The basic idea of sequestering carbon by planting trees is very simple. Trees take in carbon dioxide from the air, and they convert it (along with other elements from the soil and the air) into wood. The amount of carbon sequestered by a tree in a given year is simply the increase of the mass of the tree in a year, multiplied by the fraction of the mass of the tree that is carbon. The chemical composition of wood varies depending on the type of tree, but a good approximation for most types of wood is 50% carbon, 44% oxygen, 6% hydrogen, and trace amounts of various metal ions (see Petterson, USDA, http://www.fpl.fs.fed.us/documnts/pdf1984/pette84a.pdf). Since the atomic weight of carbon is 12, oxygen is 16, and hydrogen is 2, this means that about 42% of the mass of a tree is carbon. There is a net carbon sequestration only while the forest is growing – once the forest reaches maturity, as trees die they emit the carbon they sequestered back into the atmosphere. In steady state, a mature forest gives off as much carbon as it gathers.

Our approach will give credit only for the amount of carbon a tree sequesters per year. You contribute to Bo-Dog Carbon Lock Up (BD CLU) to plant trees, and we give you credit for all the carbon the tree will sequester for your desired term. This approach is justified by the fact that carbon dioxide persists in the atmosphere for several hundred years, so what is important is not how much carbon the tree will sequester immediately, but rather how much it will sequester over and extended period.  To estimate the amount of carbon sequestered, we used the tables provided in the United States Department of Energy document "Technical Guidelines for Voluntary Reporting of Greenhouse Gas Program", March 2006, (http://www.eia.doe.gov/oiaf/1605/Forestryappendix[1].pdf). To compute the carbon offset, it is first necessary to decide on the period of time you want. Estimates for the amount of carbon sequestered after 100 years range from about 75 metric tons/hectare for southwestern Ponderosa pine to about 200 metric tons/hectare for Northeastern oak/hickory. Under the assumption that available water and type of tree are the most important factors.  According to the tables, this type of forest fixes about 130 metric tons of carbon per hectare over a hundred-year period. Note that we are neglecting the fact that carbon sequestration is not linear, and that we are simply taking an average over the hundred-year period.

To see that this estimate makes sense, recall that a hectare is 100 meters by 100 meters, and a metric ton is 100 kilograms. Imagine a mature cottonwood bosque with trees on a grid at intervals of 5 meters. (Of course, BD CLU will not plant the trees on a grid, but we are just making an estimate!). This would correspond to 400 mature trees, with roughly three hundred kilograms of carbon in each tree. Since roughly 42% of the mass of a tree is carbon, this corresponds to each tree weighing about 0.7 metric tons, or about 1600 pounds.

How much C02 emission does this correspond to? The Department of Energy website http:/www.eia.doe.gov/environmental.html states that a gallon of gasoline emits 8.9 kilograms of CO2, which corresponds to roughly 2.4 kilograms of carbon. This corresponds to the carbon sequestered in converting1.8×10−5 of a hectare of land into mature forest. In other words, by planting a hectare of riparian forest, over the next one hundred years, one can expect to offset the carbon emissions caused by about 54,000 gallons of gasoline. If you make a trip of 1000 miles in a car that gets 20 miles to the gallon, you consume 50 gallons of gas, which corresponds to 50×1.8×10−5≈10−3 hectares of forest that need to be planted. This corresponds to a patch of forest that is 10 square meters, i.e. 3.5 x 3.5 meters. The resulting formula is

square meters of forest planted =0.18×(distance travelled)miles/gallon

BD CLU uses approximately 150 trees per square acre, or 370 trees per hectare. One hectare is 10,000 square meters. Thus, we plant 0.037 trees per square meter, i.e. we plant one tree for every 1/0.037 = 27 square meters. In other words, the number of trees that need to be planted equals the number of square meters divided by 27. This is the figure for the number of trees we use. 

Trees planted = 0.0667 x ((distance traveled)/(miles per gallon)).

This is the formula used in our calculators for individual travel. If you want to offset all the carbon you will produce in a year, BD CLU needs to plant enough trees to sequester 5 metric tons of carbon over the next 100 years. Based on the above estimates, the land area needed is 5/130 = 0.04 hectares = 0.016 acres, which translates into 2.4 trees. 

You might ask, "How does the typical American emit 5 tons of carbon? Surely I emit much less than that?" Unless you live in a yurt without electricity, grow all your own food, and completely disconnect from society, the answer is probably not much less. The typical automobile emits its weight in carbon every year, which is around a metric ton. Okay, maybe you don’t own a car and you don’t fly on airplanes. But still, if you heat your house with anything except solar energy, use electrical appliances, eat food grown on farms that use machines, or use manufactured goods of any kind, you are probably causing about four metric tons of carbon to be emitted every year. If you ride a bicycle, have a well-insulated house, take care to turn off your lights and appliances, and limit your overall consumption, you might only be responsible for three tons.

In conclusion, it is worth re-emphasizing that trees can never solve the problem of global warming. Americans emit on average 5 metric tons of carbon per person per year. To offset their lifetime carbon emissions, a person who lives 80 years would need to plant 400/130 = 3 hectares of currently unforested land. The total area of the United States is only about a billion hectares, and we have 300 million people. Thus, even if the whole country were bare, and we were able to reforest all of it, via tree planting we can only offset a small fraction of our carbon emissions. We need to take much more profound steps if we are to reduce our carbon emissions to a sustainable level. Nonetheless, in addition to all of its other important benefits for wildlife and water, BD CLU's restoration activities make a valuable contribution to reducing global warming.