Volume 14, Number 15: 13 April 2011
It was once believed that significant carbon sequestration by forests only occurs when they are young and growing vigorously. This hypothesis was formally advanced by Odum (1969); and it suggests, in the words of Tan et al. (2011), that "old-growth forests exist in a carbon-neutral state, in which photosynthetic carbon uptake is balanced by respiratory carbon release." Thus, as forests age, according to this way of thinking, they gradually lose their ability to sequester much carbon, such that those that have lived a century or more remove little to no CO2 from the air over the course of a year. But is this really so?
Tan et al. (2011) suggest that it is not really so, reporting the fact that stands of trees with ages in excess of 200 years have been demonstrated by several research groups to act as carbon sinks in both coniferous and mixed forests, citing the work of Hollinger et al. (1994), Law et al. (2001), Roser et al. (2002), Knohl et al. (2003), Paw et al. (2004), Desai et al. (2005) and Guan et al. (2006). And they go on to buttress this claim by reporting the results of their own study of the subject, in which they employed an eddy covariance technique to examine the carbon balance of a more-than-300-year-old subtropical evergreen broadleaved forest that is located in the center of the largest subtropical land area of the world in the Ailao Mountain Nature Reserve (24°32'N, 101°01'E) of Yunnan Province in Southwest China.
There, in addition to their micrometeorologically-based eddy flux carbon budget estimation, the six scientists conducted a tree inventory of one hectare of forest located within the footprint of the eddy flux tower they employed in November of 2003 and again in November of 2007, after which they compared measurements of tree diameter at breast height (DBH) between the two times and employed site specific allometric equations to derive mean yearly biomass production from the measurements obtained at the two times, while they also assessed aboveground litter production via the amount captured each year in 25 litter traps that were randomly distributed within the one-hectare plot.
As a result of their efforts, Tan et al. determined that the mean annual net ecosystem production of the forest was approximately 9 tC/ha/year, which suggests, in their words, that "this forest acts as a large carbon sink." In addition, their inventory data indicated that about 6 tC/ha/year was contributed by biomass and necromass. And they report that approximately 60% of the biomass increment was contributed by the growth of large trees with breast height diameters in excess of 60 cm.
Clearly, the old notion of old trees contributing next to nothing to global carbon sequestration is manifestly invalid. They are ever hard at work, doing more good in this regard than all of the wrong-headed reductions in anthropogenic CO2 emissions ever to be conceived by the mind of man, and especially those minds housed in the bodies of U.S. Environmental Protection Agency policymakers, who have declared carbon dioxide to be a "dangerous air pollutant." Far from it, in fact, CO2 is what sustains earth's ancient trees, and what gives them the appetite to do what they do best ... and that is suck CO2 out of the air and grow!
Sherwood, Keith and Craig Idso
Reference
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