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.
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.
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.
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.
Let’s resolve to enjoy our urban ginkgo trees, but not to plant any more.