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  • Writer's pictureEdie Sonne Hall

Cookie Jar Analogy

It is surprisingly difficult to explain carbon sequestration in forests. Trees grow (at different rates) over space and time, and when they die (or are harvested) the carbon in the wood is released (either by decay or combustion) at different time scales. Terms like carbon flux, carbon stock, carbon sequestration, carbon storage are tossed around and create a headache for all trying to understand.

First, as background, let’s remind ourselves how trees and carbon are related. Yes, we all know the equation of photosynthesis- but do we REALLY understand that the weight of a tree actually comes from carbon dioxide in the air?

Figure 1: Photosynthesis

C6H12O6 is also known as glucose, which then goes on to form cellulose, which makes up cell walls of plants and trees. When a tree dies and starts decaying, or when a tree is burned, the carbon stored in the wood is released, mostly in the form of carbon dioxide.

I decided to ask my kids to try to tackle the challenging task of explaining the relation between tree growth, harvest, and mortality and carbon terms such as carbon flux, carbon stock, carbon sequestration and carbon storage. I gave them a diagram from a 2016 Forest Service report describing carbon sequestration and storage in 2011.

Figure 2: Net sequestration in US forests with attribution (from USFS GTR NRS-154)

What? You don’t understand the figure? The circle that says “FRF FCS” is the change in carbon in the forest (Forest Remaining Forest) between time 1 and time 2 (in this case 2010 and 2011). It is positive in both the Western United State and Eastern United States, which means that there is net carbon sequestration (i.e. more and/or bigger trees on landscape in 2011 than 2010). The other circles are trying to show attribution- i.e. how much was due to trees growing versus loss from fire or harvesting. The circles that say “growth” show how much total tree growth occurred (in Western or Eastern United States) during one year. The smaller “fire” and “cutting” circles refer to emissions from either fire or harvesting.

My kids tried to put this diagram in simpler terms. The 10-year old drew a wooden barrel. She showed tree growth going into the barrel and tree harvest and tree death going out of the barrel. The line in the barrel is how much additional [carbon/water/wine/apple cider- you pick] stays in the barrel between time 1 and time 2.

Figure 3: 10-year old drawing

The 7-year old drew something related to candy. I think some people are happy because they got candy and others are sad because they did not.

Figure 4: 7-year old drawing

The 12-year old described an analogy with a cookie jar. He said that everyone likes cookies at grandma’s house. When grandma bakes cookies (tree growth) there are cookies in the cookie jar. People eat the cookies and have energy and happiness (tree removal to make wood products). If they don’t eat the cookies the cookies go bad (tree death). The more cookies that are eaten the more that grandma is motivated to bake (she likes visitors!)

Figure 5: 12-year old drawing

Here is what the cookie jar analogy has turned into:

Figure 6: Cookie Jar Analogy

Figure 7: Cookie Jar Analogy rendition of Figure 2

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1 Comment
May 20, 2019

The cookie jar analogy is growing on me (ha, ha). The traditional model is a bathtub--the water coming out of the tap is growth, the water down the drain is, well, "drain," from fire or timber harvest. The level of water standing in the tub (the forest carbon inventory) can be a little or a lot, depending on how fast the input is v. the drain. If growth equals drain then the level is at least even, whether shallow or deep. What I like about the cookie jar is it's more analogous to a biological system, subject to decay just as a forest is subject to fire, insects, and other natural forces. A tub of water, not so much. Plus…

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