Why Diversity Matters to Trees

Homogeneous community forests are less able to withstand risks
The emerald ash borer (Photo: Dan West, CSFS)

Many Coloradans decide to plant trees in the spring and summer months simply because they’re outside once again, focused on their yards. However, spring is the best time of year to get new trees established, and why communities around the state host April plantings in recognition of Arbor Day.

As you engage with customers seeking your services selling, planting or caring for their trees, a key message to share with them is the importance of planting a diverse selection of trees. Whenever too many of the same type of tree are planted together, we set ourselves up for problems with insects and disease, as many tree pests target specific species. Homogeneous community forests also present a risk of losing more trees to unforeseen environmental conditions, including drought and weather extremes.

The exotic, tree-killing emerald ash borer (EAB) offers a perfect example of the need for planting diversity. EAB attacks and kills all true ash species (genus Fraxinus), and is so aggressive that trees typically die within two to four years after they become infested. Now responsible for the death of millions of ash trees and billions of dollars in costs across more than 30 states, EAB was confirmed in Boulder in 2013. Since then, the pest has been detected in Longmont, Gunbarrel and Lafayette, and further spread is anticipated in Colorado.

[Related: Emerald ash borer detected in Lyons]

One of the reasons this invasive pest is a concern for communities everywhere is that an estimated 15% or more of all urban and community trees in Colorado are ash. You and your customers can learn more about EAB and how to identify ash trees by going to the Colorado State Forest Service EAB webpage: csfs.colostate.edu/emerald-ash-borer. There you will find images and videos, and information about a free EAB/Ash Tree ID app for mobile devices.

To mitigate the potential impact that lack of diversity has on community forests, here are some factors to consider:

• No one species should comprise more than 10% of the planted trees growing in any urban or community setting. That includes the makeup of a neighborhood’s trees.
• Include drought-tolerant trees—and a variety of those that tolerate either extreme heat or cold well—to provide for future climate and weather uncertainties.
• Be sure to recognize the “right tree for the right place,” and plant trees that are adapted not only to our semi-arid Colorado conditions, but also to specific site parameters. Trees that mature to large sizes should not be planted in confined spaces or under power lines, and trees that do better in cool and moist sites should not be planted on sunny southern aspects.
• Avoid planting any true ash species anywhere in Colorado, due to the threat of EAB.

The Colorado Tree Coalition offers online descriptions of trees suitable to plant throughout Colorado, as well as a list of recommended trees to plant along the Front Range. For more information, go to coloradotrees.org. The “Find the Right Tree” option at this site includes a link to public trees growing in different Colorado communities that you can visit (either virtually or in person) to help guide your tree selection decisions this spring.

Planting trees should be considered a continuum, addressed by landowners regularly over time. We should encourage everyone to plant what they are able to maintain each year to ensure tree health, vigor and growth. Ongoing plantings will also enhance the diversity of tree ages for a more sustainable tree canopy over time.

Donna Davis is an urban and community forestry specialist for the Colorado State Forest Service. She previously served as district forester for the CSFS La Junta District, where she provided forestry assistance to southeast Colorado communities for more than 30 years. She holds Certified Arborist and Tree Risk Assessment qualifications from the International Society of Arboriculture; is a 2018 Municipal Forestry Institute Graduate from the Society of Municipal Arborists; and is a member of the Society of American Foresters and Colorado Tree Coalition.

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