Can't see the forest for the carbon absorption rates

July 24, 2006 6:49 PM

I'm trying to figure out carbon absorption rates by trees. So far the best source I have is an eBay ad.

I understand that this all depends on what kind of tree and any number of factors. But I haven't even been able to get very good numbers on how many trees it takes to offset 1 ton of CO2 or, conversely, how much CO2 100 trees would offset. This eBay ad suggests that three trees can offset 1 ton over the next ten years. How do I find better data than that? It's for a research project on the development of sustainable timber farms.

posted by andifsohow to science & nature (7 comments total)

Might get a little closer searching on "Carbon Fixation" instead.
posted by Good Brain at 6:52 PM on July 24, 2006

...or carbon sequestration.
posted by shoesfullofdust at 7:08 PM on July 24, 2006

As you are aware, there is no one answer, given that the amount of photosynthetic surface area on a "tree" can vary by several orders of magnitude depending on tree size and species, and fixation depends on resource availability, most notably sunlight and water, as well as temperature, which can also vary hugely depending on where the tree happens to be. And measuring whole-canopy gas exchange is still a technique in its infancy.

However, the Greenfleet project suggests that 17 native Australian trees will absorb the CO2 your "typical" car will produce in a year - 4.3 tonnes.
posted by Jimbob at 7:18 PM on July 24, 2006

Well, this is how I'd figure it: trees turn CO₂ and H₂O into cellulose (which google tells me is C₆H₁₀O₅) and oxygen. What a tree does is approximately this reaction, though it be convoluted and mediated by life:

6CO₂+5H₂O => C₆H₁₀O₅ + 6O₂

Atomic weights are (very round numbers): C=12, H=1, O=16. To produce 1mol of cellulose (162g) requires 6mol of CO₂ (264g) and 5mol of water (90g) and releases 6mol (192g) of oxygen.

Therefore, you can figure out the quantity of CO₂ absorbed from the mass of the tree. 1000kg of tree soaked up 1630kg of CO₂... all that remains is to figure out how fast your trees grow and how dense they are. Different trees have very different growth rates and wood densities, but you knew that. You can probably google the density of various species and their growth rates in terms of height and girth and go from there.

Though nitrogen is an important component of vegetational growth, I'm assuming that it is present only in small amounts (<1%) and I'm fairly sure that's a decent assumption. Likewise all the other trace elements.
posted by polyglot at 12:01 AM on July 25, 2006

It should also be pointed out here that it is important to consider what happens at the end of the tree's lifetime: they are only a temporary sequestration mechanism. When the tree dies, the carbon it has sequestered will be released again into the atmosphere, whether it be by combustion or decomposition.... unless we have an invasion of mad carpenters and significant quantities of consumer items are made of wood :)

Because we're digging up & burning the compressed trees of millennia past, we're increasing the amount of carbon in the biosphere. Planting trees only makes it solid instead of gaseous and for only as long as those trees exist. I'm assuming here that the rate at which we form peat bogs is negligent... growing forests and burying them would effectively sequester the carbon until someone digs it up for oil in a few million years because all the uranium has run out.

The flipside of this "inadequacy" of trees as a sequestration method is that burning wood for heating purposes has zero net release of CO₂ - they're only releasing what they absorbed while growing. If you grow wood and burn it cleanly for heat, you're using trees as chemical batteries for solar energy.
posted by polyglot at 12:11 AM on July 25, 2006

Polyglot provides the numerical method for quantifying carbon sequestration. You just measure the rate that a tree grows, estimate the volume, then weight of wood and crunch the numbers.

A couple of other things to keep in mind. Live wood is about 40% water. Assuming 60% is cellulose you need to adjust the 1630 kg number polyglot gave -- so rounding, 1000 kg of tree represents roughly 1000 kg of CO2, an easy number to remember.

Second, be careful when you gather numbers from the links recommended by Shoesfullofdust and Good Brain. Biologists often speak more strictly of the carbon cycle so they will give sequestration numbers in terms of tons or kilograms of carbon per year, not carbon dioxide. 12 mols of carbon in a tree uses up 44 mols of carbon dioxide, so multiply the tons of carbon by 3.67 to get tons of carbon dioxide.

Last, the rate of sequestration varies over the life of the tree. Larger trees make more wood per year than smaller trees. The rate of growth is a curve that starts low, peaks at 10 to 70 years, depending on species, and then slowly declines.
posted by JackFlash at 8:10 AM on July 25, 2006

Try Rattan Lal at Ohio State University.
posted by UnclePlayground at 10:25 AM on July 25, 2006