The Importance of Tree Diversity

By Jesse Eastman

It is a pretty basic human trait to see something a fellow human has, and if we like it, we want it too. Usually this sense of “keeping up with the Joneses” is just a funny social quirk, a sort of communal benchmarking. Sometimes, though, it can have devastating consequences. One of those potential consequences can be seen in the outcomes of our collective choices of which trees to plant.

Throughout modern history, certain trees have grown in popularity to the point where they are found absolutely everywhere. In certain notable cases, that fad comes to an abrupt and brutal end as specialized pests gain a foothold and, presented with a nearly endless supply of food and no natural predators, sweep through the region, leaving a path of dead and damaged trees in their wake.

The first such incident that comes to mind is the Dutch Elm Disease epidemic that struck North America full force in the mid-20th century. This disease is native to Asia (in spite of its name) and is spread by various species of Elm Bark beetles. When it arrived in North America in the early 1900s, it spread quickly, encountering nearly no natural resistance in the North American elm population. Soon, nearly all American Elms were gone, leaving only the significantly less-desirable Siberian Elm. While American Elms are slowly making a comeback, they are still few and far between.

Black Walnut failed to leaf out this year, will be removed

Similarly, Black Walnut trees have recently come under assault from a problem known as Thousand Cankers disease. Once widely planted as a resilient hardwood ornamental tree with various uses for timber and nuts, healthy black walnut trees are now a rare sight. Both of my parents’ houses were once home to beautiful black walnuts. Today, one is gone and the other will likely need to be removed this year.

This leads to the most pressing issue we see affecting Colorado. Emerald Ash Borer (EAB) has finally made the jump across the Great Plains and has taken up residence in the Rocky Mountain State. Although it has only been positively identified in Boulder County, the spread of EAB to all urban areas of the state is considered by experts to be inevitable. An infestation is a guaranteed death sentence if untreated. Treatments must be done on an ongoing basis for the life of the tree, and get more and more expensive as the tree ages.

The biggest problem here is that ash trees are absolutely everywhere. It was recently estimated that roughly 1 out of every 10 urban trees was an ash. 10% is a pretty large market share, but losing 1 out of every 10 trees is not in itself a complete loss for our urban forest. The real problem lies in the size of ash trees. They are large trees, providing a tremendous amount of shade, animal habitat, water runoff capture, air cleaning, the list goes on. While ash only compose 10% of the trees in our urban forests, they account for around 30% of the total urban canopy. Now we are talking about major impact on our cities and towns.

These three examples demonstrate what can happen if we get too focused on a small selection of trees. The more food source for the pest, the faster it can reproduce and spread. If the pest is not native, it is likely to spread rapidly. If we are highly dependent on the particular tree in question, the results will be felt deeply. By choosing a diverse palate of trees, we minimize the risk of one particular pest being able to run roughshod over our neighborhoods.

Beyond the practical reasons for selecting a diverse range of trees, there is also aesthetics. Personally, the idea of an entire street lined with nothing but one type of tree, no matter how beautiful, is mildly disturbing. I find nothing appealing about the image of extreme uniformity represented in the song made famous by Pete Seeger: “…And they’re all made out of ticky tacky, and they all look just the same.” When I’m walking my dogs or driving to work, I enjoy the variety of a myriad of different trees and plants. Sometimes the differences are subtle – varying shades of green, different branch angles, different bark colors – and sometimes the differences are stark, but they make the visual landscape a thing of beauty, keeping me engaged and interested, and they don’t all look just the same.  

Northern Catalpa is a great large shade tree with beautiful white spring blooms

The Colorado Tree Coalition is a great place to start doing research from home if you’re considering planting a tree. They list trees by all kinds of different characteristics, including soil condition tolerances, overall size, foliage and bloom color, growth rate, and more. Visit to see their complete list. Our expert staff is also here to help, and believe me, we love helping people find their way out of the tunnel vision that can lead to the monoculture approach to urban forestry that has historically resulted in devastating losses. Don’t settle for keeping up with the Joneses. See their ash and raise them a Turkish Filbert, or a Hackberry, or a Linden, or a Norway Maple, or a Honeylocust, or a Buckeye, or a Catalpa, or a Cottonwood, or a Kentucky Coffee Tree, or…. You see where I’m going here? Khalil Gibran wrote “Trees are poems that the earth writes upon the sky.” So get out there and start painting, just be sure to use a wide variety of colors.

For more information on Emerald Ash Borer, including tips for identifying the pest as well as prevention and management strategies, visit

Originally published June 5, 2018


Originally published on June 5th, 2018. Updated on September 5th, 2019.