Diversity, lessons from the trees

This planet is a wonderful teacher. Explore anything in nature closely and you will find all sorts of intriguing lessons to ponder. A couple months ago I shared the story of the largest and oldest tree in the world, Pando. A single clonal organism of Aspen that is a small forest in itself. Pando reminds us of the concept of diversity, especially as it relates to trees and natural ecosystems. Pando is a clone and has no genetic diversity. Overgrazing of young sprouts by cattle and deer has reduced the age diversity of Pando’s stems, meaning there are very few young stems taking over for the old stems that are dying. If there were more age diversity of its stems, it would be less vulnerable to shifts in climate like drought and prolonged high temperatures.

Of course we don’t have to look all the way out to Utah for lessons of diversity. We are experiencing one right now with Emerald Ash Borer. This insect is progressively killing off billions of ash trees across the country, including thousands of trees here in Boulder County.

Ash only make up about 10 percent of the urban forest canopy in the Boulder area. That percentage is higher in Longmont and Denver. A number of local neighborhoods have populations of ash trees that make up more than 30 percent of the canopy. Having to remove almost one-third of the trees in a neighborhood will have a major impact. It’s not just the cost of removal and replacement, but also the loss of shade and loss of aesthetic appeal, not to mention the environmental benefits. For those more practically minded, a tree-lined street can increase property values as much as 18 percent, as indicated in several studies.

The loss of ash reminds many people, from a previous generation, of the plight of elm trees in our country. Almost every city had an Elm Street and elm was thought to be the perfect city tree, with large swaths of urban areas planted exclusively in elm. These trees are adaptable to poor soils, poor air quality and limited root zones. Plus they grow into a beautiful vase-like canopy over streets.

So what happens when you plant too much of a good thing? Something like Dutch Elm Disease comes along on the back of elm bark beetles and wipes out all the elm trees throughout entire cities and much of the country. So much so that we stopped planting them for a while until finding and breeding disease resistant varieties of elms.

Following Dutch Elm Disease, ash and maple trees became the most commonly planted urban trees across the country. Today two pests, Emerald Ash Borer and Asian Longhorned Beetle specifically target ash and maple trees.

We have an opportunity to learn from these events. A great way to avoid past mistakes and support a healthy ecosystem on your own property and your community is to plant a diversity of tree species and shrub species. There have been numerous studies and continual evidence that supports this practice. Properties with more plant diversity support more resilient ecosystems, have more beneficial insects and experience fewer pest outbreaks. What other lessons do the trees have for us?