Planning grant sows seeds for future urban tree diversity

Two street scenes with trees. On the left, trees leafed out in green. On the right, trees with leaves denuded.
An ash tree lined streetscape in Ohio before (left) and after (right) an infestation of emerald ash borer shows the potential risk of low tree species diversity to communities. Images used courtesy of Dan Herms, formerly with The Ohio State University.

An Iowa State University researcher is leading a national USDA Specialty Crop Research Initiative (SCRI) planning grant to dig up the dirt on low tree diversity in urban areas.

Only six species comprise the majority of trees planted along sidewalks, around public building greenspaces, and in parks for an average U.S. community. Low urban tree diversity makes the urban forest more vulnerable because a larger proportion of the overall number of trees is susceptible to the same pests and diseases. This often results in tree loss and reductions in the air filtering and shading services they provide, which can negatively impact the health and well-being of those nearby.

With this one-year, $50,000 award, Grant Thompson, assistant professor of horticulture, will conduct planning research to guide development of future studies on low urban tree diversity. He and Andrew Koeser, associate professor of environmental horticulture at the University of Florida, are teaming up to co-direct a project that will survey urban tree preferences and production from coast to coast.

Thompson and Koeser, along with a set of national collaborators, will facilitate focus group discussions made up tree growers, landscape architects, urban foresters, governmental authorities, and representatives of government and nonprofit organizations from seven regions in the U.S. The goal of these conversations is to identify the root causes of why the same tree species always seem to be planted in cities across the country.

“Is it a communication problem? Is it a biology problem? Is it an economic problem?” Thompson questioned. “It’s all of those things, probably, but we need participants to tell us that so we can turn these problems into research questions that we can actually study to see if there are solutions.”

Thompson predicts participants will voice concern about tree supply and demand. He said a grower’s land can be tied up in production for five to seven years while a tree transforms from a seed, cutting or other propagule to something saleable. This makes it difficult to allocate land to different varieties of trees growers aren’t familiar with or that may be less profitable. As a result, the market continues to be saturated with the same type of saplings.

“If I buy a tree that there are too many of just because it’s all I can find, it signals to growers to keep growing that same over-produced tree,” Thompson said. “One of our ideas is because of the long production cycles and risk-aversion in forestry and nursery production, the industry doesn’t have the same feedback process that you get in other producer–consumer relationships.”

Paul Tauke, Ames City Forester, agrees: Communication between buyers and growers can make tree diversity in urban areas more challenging He said citizens often purchase trees according to what color their leaves will change come fall. Landscape architects also prioritize aesthetics and tend to only choose one or two tree species to plant.

“The nursery industry produces what people like to buy, and people can only buy what the industry produces. It can become a bit of a vicious supply and demand circle,” Tauke said. “Also, when economies of scale are considered, it is easier and more cost-efficient to grow more of fewer species than fewer of more species.”

Another barrier to improving diversity could be the logistics of planting new trees. Thompson said unless a major weather event or natural disaster wipes out a swath of the current population, cities usually only have the capacity to replant a few trees at a time. Deciding which species to replant calls for consideration, too, especially because some parts of the country are better suited to support tree diversity than others.

“For example, there are more species that might work in Florida, so having a higher diversity there might be easier than somewhere in northern Minnesota where fewer species can tolerate those conditions,” Thompson said.

Despite these predictions, participants will not be prompted before focus group discussions in order to avoid bias. Researchers will hold an initial meeting with everyone involved to overview the project, then arrange virtual break-out sessions of 10 to 12 people from each of the seven regions. An advisory committee for the project has also been created. When all is said and done, the thoughts and opinions of nearly 100 people with an array of expertise on low urban tree diversity will have been shared.

In addition to the range of knowledge that will be represented in the focus groups, researchers on Thompson’s team come from a variety of disciplines. He and Koeser have backgrounds in horticulture and forestry. Other investigators include plant breeders, nursery experts, social scientists and an economist. This will ensure a well-rounded approach on future research.

Thompson looks forward to seeing if he and his team’s theories will be proven true. He is also eager to uncover issues behind low urban tree diversity his team has not yet considered.

“That’s really the goal of these planning grants through the USDA. Prior to making bigger investments of public funds on big problems like this, and they want to make sure those funds are as directed and impactful as possible,” Thompson said. “Taking the time, asking the questions, and doing the work to get it right is what matters.”