This is the third part of a three-part story. The first part is about urban forestry and the urban heat island effect. The second is about best practices in site selection and planting to ensure a long life for urban trees. In this installment, we will discuss some of the problems that are preventing many cities from increasing their canopy cover and therefore mitigating the effects of climate change. We can reduce the problems to three major ones: Site selection, Tree selection & Biodiversity, and Maintenance. The examples I will use come from my home town of Lexington, Kentucky, but they are applicable everywhere.
The biggest failure in urban forestry is poor site selection. In new housing construction, developers are often required to plant trees between the sidewalk and street. Choosing a potentially large tree, such as red maple, to plant in a narrow area, often with poor soil, will doom the trees to a short life. Street trees in new developments often survive for less than 10 years. Even if space is adequate, poor soil quality, especially when the soil consists of construction debris and clay, will limit tree growth and longevity.
Sometimes smaller trees are recommended, but small trees yield small benefits. A better strategy would be to design our streets and developments to be more tree-friendly. Developers often make little effort to ensure that soil volume and quality is adequate for large trees. As we saw in the second part of this story, planting in the yard behind the sidewalk will often give the tree more room, but only if the soil quality is adequate. Instead of planting cheap trees in poor sites, developers should be required to improve soil conditions and plant better trees in better sites.
Creating tree-friendly cities requires us to create planting spaces for trees from the beginning of design, not trying to cram trees into whatever planting spaces are left after construction.
Tree Selection & Biodiversity
Creating sites suitable for large trees opens up a lot more possibilities for tree selection. When soil quality and quantity is adequate, we are no longer constrained to very tough trees like ginkgo. Instead, we can choose from a wider variety of trees whose mature size is suited to the site.
This is a fundamental problem with the design process. All too often, when we see design drawings for a site, the trees are mid-size, not at their mature size. It makes a huge difference in the suitability of a tree to a site what that tree will look like when it is mature. Drawings of a 20 foot tree tell us little about what the site will look like when the tree is 100 ft tall.
As a consequence, by the time the tree reaches its mature size, it is far too large for the site and represents at least a nuisance and at worst a hazard. The tree is likely to be removed or to die, shortening the time in which it can contribute its environmental services.
A second problem with tree selection is failure to choose trees that enhance urban biodiversity. In my previous article about ginkgo, we saw that while ginkgo may have some merits as an urban tree – it is certainly beautiful – it is almost entirely lacking in wildlife benefits. The number of insects, birds and small mammals that use ginkgo trees is far less than the number that use a native tree such as an oak.
I am not arguing here that non-native trees do not have a place in our urban landscape. Cities provide opportunities to grow trees like ginkgo that have little or no niche left in forests. The city can be a refuge for trees that are imperiled in their native habitat. Because cities are substantially warmer than the surrounding countryside, trees from warmer climates will often thrive. In my city, for example, cherrybark oak, Quercus pagoda, grows very well even though its native range is hundreds of miles south. It is likely that many invertebrates that inhabit native oaks can also inhabit a non-native oak, along with the birds and mammals that seek out invertebrates. Cherrybark oak may contribute as much to urban biodiversity as a native oak, though we don’t know for sure. It is probably safest to say that cities should place a lot more emphasis on native trees or the close relatives of native trees.
Many years ago, most urban trees were grown as seedlings in nurseries and transplanted into cities. Today, industrial horticulture has largely done away with seedlings in favor of clones of select trees. Instead of planting red maple seedlings, an entire neighborhood may be planted with a single clone of a commercial red maple selection like ‘October Glory.’ This provides a uniformity to the landscape that some may find appealing but dramatically lowers tree diversity.
There is a simple reason why the horticulture trade favors clones. Although horticulturists may claim that select clones provide better characteristics such as color, or more uniformity, the simple reason is that, under US law, a nursery cannot patent seedlings but can patent clones. That means that payments in the form of royalties go to a nursery that has patented a particular horticultural variety. There is no question that the passage of the Plant Patent Act of 1930 was a tremendous benefit to the horticulture industry. When it comes to trees in urban areas, I believe that the increased risk of uniform clones has outweighed the benefits.
Foresters learned a long time ago to maximize the genetic diversity of a forest. Even pine or poplar plantations created from clones are planted in such a way as to maximize genetic diversity, not to minimize it. There is a fundamental difference in considerations of diversity between foresters, who work to maximize genetic diversity, and horticulturists who tend to favor uniformity and low diversity.
Many trees are still planted from seedlings. In the previous installment of this series, we saw an area planted almost entirely in seedlings. Seedlings have other advantages as well – they often grow better and survive longer than clones, especially compared with root-grafted clones. They can be grown or collected locally. For example, nearly all the bur oaks planted in Lexington, Kentucky, are grown from seeds collected from old native trees within the city.
There is an aesthetic argument to be had over the uniformity of landscape offered by clones compared with the greater variation provided by seedlings. Personally, I prefer a diverse landscape.
This is not an argument against using horticultural clones, but in favor of a) increased use of seedlings; and b) higher diversity of clones within a single planting area. It is also an argument in favor of practicing real forestry in urban areas, using the principles of maximizing diversity that foresters have used for over 100 years.
If you buy a car, you know that you have incurred the expense not only of your purchase, but of maintenance. This is rarely done with trees, even though they require regular maintenance and care if they are to survive and thrive. In housing or commercial real estate projects, money is spent to plant trees, but no money is set aside for their care. The same is true of many street and park trees planted by cities – money is allocated in the city budget to plant trees, but no money is set aside to maintain them.
The greatest maintenance challenge with urban trees is the use of heavy lawn care equipment, chemicals and pesticides. Modern lawn care uses large, heavy machines that frequently strike trees and dramatically shortens their lives.
Urban forestry as it is now practiced, at least in the United States, is failing us. As this series has shown, canopy cover in most cities is declining, the livespan of urban trees is too low and current practices are reducing biodiversity.
We urgently need to change the practice of urban forestry and of urban design. We need to design spaces that are hospitable to trees, not merely try to shoehorn trees into whatever space is left. We need to plant and maintain trees so that they can live their full life and contribute maximum benefits to our city environment. And we need to adopt the principles of good forestry to cities to maximize biodiversity not only of trees but of all the organisms that depend on them.