Diversity is Good for City Trees
June 14, 2018
Our urban forests are very important and the benefits to us urban dwellers are many. Trees reduce energy consumption during our hot summers, reduce noise pollution, help filter pollutants in the air, and even increase the value of your home. But, our urban forests are under threat from many insect and disease pressures, which makes it important to plant a diverse mix of tree species.
Tree Monocultures - what’s the problem?
When you look at trees planted along city streets, you’re usually looking at monocultures. What is a monoculture? It is defined as the cultivation of a single crop in a given area - meaning one tree species. Why does this matter to you? Because when we plant a monoculture, we are also creating the opportunity for mass problems or mass failures. One such example is Dutch Elm Disease (DED). In the beginning of the 1900’s, Elm trees were widely planted as monoculture street trees. Since Dutch Elm Disease was introduced in the 1930’s, more than 40 million American elms have died.
DED is not the only plant disease or pest to be concerned about. Emerald Ash Borer is targeting Ash trees. The Hemlock Woolly Adelgid enjoys attacking and sucking sap from Hemlocks and spruce trees. The Asian Longhorned Beetle prefers to feed on maple trees. The Rose Rosette virus is wiping out many Knock Out roses (and all types of roses) in landscapes. And the most significant to our area trees, Live Oaks and Red Oaks are becoming infected by oak wilt. All of these plants have been planted en masse as monocultures, increasing the spread of such pests and diseases.
Oak Wilt in Texas
Certain oak trees are able to resist oak wilt disease better than others. Unfortunately, Live Oak and Red Oak are the most susceptible to the disease. Guess which species of oaks are planted most often in the DFW area? You guessed it: live oaks and red oaks.
If you need a large shade tree in your yard, think about shaking things up a bit with a different oak species. Mexican White Oak (Quercus polymorpha) is a great choice as it is not as susceptible to oak wilt. Sometimes called the Monterrey Oak, these trees are very drought tolerant and grow to be a medium sized tree, around 40’ tall - perfect for most urban landscapes. They grow relatively quickly (for an oak), and are native to Mexico—so we know they can handle our heat.
Other alternatives to Live Oak or Red Oak are Lacey Oak (Quercus laceyi), Chinkapin Oak (Q. muehlenbergii), and Bur Oak (Q. macrocarpa). You may even plant a tree such as Ginkgo (Ginkgo biloba), or Chinese Pistache (Pistacia chinensis) for striking fall color and large size.
In order to keep another widespread tree death from pests and diseases such as DED and Oak Wilt from occurring, tree diversity is key. Diversity in nature is always a great defense against pests and diseases. By selecting different species of trees that are better able to resist a certain insect pest or disease, you can reduce losses.
Keeping the urban forest diverse and in turn, healthy is up to all of us. If you need help selecting the right tree for your location, contact us and we’d be happy to help.