands to lose more trees than any other NAP park (though UCSF’s Mt Sutro, also a dense historic eucalyptus forest, is threatened with even more tree removal).

TEN KEY POINTS

Stable Boundary between Forest and Grassland

Stable Boundary between Forest and Grassland

Professor McBride’s assessment differed sharply from what we’ve been hearing from supporters of the tree-cutting.  Those talked about invasive and overcrowded trees strangled with ivy, a lack of biodiversity, and imminent forest destruction. By contrast, here are the main points Professor McBride made.

1) Historic importance and Visual Value. The eucalyptus forest on Mount Davidson  was planted under the direction of Adolph Sutro, philanthropist and former Mayor of San Francisco.  The hilltops covered in eucalyptus trees and Monterey cypress are a distinctive feature of San Francisco’s landscape. They’re been there for a hundred years and are an important historical heritage.

2) Eucalyptus is not invasive. The Plan frequently refers to these trees as “invasive.” Prof. McBride’s studies indicate that eucalyptus does not invade adjacent grasslands; and this is also obviously true on Mt Davidson, where a stable boundary exists between the forested and unforested areas.

3) Eucalyptus groves are biodiverse. Eucalyptus groves are richer habitats for vertebrates than either redwood or Monterey cypress/pine forest; and are similar to dry chaparral and grasslands.

Three of the Four on Mt Davidson

Three of the Four on Mt Davidson

4) More Pesticides. Removing the number of trees shown in the Plan will expose the ground to more light than existing understory plants can tolerate. In the disturbed ground and increase light conditions, existing exotic species will proliferate and will have to be controlled by using even more pesticides.

5) Increased wind-throw and breakage of remaining trees.  Removing trees in this windy area will affect the trees that remain, which are not wind-hardened. More trees will go down.

6) Reducing a wind-break. This is a very windy part of the city, with winds blowing in straight from the ocean. Walking recreationally on Mt Davidson will be a less pleasant experience.

7) Reduction in habitat. The Plan’s assumption that birds will quickly adjust to removal of 1600 trees is unfounded. Many birds return to the same nesting site each year. Cutting down large numbers of trees displaces these birds, and also causes a great deal of disturbance. Bird protection plans usually call for a 300-foot radius of protected area around a nest.

Girdled tree Mount Davidson8) The forest is healthy.  The dead trees in the forest have been girdled by someone/s with a vendetta against eucalyptus; few trees – if any – have died naturally.

9) Ivy is not a problem. English and Algerian ivy climbs up the trees, but cannot smother the trees by growing into the canopy. The only snags covered in ivy were those that had been girdled.

10) Regeneration is a 22nd Century issue. It’s been argued that the understory of  ivy, Cape ivy, and Himalayan blackberry may restrict the establishment of eucalyptus seedlings. If so – and it’s possible – this is a problem for the next century. The forest, though 100 years old, is comparatively young.  This could be revisited in another 100 years or so. Meanwhile, the understory provides an excellent food source and cover for wildlife.

CONCLUSION

Here’s the conclusion to Prof McBride’s letter:

I conclude that the Significant Natural Resource Areas Management Plan for the removal and thinning of different portions of the eucalyptus plantation on Mt. Davidson is not justified.  The plantation serves an important role in the history and visual characteristics of the city.  Trees and the existing understory provide habitat for wildlife and wind protection for walkers.  The justifications for the management prescriptions have not been properly developed.  Furthermore, the cost of removal of the trees seems unjustified in view of other priorities in the San Francisco budget.

Enchanted Cloud Forest

Enchanted Cloud Forest