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City Jungles


Greg Dahle at the arboretum.

Questions by Diana Mazzella
Photographed by Raymond Thompson Jr.

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When an exchange student from Brazil asked Greg Dahle for a summer independent study class, they ended up putting dollar values on West Virginia’s urban forests. Angela Sakazaki used software to study the 17 Tree City USA entities in West Virginia, as designated by the Arbor Day Foundation. What they found is that the trees on sidewalks, urban yards and parks provide the state value that is worth more than $6.4 million annually by capturing 4.3 million pounds of pollutants. These trees have sequestered more than 2.8 million tons of carbon, which is valued at around $53 million. To get the whole story, Diana Mazzella talked with Dahle, associate professor of arboriculture and urban forestry in the Davis College of Agriculture, Natural Resources and Design.

What do you think were the most impressive findings from this study?

There is a huge return on our investment in our urban forests. While I expected this to be the case, I did not realize the returns would be so large, and recall that these numbers are only for 17 cities and communities. I was also very happy to see that nine of these communities are above the target goal of about 35 percent canopy coverage. I think this says a lot for the Tree City USA program in West Virginia. Yet this might be more of correlation and not causation, as caring for urban trees is a long-term management issue and many of these cities have been concerned about their trees for longer than the 33 years the state has been involved with the Tree City program.

The amount of carbon that is captured on an annual basis surprised me. With global warming becoming important to many, it is good to see that trees can help reduce our carbon footprint while beautifying our cities and towns. 

How does having a dollar value on urban forests help cities make a tree plan?

New York, Los Angeles and Miami-Dade County all have a million tree campaign that is actively increasing the urban canopy cover. Chicago elected to increase their tree canopy to a given percentage. These citywide campaigns have been embraced by mayors and managers who realized that trees reduce stormwater runoff as well as provide numerous other ecosystem services. Being able to present dollar figures helps get the attention of policy makers, and thus increase the interest in investing in their urban trees. 

Are you seeing differences in how Americans approach trees now?

Yes, but it’s too slow. I think over the last 20 or 30 years, the public policy arena is starting to value urban green resources. We’re starting to understand the value in not solely ecosystem services — the monies that we get back or we save because of green resources — but also about how it makes our life better. 

We live in this jungle that we call the city, yet we’re creatures of nature that seek to have some of that nature around us. So I think the emphasis on improving the urban green infrastructure is really helping us as citizens feel better in our cities and built environments. I think people are starting to take notice of that. The green infrastructure is broader than only trees, it includes landscape beds, flower gardens, herb beds, urban agriculture and fruit trees to provide us home-grown produce. There is a growing movement to plant more fruit trees in our yards. I don’t know why it’s taken Americans so long to return to where we once were. On a drive through the countryside, you can often see old homesteads with a number of fruit trees. There is no reason we cannot do the same in our own backyard. 

Why do cities make the decision to plant trees?

An important component of why many cities are requiring the planting of trees, is stormwater benefits. Trees capture water and slowly release it back into the environment. In a light or moderate rainfall, the branches, the stem and the leaves catch the water and it slowly sinks down. I think of the games we used to play growing up of grabbing the tree branch and showering it down on our friends. All that wetness is the captured water that is not going into our sewer drains and our stormwater management systems right away. It slowly moves back into the soil. This provides more water that is available for plant growth and reduces the influx of stormwater that our city systems have to use. The U.S. Environmental Protection Agency has pushed to reduced stormwater nationally, the amount of water that can be released into storm drains and into our rivers and our riparian zones. Consider the Chesapeake Bay: all the water that comes in off our asphalt is polluted and it hurts the crabbing industry. By planting a lot of trees we reduce that pollution into our riparian zones and help clean up our waterways, improving the habit for all the plants and animals, including us. So there’s a significant benefit to stormwater capture and that’s why a lot of cities have pushed for an increase in not only street trees but also tree planting in general.

Many Tree Cities are aesthetically inclined and while trees help beatify the city we can realize other benefit from trees. An interesting study found that people tend to drive more slowly on a tree lined street. Which if you’re a mom and dad, that’s not a bad thing, is it? As a teenage driver, I may not have liked that idea, but as a dad now I certainly like the idea of people going slow on my street, so there’s a safety aspect. Finally, shade on a city street can reduce how often you need to resurface it. And resurfacing is quite expensive so perhaps we can extend it five or 10 years because of shade that can save us money on our road maintenance budgets.

Do you think having a dollar value attached to urban forestry is helpful when cities across the country are looking at how to make a tree plan?

Absolutely. And I think it makes a difference in a couple ways. Let’s go back to what spurred our discussion, which was Tree City USA designations. Our team looked at the cities and communities in the state that are part of the Tree City USA program. In order to do that they have to meet four criteria: They have to celebrate Arbor Day and proclamation it, annually spend $2 per capita on tree care and tree maintenance, have to have a tree ordinance, and have tree board in place.

Most tree boards are comprised of volunteers who are interested in helping their community take care of the trees. Having a dollar value helps the tree board members feel good about what they do as volunteers. Additionally, estimating the dollar value of the ecosystem services received from trees can be used by board members to demonstrate to the city managers and the town’s residents why they should invest in their trees and create a plan that addresses the long-term health of their trees.

Can you talk about how greenery has made your life better?

In our yard, we have apple, cherry, and pear trees, as well as grape vines. So far we have made sour cherry preserves as well as grape jelly that we’re eating right now. We’re getting ready to harvest the apples and pears. What is better than going out as a family and discovering the bounty of your own backyard. I love to cook and typically get in the kitchen on the weekends. There’s been nothing better than saying to my daughters “Can you go out and dig up some carrots for dinner? And while you’re out there, grab some basil and cilantro so I can finish off the meal.” We live on a small quarter acre lot in a city and my daughters are learning to be an integral part of the process of preparing our meals. That’s called heaven.

Planting trees involves taking action for the future, right?

Absolutely, when we go out to our nurseries and purchase a tree, and it’s usually quite small. I have a Family Circus cartoon that I show my students when I talk about planting trees. The cartoon depicts the family returning home dreaming about a large shade tree, but they are carrying a small tree in a pot. I think we all realize it takes a while for trees to grow to size, and we must also realize that we need to plant now for the future. Yet be sure to make sure that we include management funds to prune the trees as they develop so they can survive and provide the long-term benefits we want from them.

In my backyard – my wife and I were just looking through pictures the other night – five years ago we planted the tree that was about an inch and a half in diameter, and my daughters could easily stand underneath it and it provided very little shade. Now the tree is hard to stand underneath and we can begin planting a shade garden. While benefit we receive will grow in time with the tree, we are already benefiting from the tree. So yes, we do plan for the future, yet we can take actions that help us today as well.

Planting a tree is like being a parent. The first 20 years you need to give it maintenance, you need to prune and you need to adjust its form just like you do with a toddler, a preteen and a teenager. You’re guiding the development and doing little things to correct them so that when they get to be 18 or 20 they’re ready to go out and really start maturing. With our children, building a strong foundation is so important, and the same is true with our trees. Especially the first 20 years: it’s critical to provide water for the first couple of years while the root system establishes. Then lightly pruning as it’s growing so it has the desired branch structure is developed. I would like to stress that maintenance is important in the beginning of the tree’s life so that the long-term health and stability is strong; rather than trying to do corrective maintenance in the future when it might be too late.

Why do you think people plant trees in cities?

Aesthetics. It makes my house and yard feel good. It makes my property look the way I want it to be because that’s how I grew up or that’s how I perceive it should be. Some people plant them because they provide the shade and thus we can reduce the need for air conditioning. Conversely some people don’t like it because they get pollen. While I am allergic to tree pollen, I could not fathom a city with trees. Certainly some people, incorporate trees as they realize they increase their property value. I think aesthetics and shade are probably the two biggest reasons individuals plant yard trees.

What is the value of shade that trees provide?

In the warmer climates, many of us think about the cost of running our A/C units. This is a wonderful tool we have that makes our hot humid summers comfortable. Not only do shade trees shade reduce the temperatures inside our houses and thus lower your electrical bill, if you make sure that you’re A/C unit is shaded it will work more efficiently further saving you money. Additionally, trees and shrubbery can block the winter wind that hits your house during the winter and reduce heating costs. Finally, shade trees can lower the temperature in parking lots, and during the summer shaded parking spaces typically fill up first. Returning to a cooler car is always welcomed after shopping.

What do you teach?

I teach a junior level arboriculture course and then a senior level urban forest management course. The two courses approach trees from the individual property level (arboriculture) to the management of our green infrastructure at the municipal and regional level (urban forest management). I spend a fair amount of time teaching the growth and development of trees so the students understand how to manage trees in order for them to survive over decades and centuries to meet our landscape design goals. A lot of my research revolves around biomechanics, or how trees withstand gravity, or wind, snow and ice loading from storms. Hence a lot of what I do is teach the students how to prune and maintain trees so that they’ll be stable, they’re not likely to fail and cause damage.

I was hired by WVU to develop our arboricultural program. After input from industry representative, the Division of Forestry & Natural Resource decided to bring arboriculture to the Davis College. While many of my students may initially take my class as an elective for their Forest Resource Management degree, the come away with the knowledge that the arboriculture profession offers a lifelong career path. The can work as arborist with a local or national tree care company or the urban forest manager in cities throughout the country.

In the future, they might be those people who are hoping to make those decisions on where to plant trees in our cities?

Absolutely. Our alumni are already working as arborists in the Mid-Atlantic region and Ohio provide tree care and directly working with Mr. and Mrs. Smith to plant trees in front and back yards. One of my former students works with the West Virginia Division of Forestry, Urban and Community Forestry program. He works directly with our cities and communities to increase the state’s urban canopy as well sharing knowledge on how to manage our existing trees. This is the exciting thing about the Tree City USA study, we are all working together to help the citizens of West Virginia.