How much carbon does our forest sequester?
The cover story in the current (Spring 2008) issue of OnEarth magazine, called The Giving Trees, includes some important information about the value of intact, mature forests. The author, Sharon Levy, describes something called the eddy flux method of measuring the flows of carbon dioxide and water vapor into and out of forests. Devices mounted on towers in forest stands measure winds and gas exchanges at incredible levels of detail and accuracy.
For anyone who might be a little fuzzy on the chemistry involved here, Levy offers a brief refresher course:
Plants take in CO2 and harness the energy of the sun to drive the chemical reaction that melds carbon with water, producing the substance of stem and leaf and releasing oxygen. When darkness or drought bring this process of photosynthesis to a halt, plants respire, just as humans do. That is, plants breathe in oxygen and exhale CO2. But over the long life span of trees in an undisturbed forest, huge reservoirs of carbon are stored for great stretches of time in the organic matter in soil as well as in living wood.
Most relevant to Plummer’s Hollow, Levy describes measurements of the intake and storage of carbon done at the Harvard Forest, in Petersham, Massachusetts, starting in 1989. The stand that scientists measured, predominantly an oak-maple forest, had been flattened by a hurricane in 1938. In the first year of the study, the 50-year-old forest was absorbing 0.8 tons of carbon per acre per year.
Previous calculations by ecologists had suggested that a forest of that age should be reaching its maximum ability to absorb carbon, but measurements at the Harvard Forest 15 years later showed that the rate of carbon sequestration had doubled. In other words, a 65-year-old forest absorbed 1.6 tons of carbon per acre per year. Other studies suggest that much older forests may continue to store carbon as they age — the older the trees, probably, the more and more carbon they store.
The idea the author is driving at is that there may be some very convincing arguments, in addition to familiar ones about wildlife habitat and water conservation, for preserving a lot of forest lands uncut. Older forests help in the fight against global warming.
The Harvard Forest is of course not Plummer’s Hollow, but we also own a mostly oak-maple forest. Excluding about 80 acres out of our 650 acres of land, where a savage cutting was performed 16 years ago before we could buy it, and excluding another 70 acres of recent blowdowns, open meadows, talus slopes, and places that have been selectively logged in the last 30 years, we still have at least 500 acres of forest ranging from 80 to 120 years old.
A 15- to 20-acre section of Laurel Ridge inside and above the large deer exclosure is closer to 200 years old, but much of the remaining 500 acres was last cut in the late 19th or very early 20th centuries. Thus, if the comparison to the Harvard Forest is roughly valid, I would speculate that the forest land in Plummer’s Hollow may be capturing 800 tons of carbon per year, and perhaps quite a bit more.
But other than showing that the property captures so many tons of carbon per year, how does this stack up against the amount of carbon we as a family contribute to the atmosphere through our annual activities? A variety of websites provide simple calculators so people can input data relating to their daily lives — home heating, transportation, consumption of goods — and get an estimate of how much carbon they contribute to the global atmospheric problem.
Ignoring the carbon footprint of the Guest House and its occupant, but including our one jet flight this year, the Carbon Footprint Calculator adds together a variety of estimates and comes up with a figure of 14.134 tons per year. The calculator provided by the Nature Conservancy returns a figure of 42 tons of carbon per year. A third calculator shows that we contribute 10.2 tons per year. Averaging those three calculations we come up with 22 tons per year.
The conclusion: our (mostly) healthy, moderately old, primarily hardwood forest offsets the carbon footprint of roughly 36.3 households with a reasonably low-consumption lifestyle like ours. Or to express it another way, we could live 25 times more extravagantly, wasting resources wildly, and still be net savers of carbon simply by preserving our private forest from being logged.
Not to sound greedy, but if state and federal governments are serious about combating global warming, perhaps forest landowners should get tax credits for not cutting their woods, comparable to the subsidies long enjoyed by farmers who enroll arable land in the Conservation Reserve Program.
— Bruce Bonta