Skip to content

The Tree is Not Trying to be an Asset

Post date:

Everybody likes trees. This is a tree (in the front.)

IMG_0039It’s an overcast day, and almost evening so the tree’s not doing much in the way of photosynthesis or the other stuff that trees do incidentally as a result. Here’s a list of that other stuff from The Encyclopaedia of Earth.

Mitigation of heat islands effects is something we knew about already, if not necessarily from Paul Gut & Dieter Ackerknecht’s “Climate Responsive Buildings” (1993). What I didn’t know was that the rates of cooling for different tree species had been quantified. The given figures represent the reduction in radiation intensity compared with the unshaded situation.

tree cooling factor

Removal of air pollutants we know of. It’s a good thing. Did you know that

  1. In 1994, trees in New York City removed an estimated 1,821 metric tons of air pollution at an estimated value to society of $9.5 million?
  2. Standardized pollution removal rates differ among cities according to the amount of air pollution, length of in-leaf season, precipitation, and other meteorological variables?
  3. Large healthy trees greater than 77 cm in diameter remove approximately 70 times more air pollution annually (1.4 kg/yr) than small healthy trees less than 8 cm in diameter (0.02 kg/yr)?
  4. Air quality improvement in New York City due to pollution removal by trees during daytime of the in-leaf season averaged 0.47% for particulate matter, 0.45% for ozone, 0.43% for sulfur dioxide, 0.30% for nitrogen dioxide, and 0.002% for carbon monoxide?

CO2 removal is just what trees do. It’s good for them, good for us. Trees don’t see it as “removal” though.


Mitigation of stormwater runoff is something we’ve heard of too. I’d have thought they did this by increasing the % of humus in the soil but Encyclopaedia of Earth claims that

evergreens, conifers, and trees in full leaf can intercept up to 36% of the rainfall that hits them.

I never thought of that. But how much and for how long?

3893367448_368303cf86_zQuality of Life is subjective and difficult to quantify but, especially when property values are concerned, it won’t stop people from trying.

  1. Trees and vegetation can help reduce noise, which may be highly valued in urban areas.
  2. They also provide shade from harmful ultraviolet radiation, particularly in playgrounds, schoolyards, and picnic areas.
  3. In addition, trees and vegetation may increase property values, as several studies have shown that home values are higher on tree-lined streets.
  4. Lastly, community gardens and neighborhood parks can
    1. help reduce physiological stress,
    2. aesthetically improve an area, and
    3. provide an urban habitat for
      1. birds,
      2. animals, and
      3. insects.

All good, although from first-hand experience I can say that a thriving urban habitat outside one’s bedroom window is not always a blessing at sunrise.

Effects on volatile organic compounds would be similar to CO2 I thought but no. Trees are the bad guys because they produce VOC’s such as monoterpenes and isoprene that are responsible for those smells such as fresh pine and cut grass, but also help to form ground-level ozone which is a major component of smog. OMG! It’s no problem if there aren’t any nitrogen oxides but, since NOxs are formed by the combustion of fossil fuels, this is actually a problem. The US EPA forecasts a 5% global increase in NOx emissions between now and 2020, mainly due to agriculture, of all things.


Nitrous oxide molecules stay in the atmosphere for an average of 120 years before being removed by a sink or destroyed through chemical reactions. The impact of 1 pound of N2O on warming the atmosphere is over 300 times that of 1 pound of carbon dioxide.


Indirect energy use also isn’t something I’d have thought of, but trees in urban areas don’t just grow there on their own. They require guys in trucks to go around and water them and prune them and other people to rake up leaves and clean up after them. In return, trees do a lot for us and that’s why efforts to quantify and monetize their value are needed. People and municipalities have been known to chop down trees rather than pay people to prune them.  

Reduction in energy use of course refers to the reduction of energy use in buildings that have trees nearby. Quantifying this is a growth area and James Simpson has developed a simple method for doing so. His method provides a % value for energy use that can of course be easily converted to $.

simpson tree energy calculator

Data isn’t given, but one study by the Lawrence Berkeley National Laboratory (LBNL) and the Sacramento Municipal Utility District (SMUD) placed varying numbers of trees in containers around homes to shade windows and walls. Savings in cooling energy ranged between 7% and 40% and were greatest when trees were placed to the west and southwest of buildings. Another study found that with a 20% tree canopy – roughly equivalent to planting one tree to the west and another to the south of a home – buildings could achieve annual cooling savings of 8% to 18% and annual heating savings of 2% to 8%. Plantimg trees to the west and south seems to be the conclusion. This clearly isn’t rocket science.


And there’s no reason why it should have to be. The problem is that we’ve become so detached from our environment that we can only understand what’s best for us in terms of economic value rather than deducing it directly from our (now largely lost) experience. The people living in those houses in the image above don’t need to be told of the benefits of trees or how much more comfortable their living spaces are as a result. Trees are as much a part of their life as a window, a door and a courtyard. In fact, the tree, window, door and courtyard all function as components of the same “cooling system” that has no cooling bill and thus needs no reduction in one to justify the presence of the tree.

I think most of us appreciate the many things trees do. In principle, it won’t hurt to know the dollar value of trees but our world is one where all kinds of value get reduced to this same metric of worth. Seeing everything in terms of economic value is still a modern disease, even if people and communities might be a bit more prepared to plant trees, care for them and pick up after them once they know exactly how much in it for them.

i-Tree steps in to address this unfortunate truth. As with many modern products, it’s difficult to tell if it’s part of the problem or the solution.


Within the i-Tree software suite, street tree populations are assessed using i-Tree Streets, which is an analysis tool for urban forest managers that uses tree inventory data to quantify the dollar value of annual environmental and aesthetic benefits: energy conservation, air quality improvement, CO2 reduction, stormwater control, and property value increase.

It’s an easy-to-use, computer-based program that allows any community to conduct and analyze a street tree inventory. Baseline data can be used to effectively manage the resource, develop policy and set priorities. Using a sample or an existing inventory of street trees, this software allows managers to evaluate current benefits, costs, and management needs.

* * *

To close (for it is the end) – a poem, in memory of a third way of relating to trees.

Tree At My Window by Robert Frost

Tree at my window, window tree,
My sash is lowered when night comes on;
But let there never be curtain drawn
Between you and me.
Vague dream-head lifted out of the ground,
And thing next most diffuse to cloud,
Not all your light tongues talking aloud
Could be profound.
But tree, I have seen you taken and tossed,
And if you have seen me when I slept,
You have seen me when I was taken and swept
And all but lost.
That day she put our heads together,
Fate had her imagination about her,
Your head so much concerned with outer,
Mine with inner, weather.


  • This is a gold mine of information I’d always wondered about quantifying. Excellent work, thank you.

    One interesting thing that isn’t on the list though. In planting trees in urban spaces, the plants tend to need MUCH more root space than they are given at planting. This leads to a few stability issues, as well as premature death of branches and trees, and additionally doesn’t make as much use of the trees potential in catching run off (and so decreasing urban flash flooding). I found a reference in an oldish landscaping architecture book a year or so back. will see about digging it up tommorrow 🙂