Urban environments are the most stressful for trees but where they are most wanted and needed. Especially in light of climate change, trees make our cities and towns much more habitable, not just for shade and beauty, but also ecosystem services for wildlife and pollinators.

As with so many problems affecting the natural world, biodiversity is key, even though human domination of the planet is having the opposite effect. But that’s a larger story.

Here’s a summary of this year’s discussion of strategies, successes and solutions for creating a diverse urban environment.

1) Henrik Sjöman is Senior Researcher at the Swedish University of Agricultural Sciences & Scientific Curator at Gothenburg Botanical Garden. He stressed the importance of tree selection asking, What kind of plan does the plant (tree) have? How well prepared is it to take the punches of heat, drought, flooding, etc? The book he co-authored with Arit Anderson, The Essential Tree Selection Guide for climate resilience, carbon storage, species diversity, and other ecosystem benefits contains an A-Z list of over 550 trees with these benefits. (This is the same book mentioned by Kenton Seth in his piece, Trees in the Apocalypse in our Harvest Issue.

Sjöman said that we do our research when buying a car so why not be at least as thorough with tree selection? It’s not just about winter hardiness and autumn color. Genetics is important. There is different genetic material within the same species, as well as much variation in the wild, even in the same growing conditions. There is a new era of plant hunting to find natural growing habitats that match urban environments, and a lot of work to do as we are losing these habitats. For example, trees growing on the south side of a ridge have developed strategies to take the heat while the same trees on the north side like it cool and moist, and wouldn’t be good street trees. And some trees want to be alone while others grow in groups – the “social capacity” of trees. What makes trees able to manage or too fragile to cope? On plant hunting trips it’s important to meet with local farmers and foresters; they hold the knowledge. He also talked about the importance of creating teams, “working with the fungi people, the bug people, the soil people (the hidden half)” and with nurseries.

2) Nina Lauren Bassuk, professor and program leader of the Urban Horticulture Institute at Cornell University, gave a presentation on Evaluating Your Site to Increase Diversity in the Urban Forest. She showed slides from a list of smaller trees with attributes that might work in Colorado including ‘Winter King’ Hawthorn (C. viridis), Acer miyabei, Zelkova serrata ‘Wireless’ (for beneath powerlines), Golden Rain Tree (Koelrenteria paniculata), Lindens Tilia americana ‘Redmond’ and Tilia tormentosum, and many others.

The cooling shade of trees can be critical on a sunny day in cities where temperatures can reach 135° F. But different sides of the street vary considerably in the amount of sun and humidity.

Compaction and rubble from construction as well as sidewalks and salty streets impact soils that become as hard as brick. Tree roots often can’t make it out of their initial planting area. While we can select trees for many factors like heat, cold, insect & disease resistance, salt to some extent, we can’t choose trees for soil compaction so we need to do something about it. She said the “Scoop & Dump” method of soil remediation has worked well and showed before and after photos to illustrate the point.

Scoop & Dump

• Apply a layer of 6-8” of compost to compacted soil

• Use a backhoe bucket to dig down 18”

• Lift bucket with topsoil/compost mix 3 ft into the air

• Soil/compost mix is dropped onto the ground and smoothed

• Landscape plants are directly planted in the soil

• Surface mulch is added every year to replenish organic matter until there is canopy closure

2) Elizabeth Judd, Urban Forestry Planner for the City and County of Denver, presented results from a 2023 survey in which Denver residents were asked what they think about trees as part of an effort to create an Urban Forest Strategic Plan with citizen input. The top 5 answers (and their order) were a surprise to city foresters:

1. Beauty

2. Environmental Benefits (air quality, stormwater, etc)

3. Feel Better (mental health)

4. Wildlife

5. Property Value

Perhaps the main takeaway is that “trees need to be on the list for development project budgets,” Judd explained.