This past semester, I took an Arboriculture class, and it was great - I learned SO much. As part of the class, we had to pick a tree we felt had some structural or health issues and write a report on it. It was a cumulative project; we had to know how to measure the health of the tree, how to do risk assessments, how to calculate value if the tree was to be replaced, etc., all of which sounds fairly simple but each step sort of builds on the one before. It was fascinating. I chose the valley oak in the photo above, for my project.
The benefits of trees, particularly in an urban setting, are obvious and numerous: Trees save energy because they provide shade, they intercept stormwater and hold it on the property, they increase property value, and they provide benefits for local wildlife. There are many studies, both recent and ongoing, that show trees can improve the actual physical health of humans. They can intercept allergens and filter air, for instance, helping with breathing and air quality. Mental health and trees is also being studied. It's hard to quantify, but they can improve mental outlook and provide a very real sense of solidity and peace. Trees are also beautiful, a value hard to measure, but one we wouldn't want to live without.
Another unseen way trees improve our environments is by sequestering CO2, which they use for making food through photosynthesis. Then they take those foods and store them deep in their root systems, in the soil. This kind of storage of carbon, or sequestration, is very important to the health of our planet. I won't go into the 'why' here, but if you're interested, see this article by the USGS, which explains it nicely.
It's rather interesting to take the trees on your property and determine how much carbon they are sequestering every year. There are several ways to do this. The industry standard is a tool called CUFR, which is provided by the US Forest Service. They have an excel spreadsheet which will do this calculation for you. I had a little trouble with this interface, so I went searching for another way. I found another online calculator which is good if you have a common (not unusual) tree, called National Tree Benefit Calculator. You simply enter your zip code, then choose the kind of tree you have, and enter the diameter of the tree.
***A word on diameter: This is a trunk measurement you take at breast height, about 4.5 feet up from the soil. It's called 'DBH' or 'diameter at breast height.' You'll see this in all the calculations; it's an important measurement and helps us to know the age of the tree. And remember (I needed a refresher course myself), diameter is the length across one side of the tree trunk; radius would be half that; circumference would be the length all the way around. (Hey you math people, stop giggling.) So for diameter, just take your tape measure or yardstick or ruler out to the tree and estimate about four and a half feet up from the soil, and then just put your ruler up to the trunk and see how wide it is.
For those of you with multiple tree trunks at breast height, you'll need to do a different calculation (I'm thinking of your Coast Live Oak, Mom and Dad): please see this nifty explanation by the City of Portland. This also explains how to measure if your tree is on a slope.***
The Tree Calculator website is interesting, because it gives more data than just CO2. When you put in your info and it does the calculation, first you will see a pie chart overview; if you go to each tab above that, it will tell you more about each single factor. For instance, when I put in my oak tree, and I then hit the CO2 tab above the pie chart, it reminds me that "most car owners of an average car drive 12k miles generating about 11k pounds of CO2 every year." That's very helpful in context. According to them, my oak is sequestering 660 pounds of carbon per year, but I'm putting a much larger amount into the atmosphere by driving my car. I would need 20 oaks of this size to make up for the emissions from my car.
So you start to see why trees can be very important even in unseen ways.
in full leaf, in September, a beautiful canopy
For my project, I wanted to actually DO my own calculation, so I found a research paper which explains how. For this calculation you will need to know the DBH and the approximate height of the tree, which can be difficult to estimate. I usually try to look at buildings near by, as one story is usually about 10 feet. I estimated my oak to be about 50 feet tall.
Click on the box below to get a printable of how to do this calculation. It looks daunting but please don't let that dissuade you - I am the least mathematical person in the world and I was able to do it. It explains the 'why' of every step, but unless you really want to know that, you can just skip down to the bottom of each paragraph to learn how to do the next calculation.
With this process, I determined that my oak sequesters 488 pounds of carbon per year, a bit less than the online calculator said. I think this is because the physical calculation only takes into account the age and size of the tree, whereas the online calculator takes into account the type of species and where it grows, which is important for several reasons. Certain species sequester more carbon - generally trees that grow slower are the ones who store the most. (Side note - slow growing trees are also stronger and live longer.) Plus how the tree is adapted to your climate is important, hence the reason you put in your zip code for the online calculator. If the tree is native to your area, it's naturally going to grow better and store more carbon. For instance, you've already heard the details about my oak tree, which is native here and well-adapted to this climate. However I have a large southern magnolia on the property (Magnolia grandiflora) - it's not quite as large, but it is significantly older than the oak. When I put that tree into the online calculator, I find it's only storing 180 pounds of carbon per year. I would be willing to bet that a similarly sized magnolia in Georgia would be sequestering much more carbon than it is here in California. It's also interesting to see the difference between these two trees when looking at the other factors: the oak is intercepting and storing much more rainwater (very important here in our dry climate), it's doing a better job of cleaning my air, and it raises my property value quite a bit more than the magnolia. I didn't plant either one of these trees, but it sure helps me decide which one I'd rather have going into the future on this property.
This would be a fun exercise to do with kids, leading naturally into a conversation about how the trees on your property are improving it. It's also worthwhile to spend some time observing the tree and how the wild creatures use it (I spent so much time watching the songbirds in my oak that it made me realize what a value it is to them, not just to us). You can discuss how the tree increases habitat for many different creatures, providing food for some, shelter for others. You can discuss how the tree shades the house, decreasing the need for air conditioning. If the tree is native, you can read up on how the native peoples would have used it hundreds of years ago. You could take the leaves and make some art with them.
But even if you don't have kids, it's worthwhile information to know and absorb. We often pay little mind to the trees on our property, unless there is a problem with them. This helps us appreciate what they are doing, in seen and unseen ways, to improve our immediate and greater environment, every day.