Yesterday was Arbor Day but for me, every day is a great day to hug a tree. There is an important concept to keep in mind when it comes to trees, however: biodiversity is critical in the urban forest/forest. History has provided a perfect example of what happens when there is a lack of diversity in the plant world.

Back in the 1920s, streets and parks were lined with American elm trees that possessed the perfect growth habit and shape. These upright trees arched gracefully across streets, touching in the center, creating a beautiful canopy and shade for the neighborhood.

These stately trees were planted everywhere. They have a fast growth habit, are easily transplanted and tolerant of harsh urban environments, and had very few pests (at the time). It was promoted as a “tree that would last centuries.”

One resource noted that Park Commissioners in one community (I won’t reveal the town — not in Ohio though) called for planting more than 2,100 trees, all elms.

Fast forward to the 1950s, 60s and 70s. By this time, Dutch elm disease was pretty rampant in Ohio. The elm leaf beetle carries or vectors the disease and infects the tree through feeding, resulting in the tree dying.

Researchers say DED has proven to be the most devastating shade tree disease in the United States.

Since this tree was so popular in the nursery and landscape industry and was used extensively, many streets and parks were bare. In addition, a lot of money was spent on removal of the dead trees or on the prevention of the elm leaf beetle.

Fast forward again to the 70s, 80s, and 90s. If you would have asked me what type of tree to plant that was fast-growing and had very few problems at that time, I would have surely suggested an ash tree as one of the choices.

This is another great tree and unfortunately has been planted everywhere. Just like with DED, communities and individuals are faced with the daunting task of removing dead ash trees that have been killed by emerald ash borer.

When a landscape or urban forest lacks biodiversity and problems such as the above occur, they can be totally wiped out.

I have only heard about the devastation of DED; however, I am seeing the results of EAB firsthand. In Toledo, you can walk down streets that used to have a lush canopy of ash trees that are now completely bare and heated by the summer sun. The results are pretty stark and devastating.

When you have one species planted and a major pest comes along, all are usually wiped out. With a diverse planting or forest, only those affected by the specific pest will be lost and the damage is minimized because other species still stand.

A good, long-standing rule of thumb for biodiversity is the 10-20-30 guideline. This suggests that no more than 10 percent of the trees planted are of the same species — for instance, red, sugar, black, hedge maple. No more than 20 percent of the trees planted are of the same genera, for instance, maple, oak, crabapple, etc. No more than 30 percent of the trees planted are of the same family, for instance oaks and beeches are in the Fagaceae family.

By following this guideline, a community can have a diverse forest that minimizes the impact of losing all trees due to a pest problem such as DED or EAB.

Take a look around your neighborhood; do the trees in your community fit into these guidelines?

Pam Corle-Bennett is an Ohio State University Extension

horticulture educator and the state Master Gardener volunteer coordinator.