The Natural History of America’s Urban Forests

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 20^th 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.

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.

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:

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.

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.

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.”

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.

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