Expert Q&A: David Nowak, U.S. Forest Service

How do you define an urban forest and the components of an urban forest?

I define urban forest as all trees within the urban landscape. Street trees, park trees, and backyard trees are just subsets of the woody vegetation that makes up the urban forest. i-Tree can subdivide the results by whatever category is most useful to the user. Within cities, that is typically land use: residential, park, open space, commercial, industrial, etc. People can use other categories, such as “ecological zones,” but if those categories are not of practical use to managers, they’re going to have a limited impact on affecting management.

How would you describe the state of urban forests in the US today?

Our studies showed that there was a decline in tree cover within cities from around 2004 to 2008. Of the 20 cities we studied, only one increased in canopy cover, while 17 decreased. Overall, we estimated that we are losing four million urban trees per year. However, nature cycles up and down, and two points do not indicate a long-term trend. We are currently finishing a state-by-state study of urban tree cover change in the entire U.S. from around 2009-2014, so we’ll know later this year what the next trend is. Hopefully we are not in a continuous downward trend.

What is your gut telling you?

You have two counteractive forces. On one side, you have development, aging trees, insects, and diseases, etc. On the other, you have replanting and regeneration. The combination of these forces through time will determine if tree cover goes up or down. In our original tree cover change study (c. 2004-2008) we specifically targeted two cities we thought would have large canopy losses: Detroit and New Orleans. In New Orleans, we wanted to see the impact of Hurricane Katrina. They lost about a third of their canopy cover.

In general, what is the return on investment in an average urban tree?

I can’t answer that. Even if you see a return on investment, it is going to be conservative. We’re good at quantifying the costs of urban trees, but we don’t yet know enough to quantify the true return from trees. Trees don’t give us cash back; the return is indirect, through improved health, reduced energy use, cooler air temperatures, etc. We have only quantified the monetary value of four or five of these benefits. We also need to value the more indirect costs associated with trees (e.g., pollen impacts on health).

My belief is that the ratio of return is greater than one if people are choosing to plant trees. Even if we do not quantify the environmental benefits, there are numerous social benefits derived trees and tree planting that people factor into their tree planting decisions.

With i-Tree, we’re trying to integrate all of these systems and responses together, so when you turn one cog, you can see how all the other cogs change. This integration can help us get to the optimal design for the forest. It’s like looking at a puzzle. Instead of looking at one piece of the puzzle, let’s put the whole puzzle together and see the larger picture.

Let’s talk about some of those puzzle pieces. Trees provide so many ecosystem services. As of today, which of those services can be quantified in terms of dollars?

There are four that we have analyzed: air pollution removal and effects on human health, carbon storage/sequestration, altered building energy use, and a secondary service from energy changes: altered power plant emissions.

We have an estimate for the water, but it’s very rough. We have most of the water flows down and the changes in water quality, but how do you value that? We use proxy values from control cost technologies, but we are hoping to find better, more locally appropriate values related to changes in stream flow and water quality due to trees. We are working with economists on that problem.

Right now, we are also working with collaborators at RTI International on how air temperature changes due to trees affect human health. We are modeling the temperature effects of trees and how they impact mortality and various health incidents.

Yes. It’s a three-step process. One is understanding your forest structure, which we can do. Two is understanding how the existing forest affects the UV index, which we can also estimate. The third step, which is the most difficult, is determining what that impact means in terms of dollars and human health. We need to make that link by looking at human health literature. We have been able to do that with air pollution using the EPA’s BenMAP model. I think we can do that with air temperature changes, and I hope we can do that for UV. There is a definite link between UV exposure and human health. Modeling that link by saying, “If the UV index drops by one, what does that mean in terms of the number of people who won’t have cataracts or skin cancer?” is difficult.

Are you collaborating with researchers in the fields of health and economy?

Our strength is coming up with the actual impact of trees on the ecosystem service: tons, degrees temperature, concentration, etc. When we go to dollars or health impacts, we must work with partners. With air pollution, for example, we work with the BenMAP modelers and for air temperature effects we have been working with RTI International. Collaborative research is critical.

Does the research suggest that any one of these services carries more weight than others among decision makers in urban communities, regardless of that community’s location, climate, or specific challenges?

When do you expect to have a dollar value associated with temperature effects on human health incorporated into i-Tree?

We have the temperature model built, and it has been through peer review. Now we’re working on the temperature effects, using Baltimore as a prototype. We will write a paper on that topic, which will also go through peer review. We hope to have the paper out later this year, but we are at least another year away from incorporating it into i-Tree. The temperature calculations in this model are pretty complicated. The calculations are mathematically intensive because they model hourly temperature changes due to trees across the landscape.

With i-Tree, building the science behind the tool and going through peer review is half the work. The other half is making it easy for the user to use. You can’t imagine how difficult it is to make something like this simple to operate. There are a lot of computations and data processing going on in the background.

Was Baltimore chosen as the prototype because it is one of the Long-Term Ecological Research (LTER) sites?

Partly because we are part of the LTER and we have a lot of field data from Baltimore, but also because Baltimore has LiDAR data. The LiDAR data, which provides object heights in the landscape, was used to develop a high-resolution cover map (e.g., trees, buildings, etc.) for the City of Baltimore by the University of Vermont’s (UVM) Spatial Analysis Laboratory. The data provide critical inputs to the air temperature model. We are also working toward developing an air temperature and health assessment in New York City, as we have a long history of collaboration there and the necessary input data for the model.

How valuable are urban trees in terms of carbon storage? How does that compare with the carbon storage capacity of trees in a natural forest?

Trees in a natural forest, per unit land area, will store more carbon, simply because there are usually more trees per acre. But if I could only plant one tree in the world, I’d plant it in an urban area because you can shade buildings, reduce temperatures, and alter building energy use. If you do that, and you get the secondary effect of reducing emissions from power plants. Trees in both urban and rural areas will give you temporary carbon sequestration as the tree grows. A tree in the city will also give you a reduction in carbon emissions.

In a talk you gave in Dublin, you mentioned the impact of urban trees on acoustic environment. I never thought about that. I was also surprised to learn that shaded asphalt streets can outlast non-shaded streets by a decade (Alabama Cooperative Extension). What do you think are some of the more lesser-known and underappreciated services—quantifiable and not–provided by urban trees?

Sound and wear and tear on infrastructure are two good ones. I was in Raleigh, NC after a major hurricane had taken down many trees, and I asked folks there, “What is the biggest difference you notice without the trees?” They said it was sound. Without the trees, they could hear freeways they had never heard before. I hadn’t even thought about that large of an impact of trees on sound. Other benefits that people may not understand are the energy conservation effects from shading buildings and altering winds, and the protection from UV radiation. The least understood and likely least appreciated benefits are the social and psychological effects—how our bodies react from seeing vegetation. Research on this subject by Roger Ulrich, and more recently by Bill Sullivan, has been fascinating. Researchers are exposing people to vegetation and measuring body reactions, in terms of chemistry and physiological responses. (See this website for details.)

Three tangible benefits that people generally grasp are temperature effects/shade, aesthetics, and wildlife.

Last summer, you coauthored a paper entitled “Where to plant urban trees? A spatially explicit methodology to explore ecosystem service tradeoffs,” which was based on research you conducted in Baltimore. Tell us about this methodology, and how it may help city planners, urban foresters, and others in their efforts to better manage urban forests?

The question we ultimately want i-Tree to answer is, “Where are you likely to get the biggest bang for your buck in terms of having trees provide a particular service?” We want users to first understand the issues that need to be dealt with at their site. Once we know that, we can start looking at the best species for that site that would help mitigate that problem. That requires asking, “Is the tree going to survive in that site?” and “Of the palette of trees that survive, which will give the greatest benefit?”

We have a tool called i-Tree Species, which is our species selection program that we will be enhancing over the next few years. It asks the users their location and which tree benefits are most important to them on a 0-10 scale. From this information, the program provides the species that best fit those requirements. We will be adding other site parameters to help best select species (e.g., species tolerances to salt, soil parameters etc.) to improve the selection process, but the ultimate species decisions should be done with local tree experts. i-Tree Species helps answer the “what species to plant” question.

To get to the “where to plant” question, we developed i-Tree Landscape and i-Tree Design. i-Tree Landscape integrates national cover data, population data, forest and human health risk data and tree ecosystem service data to help determine priority areas in the landscape to plant or protect existing trees. The finest resolution for Landscape data is now the census block group level, but we plan on increasing the resolution to 30-300 m in the future. i-Tree Design enables users to virtually plant trees on their property to see current, past and future benefits. We are working towards integrating Landscape information within Design to help users determine the best locations on their property to plant trees to enhance desired ecosystem services and values. Within four to five years, we hope to have i-Tree automatically develop management recommendations based on locally input data regarding what species and where to plant trees to maximize environmental benefits and values.

The urban forestry community now has a Ten-Year Urban Forestry Action Plan to guide the implementation and expansion of urban community forestry through 2026. The plan includes a companion piece on research needs, which are divided into six categories. In the category “Understand Ecosystem/Ecological Services,” one emergent objective is to “Study how urban forest structure and functions can best meet regulatory requirements.” Many of our readers are involved in projects aimed at improving water quality in urban communities and many of those projects are driven by stormwater management regulations. What do we know about the value of urban trees in terms of improving water quality?

Trees affect the flow of water in cities by intercepting rainfall, facilitating the evaporation of water from the canopy and the soils, and increasing infiltration rates into the soil. Trees also alter the amount of runoff reaching streams and thereby affect water quality.

We have been modeling the effects of trees and impervious surfaces on runoff that carries nutrients and pollutants into streams. Forests have a direct impact on runoff, but not as much of an impact as impervious surfaces. We’re finding that runoff reduction from approximately 10-15% tree cover is equivalent to runoff increases due to 1% impervious cover.

With i-Tree Hydro, we can model the effects of trees on hydrology including stream flow and pollutant loads. Using ‘event mean concentration’ data, we can also model how trees affect what is being carried by the runoff into streams through pipes or overland flow. Right now, with i-Tree Hydro, we can look at the effect of the whole forest on runoff in a given area or watershed, but we are not yet able to do this at the tree species level. We’re currently working with the Holden Arboretum to examine how different tree species may transpire water to help differentiate species effects and improve species recommendation in relation to water flows.

The harder part is getting to the question of crediting. How do you do that? What does a 1% canopy cover increase mean in terms of credit? What does one tree even mean? Should a tree in a riparian zone get a higher credit than a tree in a different location in the watershed? Once you go for regulatory credit, it’s a different ballgame in terms of monitoring, verification and enforcement. There are people working on this issue and i-Tree can and has provided information that help determine the tree’s impact.

An overarching recommendation in the research needs document was to expand the scale of the science. You and some colleagues just published a study in Urban Forestry & Urban Greening entitled “Residential building energy conservation and avoided power plant emissions by urban and community trees in the United States” which found that trees save our country $7.8 billion in energy use. Was that study in response to that guidance?

It is one thing to get a handle on the value of urban trees, but conveying that value is another challenge altogether. What advice do you have for practitioners who need to educate others-whether they are clients, partners, stakeholders or community members- about the value of ecosystem services provided by trees?

To get people to do something, you have to make it relevant to them. If it’s a municipality, it might be dollars. If it’s a homeowner, it might be energy use, aesthetics, or doing the right thing for their grandchildren. If it’s a client, it’s best to really know that client and their specific objectives. They might not care about some of these benefits. They might want to harvest timber or have some other main objective. Knowing what is relevant is essential. Human health and well-being is relevant to most everyone.

Speaking of harvesting timber…turning declining urban trees into products seems like a good option for a community that wants to establish a self-supporting urban forestry program. How viable an option is urban wood fiber processing for a community? Do you know of any successful municipal examples? 

In rural forestry, you often grow the trees to produce wood as a product along with producing clean water and other forest benefits. In urban areas, when a tree is discarded, it becomes a liability, and the focus is on getting rid of the waste wood. We have tons of waste wood that must be disposed of, and we often give it away!

Where, in addition to the U.S., is i-Tree used today?

We have over 120,000 users in over 120 countries. The biggest user base is in the U.S., but Canada, Australia, the UK, Italy, and China are also big users. Some tools, such as Species, Hydro, and Canopy-the photo interpretation tool-can work globally right now. i-Tree Design works in Canada and the U.S.  Landscape will only work in the U.S. because it is comprised of U.S.-based maps.

Can i-Tree be used as a tool to help project the costs of maintaining/managing urban forests?

It could be, but it is not right now. You can put costs in now, but we rely on the user to do that. With i-Tree, we can project forward the urban forest population and associated benefits. Given estimates of mortality and costs of typical management actions (tree removals, pruning, planting) we could project annual costs of the urban forest. I like that idea. I may add that to our to-do list.

What are some other new & emerging tools in urban forestry?

STEW-MAP [a USDA Forest Service program that studies a city’s or region’s environmental stewardship regime and creates publicly available maps and databases] is one such tool. UTC mapping is another, which creates high-resolution cover maps and a data layer which shows where trees are and potentially could be planted. One of our latest i-Tree tools, i-Tree Landscape, incorporates UTC data and numerous other maps to look at various ecosystem services and data within a user selected area to aid in a better understanding of the local forest. It allows users to see issues affecting the forest and local residents, and prioritize locations for management actions. We also just came out with a new phone app called MyTree, which allows you to enter local tree information on your phone and look at their value. Using your phone or computer, you can pin or enter the tree location, answer a few questions, and then it gives you a chart listing benefits of the tree.

Besides integration, we are also trying to engage kids in urban forestry. We’re working with Forest Service partners, Project Learning Tree and Casey Trees to develop teaching guides and student manuals so that middle school kids can use these tools as part of their classes.

What U.S. city/community do you think has best applied the latest knowledge about the value of urban trees to the way it has designed/developed/revitalized itself?

Several cities have utilized i-Tree in interesting ways. A few in the U.S. that come to mind are Pittsburgh, Milwaukee and New York City. Pittsburgh has their Urban Forest Master Plan, Milwaukee did a billboard campaign and used the data to develop an emerald ash borer strategy, and New York City has their MillionTreesNYC campaign.

i-Tree is just a tool. It gives people information that hopefully helps them do their jobs. It’s the people, like those in Oakville, Pittsburgh, Milwaukee and New York, who really make a difference by using it. People have been very innovative, both in using the information to create master plans and in getting the information out. For example, Minnesota DNR did a “Trees Pay Us Back” public education campaign that involved placing price tags on trees.

Is there anything in i-Tree that can help planners, designers, and decision makers get a better sense of which trees in which locations are likely to be the most resilient in the face of climate change, and things like invasive species or species migration?

We are currently working with Louis Iverson [USDA Forest Service landscape ecologist] on a paper that looks at which plant species are likely to do better or worse in future climates. In trying to make recommendations to users on species, there are three things we are looking at. One is insects and diseases: where do they exist and which tree species are susceptible to them? Two is invasive species: is it on the invasive list? Three is climate change: are the trees likely to do better or worse 50 years from now based on what the projections show? That is what we are working on now. We’re also working on incorporating climate change maps from 2020-2100 into i-Tree Landscape. Once we do that, you’ll be able to see which areas are likely to get warmer, wetter, or drier based on climate projections, and that will inform management recommendations. We are also finalizing surface temperature maps across the U.S., which will enable us to see where the hot spots are in this country.

One of our goals is to integrate the five big factors that are going to drive change in the urban forest: development, climate change, insects, diseases, invasive plants, and fire within i-Tree. We developing a system of tools to aid managers in selecting proper species and locations to sustain healthy and functional forests that enhance human health and well-being for current and future generations.

What do you think is the most pressing need for research related to urban forests?

First, I’d finish this integration work that we’re doing now, so we can get to the tradeoffs I discussed earlier. After that, I’d move to the social/psychological research because that is where the gap is. To me, the top five benefits of urban trees are related to: 1) temperature/energy use; 2) social/psychological effects; 3) air pollution; 4) water flows and quality; and 5) climate change. I’d like to know how to integrate into i-Tree information on crime effects, physiological responses and commercial impacts of green landscapes, and sound propagation effects.

Any final words of advice for our readers?

Check out the i-Tree tools and try them. It’s an open process. If there are any issues or questions, just let us know. Many of our ideas come from the people who use the tools. We talk to engineers, planners, and homeowners all the time who ask, “Can you do this?” If it’s a good idea, we put it in the queue and discuss it as a group.