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Ginkgo leaves in autumn

Ginkgo leaves in autumn

Cities are heating up because of a combination of climate change, the urban heat island effect, and a loss of urban tree canopy cover. We can mitigate some of these effects and increase the resilience of urban environments  if we plant more trees. The wrong choice of trees, though, can reduce urban biodiversity and may make cities less resilient.

There are few trees more glorious than ginkgo in the autumn. It is easy to overlook how abundant ginkgo trees have become in our cities until we see the blaze of yellow up and down the streets.

Ginkgo trees on an urban greenway

Ginkgo trees on an urban greenway

As most people know, ginkgo is an ancient tree, found worldwide in the fossil record but today only found in cities and in a tiny area of China, where its survival has probably depended on Chinese monks and botanists for thousands of years (See Peter Crane’s marvelous book Ginkgo: The Tree That Time Forgot for the whole story).

Over the last 150 years, ginkgo has found a place for itself in cities throughout the world’s temperate zones. It is tolerant of air pollution, which was probably the attribute that made it so popular. It also handles other urban stresses well, from soil compaction to excess salt. Ginkgo is almost entirely free from the insects and diseases that beset other trees in urban environments.

Ginkgo street trees

{/caption] Ginkgo street trees

And therein lies the conundrum of ginkgo – it seems to be such a perfect urban tree that we plant them by the millions.  In my city, Lexington Kentucky, we almost seem to have an obsession with ginkgo.  All the pictures on this page were taken within a few blocks of the center of town (click the pictures for a gallery).

If all we cared about was shade, ginkgo might be a good choice. But urban biodiversity is also important. Trees in urban areas can support many other species – birds, small mammals, insects. In fact, a highly diverse urban canopy can appear to many organisms as a forest.  Cooper’s hawks are forest birds that have become quite abundant in cities, and there are several nesting pairs in my urban neighborhood.

Fleshy seeds of female ginkgo

The fleshy seed of female ginkgo is not technically a fruit.

Ginkgo may not be a good neighbor for other species. I have carefully inspected a number of ginkgo trees in my neighborhood this summer. It is remarkably difficult to find insects on ginkgo trees. Douglas Tallamy, in his excellent book Bringing Nature Home, says that an urban oak may host over 500 species of caterpillars while ginkgo hosts only one.  Ginkgo seeds, with their strong odor that many people find offensive, are probably adapted to be distributed by carnivorous animals, but in urban area, only squirrels will eat ginkgo, and it is not a preferred species for them.  Acorns, hickory nuts and other native fruits are much preferred.

Not surprisingly, birds avoid ginkgo as well. Most resident birds spend time in trees where food is available. Since there are no insects in ginkgo, birds tend to avoid them. This summer, I only saw birds in ginkgo trees as casual visitors, flitting through on the way to somewhere else.

The genetic diversity of planted ginkgo trees is also low. Ginkgo in China appears to originate from only a few remnant populations, suggesting that the species has been through a genetic bottleneck of very small populations in only one or a few geographic areas.  In urban areas, the requirement for male trees, to avoid the offensive odor of females, means that only a few clones are planted, further reducing genetic diversity.  A city that contains thousands of ginkgo trees may in fact contain thousands of copies of a few individuals. While ginkgo is currently not troubled by pests and pathogens, having only a few clones is a risk for future problems.

I love ginkgo trees, as most of us do. But ginkgo is not helping to create diverse, resilient cities. Instead of endless planting of ginkgo, we urgently need to diversify our urban forests with diverse plantings of seedlings of many species.

We need to prepare our cities for climate change, and greatly increase not just our urban forest canopy, but our urban forest diversity.  We have more than enough ginkgo trees now. Let’s resolve to stop planting them.

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11 Responses

  1. Bill Fountain says:

    Thank you for your insight on avoiding the further use of ginkgo in the built environment. However, I must take exception with your position as stated. The urban environment is certainly less hospitable to both flora and fauna than native habitats. We plant trees in the urban environment for the multitude of human benefits, wildlife being only one. Urban biological systems are far more complex than simplistic statements will allow. Rather than avoiding the use of ginkgo in the urban environment, I continue to encourage the practice of matching the species to the site. Ginkgo is one of our toughest, most adaptable, most attractive and structurally stable species for sites where other species fail to thrive or fail structurally endangering human life and property. The absence of biological pests (e.g. aphids, scale, and mealy bugs) is a plus in parking areas where reflected light and heat is an issue as is dripping of honeydew and the subsequent formation of sooty mold on cars and other property. With limited municipal resources for urban tree care it makes economic sense to use this and other species that will allow us to stretch otherwise limited financial and personnel resources. With the alternative choice of having fewer trees and trees with increased potential for failure; my vote will be for the ginkgo every time. Rather than stopping planting ginkgoes we need to be more diligent about where we plant them and to avoid the use of monocultures of ginkgo or any other species. Rather than avoiding the use of ginkgo in the urban environment we should be reserving it for use in areas where we cannot tolerate diseases, insects, and the wildlife that are attracted to these food sources. It is more of a street tree than a park tree; a species for poor growing conditions where most of our beautiful and environmentally natives are unable to thrive.

  2. Tom Kimmerer says:

    Thanks for your comments, Bill (for other readers, I should point out that Bill and I live in the same city, Lexington, KY).

    Your comments about my "simplistic statements" are a perfect illustration of what is wrong with industrial horticulture. I will have a column soon about the difference between urban forestry and horticulture. Suffice it to say for now that forestry strives to increase biodiversity, while horticulture strives to decrease it.

    We can look at the many tales of woe that industrial horticulture has visited on our cities:
    – Endless duplication of a small number of clones, so that, rather than having thousands of genotypes from planting seedlings, we have a huge number of copies of few genotypes.
    – A long-term proclivity for introducing large numbers of poorly-behaved species, such as callery pear, bush honeysuckle, wintercreeper- the list is practically endless.
    – A failure to learn from past mistakes. The devastation of Dutch elm disease in cities with too many elms should have taught us the benefits of diversity, but instead, cities repeated the same mistakes over and over – pin oak being just one example. There is an old comedy routine where a man is asked if he has learned from his mistakes and he replies "I have learned from my mistakes and believe I can repeat them exactly." That is modern horticulture.

    You say that we should plant more ginkgo because the urban environment is inhospitable to other species. The answer is to make our planting sites more hospitable. You praise the virtue of ginkgo for having no insect pests, but in fact it has no insects period. I encourage you and your horticulture colleagues to pay head to Doug Tallamy's fine book Bringing Nature Home. Instead of making our cities less diverse and less hospitable to wildlife, we should be inviting wildlife, including insects, into our cities by planting appropriate trees. An oak (whether native or not) can host up to 500 species of insects, and therefore attract birds and small mammals. A ginkgo is a biological desert that happens to provide some shade.

    If we stop planting ginkgo trees, or greatly reduce the frequency with which we plant them, there will still be plenty of them because of our current overplanting. Rather than avoiding natives because it is inconvenient to find a niche for them, we should use the urban landscape to make conditions more hospitable to beneficial trees.

  3. Troy says:

    You honestly think gingko trees are over-planted in Lexington? Oaks, young & old, everywhere I look…maples galore…gingko, a rarity. If you are referring to the relatively young gingko trees planted in downtown Lexington….give the city two or three years and they will cut them all down (likely at your advice) and plant something new. I too believe diversity is the key, but I feel we have room for a quite a few more hundred gingko trees in our beautiful city…I personally pray that
    many of them survive to maturity, better yet, old age.

    • Troy, while I can’t fault you for your aesthetic preferences, I’m afraid this attitude reveals a basic misunderstanding of what Dr. Kimmerer is saying. Ginkgos, lovely though they may be, are not conducive to a healthy biome in this part of the world. Personally, I’m a big fan of lush, green grass, but here in LA that is a serious waste of water. We can’t continue planting it and watering it because we do not have the climate (or the water) to support it.

    • Tom Kimmerer says:

      Troy, thank you for your comment. I suggest that you look in the area around Rose Street/Elm Tree Lane and adjacent streets. The city has planted several hundred ginkgo trees. That makes this area essentially a biological desert, inhospitable to lepidopterans (moths and butterflies) and other insects, and also for birds. The more we create such low-diversity areas within our cities, the more the total biodiversity of the city declines. So it is not really a question of the total number of ginkgo trees in the city, but of their distribution. By the way, you may want to check your spelling – it is ginkgo.

  4. Lee S says:

    I am no expert, but I tend to agree with Bill. I am aware of a considerable number of ginkgo trees in the Ashland Park area (1 street, in particular) going back over 75 years. But outside of downtown Lexington, I don’t see near as many gingkos as I do oaks (particularly pin oaks) and maples. Now I admit, I do not claim to know what trees have been planted in downtown in recent years in areas of rebuilding. But if there has been a resurgence of the gingko in recent years, I say, good!
    Depending on the subdivision, it’s age and location, I would wager there are several types of trees that have been overused much more heavily than gingkos. If you live in the Chevy Chase area, you must be aware of all the pin oaks! Then during baby boom years, when middle income homes were spreading like wildfire, so was the use of silver “water” maples. Many homeowners replaced these over the years with red maples and sugar maples. There were also a lot of locust trees around the same time.
    Then there was the trend of planting flowering crabs and other flowering trees, plums and, of course, the Bradford pears– so desirable for awhile for there shape and blossoms– until they started splitting! Another unfortunate trend was planting magnolia trees, also spruce trees, right up against houses with no allowance for root growth.
    But I digress… Mainly I wanted to point out that I feel the pin-oaks and maples have been used much more than the gingko, and many are currently struggling. I don’t know if they suffer from a common disease or if it is a combination of age and stress, but we are in danger of losing a lot of our 50+ year old trees, If there has been a resurgence of the gingko, it may be because of its lack of problems. I know in my area, we are currently needing to replace some of our trees. It would be helpful if, rather than merely criticize the gingko, you could make suggestions of suitable replacements on your opinion.
    Your suggestions are most welcome!

    • Tom Kimmerer says:

      I don’t disagree with anything you say. However, I think you somewhat miss the point of the article. The problem with ginkgo is that, as a non-native tree in a genus unrelated to any North American trees, it is not hospitable to all the organisms that live on and in trees and thereby increase biodiversity in our urban forest. All the other trees you mention, regardless of their other failings, are of native species and genera, and therefore can support thousands of species that live on and in them. Even callery pear (also called Bradford), while not native to North America, is in a native genus, Pyrus.

      The choice of replacement trees depends very much on site conditions. You would not want to put a bur oak between the street and sidewalk because it will be too big. LFUCG has a recommended species list available, which may provide a good starting point.

  5. I notice that gingko is on the list that you linked to.

  6. Arnold says:

    Thank you for the excellent discussion. At risk of over simplification, it seems this is primarily an argument of native vs non native. And designing the overarching goals of a sustainable urban forest. As a city planner, this is a tough one. Most cities have precious few dollars to devote to trees. The choices must be well thought out and planned. Survivability, limiting your maintenance costs, promoting ecological diversity, aesthetics all factor into the equation. I always lean towards natives, ecological diversity and avoiding monoculture installations along a single street. However, a quaint neighborhood street lined with Ginkgos in the fall sure is beautiful!

    • Tom Kimmerer says:

      As I said in my article, I certainly agree that a street lined with ginkgo trees is lovely, but having all streets lined with genetically identical trees is not. In spite of huge efforts and expense to improve urban forests, the majority of cities are losing canopy cover. In most cities, street trees don’t survive long enough to pay, in environmental benefits, for their planting costs.

  7. Dave P. says:

    Is it possible to plant a vine on the Ginkgo trees to bring some bio-diversity? If so, what kind of vines won’t take over/strangle it?

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