Creating Tree Graveyards in San Francisco

At 13.7% of tree canopy coverage, San Francisco has one of the smallest tree canopies of any major city in the country.  When San Francisco’s Urban Forestry Council (UFC) announced its goal of planting 30,000 new street trees in the next 20 years, it seemed a modest goal.  Yet, Jake Sigg, the leader of native plant advocates in San Francisco, immediately objected to even this modest goal in his Nature News.  He announced the meeting of the UFC to consider the proposal and pronounced it a bad idea:

“JS:  Let’s start taking climate change seriously.  There is a prejudice—it is nothing more than that—that trees sequester more carbon than other life forms.  That is a simplistic view that, when looked at more closely, is found wanting.  To counter climate change we need to remove carbon from the air and put it where it will be for a millennium or more.  Removing it for a few decades or a century is pointless. 

“There are many reasons to plant trees on San Francisco streets, and many of our streets need them.  Climate change is not a stand-alone phenomenon; it is intimately related to diversity of biological elements.  That argues for planting native plants to invite dispossessed wildlife back into the city and you do that by planting the plants they need.  There are trees, shrubs, and perennials that ought to line our street to function in this way.  Carbon removal should not be a factor in our street plantings—biodiversity should be Number 1.”

Jake Sigg, Nature News, July 2, 2022

Yes, Jake, biodiversity is important because a diverse ecosystem is more resilient in a changing climate, but destroying all non-native plants does not make an ecosystem more diverse.  Climate change is the greatest long term threat to biodiversity, which makes addressing climate change a prerequisite to preserving biodiversity. 

I attended the Urban Forestry Council meeting of July 5, 2022, when this proposal was considered.  I was expecting to hear objections from Jake Sigg’s followers. Instead, the handful of written public comments objected to the meager commitment to plant only 30,000 new trees in San Francisco in the next 40 years. I learned more about the plan to plant more street trees in San Francisco:

  • There are presently an estimated 125,000 street trees in San Francisco.
  • Because the mortality of street trees is high, the expectation is that 50,000 street trees would need to be planted in the next 20 years to replace dead street trees.
  • According to the Urban Forestry Council it costs $1,500 to plant a tree and an additional $2,500 to water it for three years until it is established.
  • 4,000 trees would need to be planted every year to keep pace with expected tree mortality and to add 30,000 more street trees. 

These goals exist only on paper.  Between 1,500 and 2,000 trees per year are being planted in the city and no funding has been identified to increase this number.  After delivering this bad news about the sorry state of San Francisco’s urban forest, one member of the UFC spoke some much needed common sense.  Nicholas Crawford said we should “hold onto shabby trees” that are established and storing carbon.  He suggested that San Francisco should not remove trees that are at least stable because there are no trees to replace them. 

Existing trees in our urban forest are more valuable than ever.  They are storing more carbon than a replacement tree will store for at least 20 years.  They don’t need to be irrigated because they have the root and fungal networks needed to supply the tree with the moisture it needs.  Existing trees have proven themselves.  The fact that they are alive and well after 10 years of extreme drought proves they are adapted to current climate conditions.  So why destroy them? 

Jake Sigg acknowledged the value of forests to address the challenges of climate change in a recent newsletter:  “In order to have an impact on climate we need to stop deforestation and preserve, strengthen, and restore what is already here.” (Nature News, July 6, 2022)  But that principle does not apply to San Francisco for Sigg and his followers because the trees of San Francisco are predominantly non-native and they place a higher value on restoring pre-settlement treeless grassland and coastal scrub.  Because of the power and influence of the native plant movement in San Francisco our urban forest is being destroyed and planting trees is resisted.

San Francisco has made a commitment to destroying more than 18,000 non-native trees in San Francisco’s public parks.  The stated goal of that program is a landscape of native grassland and scrub.  UC San Francisco has also made a commitment to destroy most of the non-native forest on Mount Sutro.  Thousands of trees have been destroyed on Mount Sutro and more will be destroyed in the future.  The Executive Director of Sutro Stewards, the non-profit organization that is implementing the plans for destruction of the non-native forest on Mount Sutro is represented on the Urban Forestry Council, an odd choice for a citizen’s advisory council theoretically committed to the urban forest.

Tree destruction on Mount Sutro, January 2021.  Courtesy San Francisco Forest Alliance

McLaren Park:  A Case Study

Today Conservation Sense and Nonsense will visit a relatively new project in McLaren Park that has destroyed non-native trees in order to create a small native plant garden.  We drill down into the project to understand why San Francisco’s urban forest is being destroyed.  We visit this project because it is an example of many similar projects that are planned in San Francisco. 

This is one of many attempts to plant native plant gardens on Sunset Blvd in San Francisco. The functional windbreak of Monterey cypress is dying of old age. Rather than replace the windbreak, native shrubs are being planted on Sunset Blvd that will not function as a windbreak in the windiest district in San Francisco. The lack of maintenance that you see here is typical of these gardens, which makes them unpopular with neighbors.

At 312 acres, McLaren Park is one of the largest parks in San Francisco.  Fifty-three percent (165 acres) of McLaren Park is designated as a “natural area,” which means that a commitment was made nearly 25 years ago to transform it into a native plant garden.  The new native plant garden that we visit today is not actually inside one of the designated “natural areas.”  The reach of the native plant movement in San Francisco extends far beyond the 1,100 park acres of “natural areas” that were claimed in 1998. 

The new native plant garden is located in the southeast corner of McLaren, south of the community garden at the intersection of Visitation Ave and Hahn St.  This is a photo of some of the trees that were destroyed to create the native garden:

©Lance Mellon with permission.  July 2020

And this is a photo taken in December 2021, after the trees deemed “non-native” were destroyed:

© Lance Mellon with permission

The plans for the native plant garden say that 18 non-native trees would be destroyed and 6 native trees would be retained.  The plan claims that tree removals of all non-native trees were based on “professional assessments.”  Such “assessments” are routinely used by the Recreation and Park Department to justify the removal of non-native trees.  Photos of the trees indicate otherwise.  Retention of only native trees suggests that assessments aren’t even-handed.  The claim does not pass the smell test. 

Plans for the native plant garden indicate that more native trees will be planted:

The trees will need to be irrigated for at least 3 years to establish their root systems and ensure their survival.  The entire garden will need to be irrigated if it is to survive.  Let’s be clear:  an established grove of trees with an understory of annual grasses that did not require irrigation or maintenance was destroyed and replaced with new plants and trees that will require irrigation.  Is that a suitable use of scarce water resources during an extreme drought that is expected to get worse, if not be a permanent change in the climate?  That is the question we consider today.

About 9 months later, the “native plant garden” looks more like a tree graveyard:

McLaren Native Plant Garden, July 2022
Some of the newly planted trees are holly leaf cherry. Signs on the trees indicate that the project was paid for with a CAL FIRE grant. One wonders how a garden full of dead wood is less flammable than a garden full of living trees.

Granted, the native plant garden is likely to look better as plants grow.  However, it will only look better if it is irrigated and taken care of.  Why should we expect it to be taken better care of than the existing garden that required no maintenance?  Wishful thinking will not make it so.

The death grip of nativism

Climate change is the environmental issue of our time.  We are seemingly incapable of doing anything substantive to address climate change.  Political gridlock prevents us from controlling the greenhouse gas emissions causing climate change.  The Supreme Court recently ruled that the Environmental Protection Agency does not have the authority to regulate polluting emissions from power plants. 

We focus on the preservation of our forests because it is the only tool we have left to absorb carbon emissions from the fossil fuels to which we are wedded.  Native plant advocates have taken that tool away from us.  Our urban forests are being destroyed and replaced with grassland and scrub.  Claims that grassland and scrub store more carbon than forests are ridiculous.  Those claims earn native plant advocates the label of climate change deniers.  As the drought continues to plague California, established landscapes that required no water are being destroyed and replaced with native plants that require irrigation. 

The need for diverse urban forest and the obstacles to achieve that goal

Matt Ritter is a professor of biology at Cal Poly San Luis Obispo and Director of Cal Poly Plant Conservatory.  He is the author of several books about California’s unique flora, including A Californian’s Guide to the Trees Among Us.  He is considered an expert on the horticulture, ecology and taxonomy of the Eucalyptus genus.

Click on picture to view Professor Ritter’s presentation

In October 2021, Professor Ritter gave a presentation to the California Urban Forests Council, entitled “Underutilized Species for the Future of Urban Wood and Urban Forestry.”   He began by explaining why it is important to identify new tree species for our urban forest.

  • “Baja is moving to Oregon,” said Ritter to set the stage.  Within 50-80 years trees living in California now will no longer be adapted to the anticipated warmer, drier climate.  Trees killed by wildfire in California are not returning.  Forests are quickly converting to grassland and shrub.  As of 2018, California had lost 180 million trees to drought, disease, bark beetles, heat, and wildfire, which is nearly 5% of the total tree population in California.  Adding subsequent years to date, we have probably lost 7% of all of our trees.
  • Trees in urban areas will help Californians cope with warmer conditions because they cool our cities and reduce energy consumption.  Fewer trees will mean a lower quality of life, for us and for birds.  The loss of our trees reduces carbon storage, which contributes to more climate change. 

Ritter then explained why we must diversify tree species in our urban forests.

  • There are over 60,000 tree species in the world and only 7% of tree species are found in urban areas around the world.  In California our urban forests are even less diverse.  There are only 234 tree species on average in California’s urban forests.  The average number of approved tree species for planting in California’s municipalities is only 49 and few species on those approved lists are native to California.
  • Diversity of tree species ensures greater resiliency that enables our urban forests to survive changing conditions.
  • Only 9% of tree species in California’s urban forests are native. 
The native ranges of tree species in California’s urban forest.

An inventory of Oakland’s urban forest (street trees, medians, and landscaped parks only) was recently completed.  With 535 tree species, the diversity of Oakland’s urban forest is greater than average for California.  With 14% native trees, Oakland’s urban forest is more native than average. There are 59 species on Oakland’s list of approved trees, of which only 4 are native to Oakland.  The most significant finding of Oakland’s tree inventory is that our urban forest is only 64% “stocked,” meaning that of existing tree wells, only 64% are currently planted with trees.  When trees die in Oakland, they aren’t being replaced.  I don’t doubt there is a will to plant trees in Oakland.  I assume it is a question of means in a city with more pressing needs than resources.

Ritter and his colleagues at Cal Poly have created a website called SelecTree to help Californians choose the right tree for the right site and conditions.  There are 1,500 tree species described on SelecTree, using 60 characteristics, such as drought tolerance.  SelecTree rates blue gum eucalyptus “medium” for drought tolerance, the same rating as native coast live oak and bay laurel.  Ritter clarified that drought tolerance on SelecTree is a measure of how much water the tree species uses.  Claims that eucalyptus uses more water than native trees is bogus, like most bad raps about eucalyptus.  

Ritter recommended specific tree species, based on their drought and heat tolerance.  He said that when diversifying our urban forests “we have to think about Australia” because it is the hottest, driest, flattest, and oldest place on the planet, which is another way of saying that tree species in Australia have survived terrible conditions that are comparable to the challenging conditions in urban environments.

Ritter recommended oak species that are native to Texas; eucalyptus and closely related tree species; and several tree species in the legume family, especially acacia.  In each case he mentioned the suitability of tree species based partly on the quality of its wood.  Apparently, I’m not the only person in California who is disturbed by huge piles of wood chips wherever trees have been destroyed.  Ritter also thinks we should be thinking about how we can use wood when trees are destroyed, rather than building potential bonfires.  

Obstacles to diverse urban forests in California

When Professor Ritter took questions from the audience, we learned that the main obstacle to a diverse urban forest in California, adapted to our climate conditions, is the myopic focus of native plant advocates:

Question:  “Are we introducing new pathogens to our natives by importing new species?”

Answer:  There are many laws and rules that restrict the importation of plants to prevent that from happening.  We also import only the seeds of plants, not grown plants.  The seeds are sterilized and they don’t carry the pathogens that may exist on grown plants in their native ranges.

Question:  “Do we know how quickly birds and insects adapt to new species?”

Answer:  “No we don’t, but who cares?  We are facing a climate emergency.   We have 50 years before life in our cities becomes hell.  We have a responsibility to protect the quality of life in our cities.  We should stop developing the wild, but cities are different.” 

Ritter anticipated a question that is often a concern of native plant advocates by saying we should not be concerned about “weediness,” AKA “invasiveness.”  He said, “That should be far down on our list of priorities of what to worry about.  We need to be primarily concerned about what tree species will grow in our changed climate.”

Rhetorical Question:  “But insects need native plants!

Answer:  Ritter instantly recognized the mantra of Doug Tallamy.  He replied that it is not well established that there are more insects living on native plants than on introduced plants.  He mentioned a single study that inventoried plant and animal species in eucalyptus compared to oak forests, presumably Dov Sax’s study which concluded:  “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.” 

Rhetorical Question:  “We are still dealing with a legacy of blue gum eucalyptus in the Bay Area.  Why should we repeat that mistake?”

Answer:  Ritter agreed that blue gum eucalyptus is “inappropriate” in many places where it was planted in the Bay Area, but we’re not planting blue gums.  There are 800 eucalyptus species and many are ideal for our conditions.  He said, “Why not plant eucalyptus?  It would be dumb not to plant suitable eucalyptus species just because it shares a name.”

Ritter added that, “Planting only natives just doesn’t work in San Francisco.  There would be no trees in Southern California because we don’t have very many native trees in California.”  The pre-settlement coast of California was virtually treeless in most places and that’s a fact. For example, a study of historic vegetation in Oakland, California reported that only 2% of pre-settlement Oakland was forested with trees. “Vegetation before urbanization in Oakland was dominated by grass, shrub, and marshlands that occupied approximately 98% of the area.” (1)

San Francisco in 1806 as depicted by artist with von Langsdoff expedition. Bancroft Library

Oakland as a case in point

The San Francisco Chronicle recently published an article about a guerilla tree-planter in Oakland who is planting native oak trees on public land, wherever he wants.  Oakland’s Director of Tree Services, David Moore, gently suggests that many of these tree plantings are ill-advised:  “‘There is a part of all of us that loves with our hearts the coast live oak tree because of its heritage, the symbolism of our city, and just the legacy that they have,’ Moore said. ‘But we have to diversify, and we are diversifying to other ones that are recommended to be more adaptable to climate change…The reality is that we have created a world that is not the native conditions of these plants,’ Moore said. ‘If we want trees to survive in these non-native conditions, we have to pick trees from around the world that can survive these conditions.’…Moore said oaks, while beautiful, are not the ideal tree for today’s hot, dry and cramped urban landscape. Without careful and costly maintenance, he said, oaks could destroy sidewalks, block light from street lamps and grow their branches into streets and walkways, creating hazards for motorists and pedestrians. The city still plants oaks, but mainly in parks rather than streets because that’s where they do better, Moore said…”

Stalemate

So, here we are.  We have a pressing need for a more diverse urban forest that is adapted to present and anticipated conditions, but we are paralyzed by the ideological commitment of native plant advocates who are demanding that we destroy our urban forest because it is predominantly non-native.  In a recent edition of Nature News, Jake Sigg said, “Hysterical tree planting is worse than a waste of time and resources…”

I am grateful to Professor Ritter for being bluntly frank with members of the arborist community who should know better.  Dare we hope they learned something from that presentation? 

I wish you Happy New Year.  Please join me in my hope for a more peaceful year.  Thank you for your readership.


(1) Nowak, David, “Historical vegetation change in Oakland and its implications for urban forest management,” Journal of Arboriculture, 19(5): September 1993

The Natural History of America’s Urban Forests

American chestnut

The story of America’s urban forests, as told in a book by the same name (1), is a story of loss and redemption.

The loss of America’s trees

The American chestnut was the first of our iconic trees to be lost to us.  Between 3 and 4 billion chestnut trees were killed in their natural range by a blight caused by a fungus imported in an Asian species of chestnut (2).  With the exception of a few isolated chestnuts outside their natural range, all American chestnuts died in the first half of the 20th Century.  Over one hundred years after the epidemic began, scientists believe they have finally developed an American chestnut that will survive the fungal infection.  They have achieved this by the addition of a single gene to the 38,000 genes of American chestnut.  They are applying for regulatory approval of the EPA and USDA to plant the genetically modified tree in the wild, a process they expect to take several years.

American elm. Creative Commons

Dutch elm disease (DED) was introduced in America from Europe in logs and furniture in the 1930s.  DED is a fungal disease spread by a beetle.  The natural range of American elm was much larger than chestnuts and they were widely planted as park and street trees in cities because of their tolerance for challenging urban conditions. Therefore, their loss was devastating to many communities.  Most communities tried to save their elms by removing diseased trees in an attempt to isolate the disease.  By the 1990s, the majority of American elms were dead.  Many scientists have tried to develop a DED-resistant variety of American elm by cross-breeding them with species that are not vulnerable to the disease.  Several DED-resistant varieties of elms are being planted and their fate will determine the future of American elms.

Asian long-horned beetle (ALB) arrived in North America in the late 1990s on wood packing material, such as wooden pallets.  It is less selective about its tree hosts than the beetle that carries the elm fungus, although it has a preference for maples.  Once again, destroying infested trees was the preferred control method.  “Over 1,550 trees in Chicago have been cut down and destroyed to eradicate ALB from Chicago. In New York, over 6,000 infested trees resulted in the removal of over 18,000 trees; New Jersey’s infestation of over 700 trees led to the removal and destruction of almost 23,000 trees, but infested trees continue to be discovered.” (2) Injections of a pesticide (imidacloprid, a neonicotinoid insecticide) were also used on infected trees.

White ash. Creative Commons

Emerald ash borer (EAB) also arrived in North America in the 1990s, but it has spread far more rapidly than the long-horned beetle.  Little was known about EAB when it arrived from Asia because it does not kill Asian species of ash, which have developed a resistance to the beetle predators after centuries of co-existence.  In North America, the EAB threatens the entire ash genus: “It has killed at least tens of millions of ash trees so far and threatens to kill most of the 8.7 billion ash trees throughout North America. Emerald ash borer kills young trees several years before reaching their seeding age of 10 years.” (2)  Ironically, ash trees were widely planted in urban areas to replace the elms that were destroyed by DED.

Here in California, we have lost 5 million of our oak trees to the pathogen that causes Sudden Oak Death since 1995.  Also, over 100 million conifer trees in the Sierra Nevada have died in the past 5 years of a combination of drought and native bark beetles.

A digression:  Turnabout is fair play

Some of our readers might be surprised by our reporting about the devastating consequences of introducing pathogens and insects to our urban forests.  After all, aren’t we usually defenders of introduced species?  So, let’s take this detour to explain how this topic is consistent with the mission of Million Trees:

Asian long-horned beetle

There is a lesson in these stories about the relationships between insects and trees.  Insects introduced from distant places, such as Asia, quickly attacked our native tree species that do not exist in the native ranges of the insects.  It didn’t take thousands of years of co-evolution for the insects to find the native trees that they killed.  They came, they saw, and they feasted on our trees.  In most cases, the insects found trees here in the same genus as the trees in their home ranges that were chemically similar. And in most cases, the insects were not nearly as damaging to trees in their home ranges because their host trees had developed resistance to the insects.

Now let’s turn this story around.  Native plant advocates who wish to destroy non-native plants claim that they will become “invasive” because they don’t have insect predators here.  They claim that native insects “co-evolve” exclusive relationships with native trees and are unwilling and/or unable to use introduced plants and trees.

Anise Swallowtail butterfly in non-native fennel

Not only is there little empirical evidence to prove that native insects do not use introduced plants, it doesn’t make sense.  If introduced insects are quick to attack our native trees, why should we assume that native insects are not just as quick to attack introduced plants?

Furthermore, when introduced plants are attacked by our native insects, they are more vulnerable to the native insects because they have not had an opportunity to develop defenses against them.  The Asian insects that are killing our native trees are not a serious problem in their home ranges because the trees in their home ranges are resistant to them

In other words, claims that introduced plants are invasive because they have no insect predators are contradicted by reality.  In fact, introduced plants are at a disadvantage when confronted by the insect predators in their new home.  Once again, we find little logic in the unproven hypotheses of invasion biology.

Redemption of our urban forests

Efforts to restore America’s urban forests began in earnest in the 1960s, partly as a response to the loss of chestnuts and elms and partly as a growing interest in the environment.  These efforts are too numerous to mention, but we will pay tribute to a few of them.

Arbor Day is celebrated all over the country. This is a park in Minneapolis.

Early interest in America’s trees was reflected in the creation of Arbor Day in Nebraska in the 1870s.  Celebration of Arbor Day spread slowly across America until it was observed in nearly every state of the nation.  Sterling Morton was the organizer of the first Arbor Day and he was also instrumental in the creation of one of our most important research arboretums, the Morton Arboretum in Illinois.  Harvard’s research arboretum, Arnold Arboretum, was founded in 1872 and to this day is responsible for much of our knowledge of our trees.

Private enterprise has also made significant contributions to our knowledge of trees.  Davey Tree Experts was founded in 1880 by a tree-evangelist who was inspired by butchered trees to write practical guides to inform citizens about how to take care of their trees properly.  Bartlett Tree Experts was founded in 1907 and is also a respected provider of tree care.  Much of the damage done in our urban forests is caused by people who don’t know how to take care of their trees.  Hiring a certified arborist to take care of your trees is money well spent.

The US Forest Service is the source of much of our knowledge about the value of our urban forests.  David Nowak and Greg McPherson of the US Forest Service were responsible for groundbreaking research about the carbon stored by trees, the pollution that trees remove from the air, the contribution that trees make to the value of our properties and to our health and well-being.

Armed with this new improved understanding of the value of our trees, an army of volunteers and a proliferation of non-profit organizations are now engaged in the effort to preserve our urban forests.  Tree People in Los Angeles was among the first of these organizations.  One of their first projects was to plant one million trees for the 1984 Olympics in Los Angeles.  Since then they have planted millions of trees in Los Angeles.

We recently had an opportunity to appreciate how many non-profit organizations there are in California, devoted to the care and preservation of our urban forests.  Thirty-three non-profit tree advocacy organizations collaborated in a public comment on California’s Urban Greening Grant Program, which will give California cities $76 million to plant trees to increase carbon storage and reduce greenhouse gas emissions.  The letter of the non-profits acknowledged the importance of introduced trees in our urban forests:  “Native trees are generally not suited to urban conditions. They have difficulty adapting to the urban environment, thereby substantially reducing survivability. According to California’s Guide to the Trees Among Us, only 6% of California’s urban trees are native to California. As an example, the approved list of street trees for the City of San Francisco includes no trees native to San Francisco. In Oakland, two of the 48 allowed species are native.”

These are the forestry non-profit organizations that signed the letter to the Urban Greening Grant Program

Introduced trees are the foundation of California’s urban forests.  Efforts to eradicate non-native trees will doom us to a treeless landscape and all that implies about our environment:  more pollution, more noise, more wind, more greenhouse gas emissions, more erosion, less habitat for wildlife, and less beauty in our landscapes.


  1. Jill Jonnes, Urban Forests: A natural history of trees and people in the American cityscape, Viking, 2016
  2. Wikipedia