Friday, April 26, 2013

DC Tree Canopy Gets a "B-" from Casey Trees

by Duncan Gromko

Tree Planting (Source: Rosser1954)
Every year, Casey Trees grades progress on protecting Washington DC's tree cover. Based upon four criteria (tree coverage, tree health, tree planting, and tree protection), the city received a B- for 2012.

Before I get into the details of the report, a little background on trees in urban areas and Casey Trees.

While forests in remote areas are hugely important, urban trees play a critical role in improving well-being for city dwellers. From Casey Trees: "Most Washingtonians know that trees cool streets and our homes, but energy savings is just the start. Trees slow storm water, clean our air, increase property values, and create a less stressful environment that benefits human health. In short, trees make cities more livable." Among these many benefits, mitigating the urban heat island effect is probably one of the most important contributions that trees make. On average, cities are 2-5 degrees (Fahrenheit) warmer than rural areas. For DC residents that have experienced our brutal summers, imagine how much worse they would be without trees in the city! Another benefit that I want to highlight is absorbing water, particularly during storms. In Washington, for anyone who lives in Bloomingdale (or other low lying areas), the flooding you experience is directly related to the lack of tree cover (and high proportion of impervious surfaces).

There's a lot of research done to put a dollar sign on all these values. In New York City, the 600,000 street trees provide an annual benefit of $122 million, which is five times the cost of maintaining them. In Portland, trees increase property values by $1.1 billion - maintenance costs are an annual $4.6 million. Presence of street trees in east Portland added an average $8,870 to the sale price of a home.

In Washington, the total tree coverage is 36%. A large portion of tree canopy coverage comes from Rock Creek Park, but this statistic also counts the many street trees and smaller parks throughout the city. While more than one third may seem high, in the 1950s the canopy was 50%. The loss in tree canopy is mostly due to urban development and pests and diseases that kill trees.

Compared to other cities, 36% is pretty good. Of the 20 biggest US cities, only Albuquerque, Atlanta, Nashville, and Pittsburgh had greater canopy coverage than DC. Surprisingly, Denver, with less that 10%, has the least canopy coverage. Another important statistic is a city's impervious surface cover because it predicts how much storm water will be absorbed into the ground and how much ends up in the sewer system. Roads, sidewalks, roofs...basically anything concrete is impervious. 41% of DC is impervious. New York City has the most impervious cover with 61%, while Nashville has the least, with 18%.

One more background paragraph before I get into the report. Casey Trees is a non-profit in the city with the goal of increasing tree canopy to 40% (this goal has also been endorsed by Mayor Gray's Sustainable DC Plan). In addition to producing reports like the Tree Report Card, Casey Trees does actual tree plantings and lobbies the city to protect trees. Participating in the tree plantings is how I first got involved with Casey Trees and, on a personal level, digging in the dirt on a beautiful DC day is one of the most pleasurable ways to spend a Saturday morning. If you live in DC, I recommend checking out Casey Trees.

Now for the report. I'll go through the four categories one by one. Casey Tree gives each category an individual grade, which adds up to the overall grade.

For tree coverage, the city got an A-. 36% coverage is pretty good, and not far away from Casey Trees's goal of 40%.

For tree health, Washington got a B-. The biggest threat to trees' health is invasive insects, and mostly the Asian Longhorned Beetle (ALB). Up to 35% of Washington's trees are vulnerable to the ALB. The beetle has plagued Illinois, Massachusetts, and New York, but has not yet been found in Washington. However, given the proximity to other infected areas, it's likely that the city will be infected. The city currently has no plan to deal with ALB, which is a primary reason for the lower grade.

In tree planting, the city got an A+. 10,404 trees were planted in Washington in DC in 2012. Considering that, in order to meet its goal of 40% total tree cover coverage, the city must plant 8,600 trees per year, this is one area for celebration. Of those 10,000+ trees, Casey Trees planted 15%. The National Park Service planted 27% and the Urban Forestry Administration, of the Department of Transportation, planted 26%.

By far the worst grade is in tree protection, for which the city received an F. Clearly, this is Casey Trees' main gripe with the city government. There are three reasons for this poor grade. 1) It's too easy for property owners to remove trees. Under the Urban Forest Preservation Act (UFPA), a property owner has to give a convincing reason to remove a tree. Yet most property owners that applied to remove trees were granted the request. 2) Lost trees are not being replaced. The UFPA requires that trees that are removed be replaced. However, Washington does not monitor the survival of replacement trees so there's no way of knowing how successful this effort is. 3) Money from the Tree Fund is not being used well. When a property owner illegally removes a tree, he/she must pay a fine to the Tree Fund (which is used to plant more trees). Again, the city is not monitoring the health of trees planted through the Tree Fund.

What's to be done? Washington faces a challenge to preserve its tree canopy while population growth in the city places greater pressure on trees. If the Mayor's Sustainable DC plan is properly implemented, then 40% tree canopy is a possibility. However, there are few specifics supporting the Sustainable DC plan - it's more of an aspiration statement than an actual policy - and it's likely that tree cover in Washington will continue to decline, unless Casey Trees' recommendations are adopted.

It's a tough balance for the city, which wants to increase its tax revenues and provide space for incoming residents. While the rest of the country struggles with the recession, the federal government makes Washington relatively recession-proof; Washington has received more new denizens than any other US city for four straight years. This places huge pressures on the city and its tree cover. But if Washington and Mayor Gray want to protect quality of life and the long-term attractiveness of the city, they must balance these pressures against protecting the canopy.


  1. I love city level statistics because its one of these levels of analysis where individual actions and the broader environment are so clearly linked. For example, I just made it 10,405 trees. Do you know where people could find city level statistics on tree canopies for other cities? I'm sure other readers are curious how their home town stacks up (I see a GIS project being born).

    One thing I was curious about was the concern about pests. One random question I have is how we are supposed to prevent them: insecticides? Another related question, which is perhaps a broader question, is why pests killing trees are bad. I realize the benefits of a tree canopy, but I always am a little uncertain about the preservation dimension of environmentalism in part because I think nature has natural ebbs and flows, call it the creative destruction of the ecosystem. One place this always comes up for me is forest fires. We do everything we can to prevent, which make sense in terms of immediate safety, etc. etc., but, from my admittedly limited knowledge on the subject, occasional forest fires are an important of re-fertilizing forest soils. Now that's not to say that all destruction of the environment by other parts of the environment are 'creative' or good (see the atmosphere and the ice age, or, conversely, the human species pressure cooking the planet like pulled pork), but its always something I have difficulty taking into account. Deserts have been forests, mountains have been river beds and ecosystems evolve. In the case of something like global warming it is clear, we are statistically well beyond natural fluctuations, but in other areas I am always less clear. So I guess the question is a bit more epistemological: how do we know when do we need preservation rather letting the environment take its natural course? How should we think of the creative destruction of ecosystems and how do we measure when it is more destructive than creative?

    Sorry for the long tangent, suppose I'm a little out of it from walking on those shade-starved streets.

  2. Blue,

    These are some really good questions - let's see if I can help you with any of them (and here goes my long tangent):

    1. Casey Trees is not the only organization that does grading for environmental areas and issues (i.e. - Chesapeake Bay Foundation's "State of the Bay"), but we are currently the only city with a formal grading and publication. We're hoping to use our Tree Report Card to help other jurisdictions fulfill this need, and there are some working on it. Mark Buscaino elaborates on this point in our #TreeTalk on the TRC:

    2. In response to the growing concern over their potential impact here in D.C. of harmful pests, we're taking steps towards putting together a variety of resources - we just held a "CSI for Bugs" class last week with renowned entomologist Dr. Mike Raupp of the University of Maryland and will be hosting a online chat (#TreeTalk) on "High Consequence Pests & Insects" this coming May 9th - you should be able to get your specific question of prevention answered there, and there will be a host of resources mentioned for future research. More info:

    3. In terms of your broader question about the preservationist-dimension of environmentalism, you make a great point about nature's interconnected balance - in a traditional setting, these "ebbs and flows" do exist and are imperative to the health of the forest as well as the overall ecosystem. However, key to remember here is that the pests we're discussing are invasive species and shouldn't be considered a "natural" part of that environment. Their presence in actuality could disrupt that balance, taking down entire populations of trees well before their mature benefits have been reached and without a way for our (or other) "city ecosystems" to handle such loss in a positive, reproductive capacity.

    Does this begin to answer your questions? I hope so - if not, feel free to continue asking, whether here on Natural Capital or on our social media platforms (Facebook: || Twitter: Check out our website, too - we're constantly rolling out new resources: