Updated: Aug 2, 2021
This is a narrative about our perception of trees and the little things we can do to make a big environmental impact. To this end, we provide a model to demonstrate the exponential relationship between tree crowning and tree carbon scrubbing capacity. The topics covered include:
I am a hiker. I regularly go on hiking trips throughout the United States. Locally, my daily exercise includes hiking in an area park. This includes a relatively wild and untouched Northern Virginia area called Difficult Run. I also grew up working for landscapers and nurseries. In my adult life, I have become an avid naturalist. Today, I have a great interest in plants and our natural environment. My travels have taught me that plant life, especially trees, serve 2 very different and sometimes conflicting purposes. That is, the Aesthetics and the Carbon Scrubbers. This article explores these different purposes and makes a case for how we can enjoy tree beauty in a way that supports our environment.
For me, the beauty of the tree is found in its native state. As a tree matures, it reaches the point of crowning. This may happen at a different age for each genus but is generally associated with a great opening. The great opening is where the adolescent tree‘s competitive and resource-challenged struggle yields to an opening of branches and leaves. The weight of its branches surrenders to gravity by physics and fractal mathematics, as related to its tensile strength, growth rate, and proportion. All the while, the beauty we cannot see underground is busy supporting the crown and communicating with its neighbor trees. This unseen beauty is the root system, silently growing to support the tree's size and in proportional volume. The tree’s roots are also supported by the mycorrhizal networks, also known as the “World Wide Wood.” These fungal networks pass information between trees, helping to protect and quietly pass evolutionary information to its neighbors. (1)
Photo Credit: Suzanne Simard
The Carbon Scrubbers
Trees serve a very important ecological purpose as they are the primary oxygen-producing plant kingdom system that balances the carbon-producing animal system. I am referencing the earth’s primary systems of Respiration and Photosynthesis. (2) Each leaf is like a solar panel, collecting sunlight and carbon dioxide from the air. The photosynthetic output is oxygen, as necessary to support human life. As the leaves and biomass of a tree increase, more oxygen is produced. If you have ever stood in a Redwood forest, you likely appreciate the strength of the most powerful natural oxygen producers in the world. The Redwood produces oxygen at such a high volume that a human being can tangibly feel the impact of the oxygen beneath its canopy. (3)
Photo Credit: Wesley Everhart
The relationship between The Aesthetics and the Carbon Scrubber
Tree crowning is also associated with rapid growth and leaf volumization. As a tree crowns, its total volume increases in geometric proportion to its vertical height. As such, a tree’s carbon scrubbing capacity explodes mathematically in concert with its crowning. As such, we can individually make a significant impact on global warming by encouraging the crowning of trees.
Crape Murder (4) is a detrimental pruning approach. This relates to the practice of top pruning the Crape Myrtle tree. According to Southern Living magazine,
“Crape Murder is often committed by otherwise decent, law-abiding gardeners.”
While the Crape Myrtle tree is often subject to this practice, many other tree genera are being “murdered” in this way. I suspect there are a few reasons:
The gardener wishes for a smaller plant so is severely pruning to meet those goals.
Landscape companies are selling this pruning approach as a way to increase revenue and at a significant cost to the plant and the environment. These costs are often obscured because of 1) difficulty in accounting for the economic externalities and 2) cost discounting associated with the “tragedy of the commons” economic concept. (5)
Homeowners are “going along” with the practice, assuming it is a standard as they see other homeowners doing the same and receiving a short-term visual benefit to the pruning. While a fully crowned tree has striking beauty, one must be patient as the tree goes through its more awkward sapling and adolescent years. It is well worth the wait.
Southern Living points out this practice is detrimental to the tree for several reasons. My perspective is this practice is negative as it relates to both the Aesthetic and the Carbon Scrubber.
In terms of Aesthetics, Crape Murder is comparable to the following “Procrustes Home” (6) metaphor. Let's say someone buys a Great Dane puppy for a 1 bedroom apartment. Full-grown Great Danes are one of the largest dog species but are relatively small as a puppy. Once the dog gets too big, the owner chops off its legs below the knees so it better fits the apartment. I suspect you may be thinking, this is an extreme and off-point comparison. Extreme? Perhaps. However, if you remove the cultural differences regarding how people consider Dogs and Trees, the comparison is fitting. With that said, my opinion is, I find great beauty in mature, fully crowned trees. I would expect to have enough foresight to plant a tree in anticipation of its mature size. This certainly does not suggest others may have differing opinions of what drives beauty. I’m sure they do.
In terms of the Carbon Scrubber, clearly, extreme pruning like “Crape Murder” is damaging as it removes carbon scrubbing capacity from our planet. Further compounding the negative impact of removing carbon scrubbing capacity is that 1) in the current time (the year 2020) the climate crisis is growing and 2) carbon emissions have cumulatively overwhelmed the plant kingdom's ability to process carbon via photosynthesis. Under the reasoning challenged category of “a failure of invariance" I do find it interesting when homeowners commit crape murder and, all the while, regularly recycling household waste. To me, this is akin to going to McDonald's, buying a double quarter pounder with cheese, large fries, a chocolate sundae …. and a diet coke. Somehow, the diet coke makes them feel more healthy after purchasing the other and substantially higher calorie, higher fat, and artery-clogging fast food. Just like recycling does not offset the significantly reduced carbon scrubbing capacity of a severely pruned tree, diet coke doesn’t stand a chance to offset the McDonald's nutritional damage.
To demonstrate the impact of tree pruning on carbon scrubbing capacity, we created a simple model (7) that measures the carbon scrubbing capacity of a full-grown, fully crowned tree with no top pruning. We compare this to 2 other trees with different levels of pruning.
The following table shows the model results.
Based on Height (Tall) and Spread of the average tree (8), we developed the average tree volume and common sized it to the carbon scrubbing capacity of an average leaf. We then applied the leaf carbon scrubbing capacity to trees that were either
pruned once, so it reaches half its height and associated volume, or
severely pruned, so it only reaches 1/3 its height and associated volume.
The volume math is important. Because of the non-linear nature of volume, reducing height and spread linearly has a geometric impact on volume. A related quote comes to mind:
“The greatest shortcomings of the human race is our inability to understand the exponential function.”
- Albert Allen Bartlett
Our brains are just not wired to intuitively understand non-linear relationships, and it shows!
To add some context to the tree type comparison, we liken the carbon scrubbing capacity of the different tree types to the average carbon emissions of combustion engine cars. A mature, fully crowned tree will scrub the annual carbon produced by 2 cars. A severely pruned tree will only scrub a very small portion of the annual carbon produced by a single car.
There are several simple ways we can help the planet. Individually, we may feel our efforts barely move the needle. However, we must start somewhere. If we all do a small part, collectively, it accumulates to a significant impact. I encourage you to be a leader. If you really want to help the planet, be a good steward by growing trees and encouraging tree crowning. Regular recycling helps at the margin and I am certainly not discouraging recycling. However, a substantial benefit accrues to a mature, fully crowned tree’s carbon scrubbing capacity.
(1) Listening to the Mother Trees, https://www.ttbook.org/interview/listening-mother-trees, To The Best Of Our Knowledge, An interview with Suzanne Simard. Also, for an excellent related TED talk by Dr. Simard, please see: https://www.ted.com/talks/suzanne_simard_how_trees_talk_to_each_other?language=en
(2) Photosynthesis and Respiration, https://photosynthesiseducation.com/photosynthesis-and-cellular-respiration/, Photosynthesis Education
(3) https://www.washingtonpost.com/national/health-science/majestic-redwood-forests-are-also-amazing-fighters-against-global-warming/2016/07/11/ae7ada8a-43be-11e6-8856-f26de2537a9d_story.html, Majestic redwood forests are also amazing fighters against global warming, Paul Rogers, July 11, 2016. By the way, if you have never been to a Redwood forest, it is highly recommended. The oxygen production is extraordinary and is best understood by experiencing it.
(4) https://southernlivingplants.com/planting-care/crape-murder/, Southern Living Plants Staff, August 6, 2019
(5) George D. Santopietro (1998) The Economics of Environmental Degradation: Tragedy for the Commons, Journal of Economic Issues, 32:3, 878-880, DOI: 10.1080/00213624.1998.11506090
(6) https://en.wikipedia.org/wiki/Procrustes, see reference to the mythical “Bed of Procrustes”
(7) This is a simple model. The volume calculation is based on that of a cylinder. A tree shape is similar to a cylinder, including the symmetric round shape. One may argue each tree species will have a different shape and produce a different number and leaf sizes, which is true. The point is, generally, the geometric volume has a non-linear relationship to its height and spread. As such, top pruning has an exponentially negative impact on the tree’s carbon scrubbing capacity. This is consistent with the mathematics of cylinder volume.
(8) Source for tree data (height, spread, carbon scrubbing capacity, average leaves / mature tree, and car carbon comparison): http://www.tenmilliontrees.org/trees/