3.10 Carbon Storage
Figure 3.10: At left, the graph shows carbon storage and sequestration for Chicago, Illinois, urban trees, mostly elms, maples, ash, and poplar, of various sizes (data from Nowak 1994). For example, a tree between 47 and 61 cm diameter-at-breast-height holds nearly 1,000 kg of carbon, and increases its carbon mass by about 35 kg each year. Estimates here assume carbon mass is half the total biomass. At right, the diagram depicts the pools and annual fluxes of carbon in vegetation, as well as carbon costs for maintenance (after Jo and McPherson 1995). The pools are in units of kgC/m2, and the fluxes are kgC/m2/yr. According to these estimates, trees and shrubs have a positive net carbon sequestration because 0.53 + 0.68 > 0.08, whereas mowing grass results in a negative net sequestration because 0.09 < 0.13.
Let me clarify the difference between “stored” carbon and “sequestered” carbon in vegetation. Stored carbon represents present-day mass: Chop down a tree and you get a certain amount of firewood. Sequestered carbon represents how much a tree will grow from one year to the next: If you chop down the tree next year, you’ll get a bit more firewood than you would this year.
From a detailed study of urban trees in Chicago, Illinois, for example, trees with 47-61cm diameter (my elbow-to-fingertips measures 48 cm) sequestered 35 kgC/year, and trees with 62-76 cm diameter (my shoulder-to-fingertips measures 79cm) sequestered 55 kgC/year.[51]
Not all urban vegetation has equal carbon sequestration costs and benefits. Results depicted in the diagram in Figure 3.10 summarize measurements for grass, shrubs, and trees, from two residential blocks in Chicago.52 Connecting the graph and diagram, the crown diameter from Figure 2.13 for open-grown trees of sizes 47-61 cm diameter and 62-76 cm diameter are about 12 m and 14 m, or about 113 m2 and 154 m2, respectively. Sequestration values then run 0.31 kgC/m2/year and 0.36 kgC/m2/year, close to the 0.53 kgC/m2/year quoted here. Similarly, carbon storage values run 8.5 kgC/m2 and 11.7 kgC/m2, respectively, close to the total 8.9 kgC/m2 itemized for trees. In Figure 3.12 I’ll examine carbon costs associated with maintaining trees.
I’ve also discussed lawns. Grass needs mowing, for example, only to keep trees from colonizing in the southeastern United States. By itself, these numbers show that mowing with a gas-powered mower puts more fossil-fuel carbon into the air than the grass sequesters out of it. Trees and shrubs, on the other hand, capture more carbon than their required maintenance releases.
Besides urban Chicago, carbon sequestration rates of golf courses in Colorado and Wyoming annually sequester about 1 ton per hectare (about 0.1kg/m2) during the first 25 or so years after construction, but then reaches a balance when respiration by soils equals sequestration by the grass. Importantly, these results did not account for fossil-fuel use in golf course maintenance.[53] For comparison, the urban Chicago results estimated mowing at 0.13 kg/m2, more than the golf course grass sequestration above. If those numbers applied to the golf courses, their fairways cost carbon.
In any event, always keep in mind my assertion that all carbon sequestration numbers are absurd! The concept, if it has any validity at all, applies only in the very short term: Herbivory and decomposition release nearly all of the sequestered carbon back into the atmosphere. Only active removal from the biosphere, such as burial in a dump, truly sequesters carbon over the long term. This removal doesn’t include burning firewood and laying mulch onto your yard.
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[51]Nowak (1994) discusses carbon sequestration by urban trees.
[52]Jo and McPherson (1995) provide a detailed analysis of the sequestration benefits and maintenance costs of urban vegetation.
[53]Qian and Follett (2002) report carbon sequestration rates in golf courses.
The sequestration rates for the golf courses should show units of kgC/m2, though it is implied in the context of the sentence.