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Old Aspen Forest Not Feeling Its Age
Binkley, D., White, C.S. and Gosz, J.R.  2004.  Tree biomass and net increment in an old aspen forest in New Mexico.  Forest Ecology and Management 203: 407-410.

The planting and preservation of forests has long been acknowledged to be an effective natural means for slowing global warming based on two well-established facts: (1) the carbon trees use to construct their tissues comes from the air, and (2) its extraction from the atmosphere slows the rate of rise of the air's CO2 content.  Although the concept is so simple a child can understand it, this partial solution to the putative global warming problem has been attacked by people who seek to address the issue solely on the basis of forced reductions in anthropogenic CO2 emissions.  The tack they take in this campaign is to claim that carbon sequestration by forests is only viable when forests are young and growing vigorously.  As forests age, they say they lose their carbon sequestering prowess, so that trees more than a hundred years old become essentially useless for removing CO2 from the air, as they claim such ancient and decrepit stands yearly loose as much CO2 via respiration as they take in via photosynthesis.

What was done
In exploring this issue, Binkley et al. revisited an aging aspen (Populus tremuloides Michx.) forest in the 3.4-ha Tesuque watershed of northern New Mexico, USA, which between 1971 and 1976 (when it was between 90 and 96 years old) was determined by Gosz (1980) to have had a negative net ecosystem production rate of - 2.0 Mg ha-1 yr-1.  Specifically, they measured the basal diameters of all trees in the central 0.01 ha of each of 27 plots arrayed across the watershed and used the same regression equations employed by Gosz to calculate live tree biomass as of 2003.

What was learned
"Contrary to expectation," as they describe it, Binkley et al. report that "live tree mass in 2003 [186 Mg ha-1] was significantly greater than in 1976 [149 Mg ha-1] (P = 0.02), refuting the hypothesis that live tree mass declined."  In fact, they found that "the annual net increment of live tree mass was about 1.37 Mg ha-1 yr-1 from age 96 to age 123 years," which is only 12% less than the mean annual increment of live tree mass experienced over the forest's initial 96 years of existence (149 Mg ha-1 / 96 yr = 1.55 Mg ha-1 yr-1).

What it means
In response to the question they pose in their opening paragraph - "Do old forests gain or lose carbon?" - Binkley et al. conclude from their research, as well as the report of Jones et al. (1985), that "old aspen forests continue to accrue live stem mass well into their second century, despite declining current annual increments," which, we might add, are not all that smaller than those they exhibited in their younger years.  Could it be that the dramatic rise in atmospheric CO2 concentration over the past half-century has played a role in this phenomenon?  Perhaps this is another question that needs investigating.

Reviewed 8 December 2004