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Will Forest Carbon Sink Capacity Fade Away as Trees Age?
The planting and preservation of forests has long been acknowledged to be an effective natural means for slowing climate-model-predicted CO2-induced global warming.  This prescription for moderating potential climate change is based on two well-established and very straightforward 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 so simple a child can understand it, this potential partial solution to the putative global warming problem has recently come under attack from 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, say the regulatory-minded pundits, they gradually lose their carbon sequestering prowess, such that forests more than one 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 what they take in via photosynthesis.

Although demonstrably erroneous, with enough jaw-boning and repeated telling the storyline of the twisted tale actually begins to sound reasonable.  After all, doesn't the metabolism of every living thing slow down as it gets older?  We grudgingly admit that it does - even for trees - but the trees of some species live a remarkably long time.  In Panama (Condit et al., 1995), Brazil (Chambers et al., 1998), and several areas of the southwestern United States (Graybill and Idso, 1993), for example, a number of different trees have been shown to live for nearly one and a half millennia.  At a hundred years of age, these super-slurpers of CO2 are mere youngsters.  And in their really old age, their appetite for the vital gas, though diminished, is not lost.  In fact, Chambers et al. indicate that the long-lived trees of Brazil continue to experience "protracted slow growth," as they put it, even at 1400 years of age. And protracted slow growth (evident in yearly increasing trunk diameters) of very old and very large trees can absorb a heck of a lot of CO2 out of the air each year, especially when, as noted by Chanbers et al., "about 50% of the above-ground biomass [of the Brazilian forests they studied in the central Amazon region] is contained in less than the largest 10% of the trees."

As important as are these facts about trees, however, there's an even more important fact that comes into play in the case of forests and their ability to sequester carbon over long periods of time.  This little-acknowledged piece of information is the fact that it is the forest itself - conceptualized as a huge super-organism, if you will - that is the unit of primary importance when it comes to determining the ultimate amount of carbon that can be sequestered on a unit area of land.  And it seems that a lot of climate alarmists and political opportunists just can't seem to see the forest for the trees when pontificating upon this subject.

That this difference in perspective can have enormous consequences has recently been demonstrated by Cary et al. (2001), who note that most models of forest carbon sequestration wrongly assume that "age-related growth trends of individual trees and even-aged, monospecific stands can be extended to natural forests."  When they compared the predictions of such models against real-world data they gathered from northern Rocky Mountain subalpine forests that ranged in age from 67 to 458 years, for example, they found that aboveground net primary productivity in 200-year-old natural stands was almost twice as great as that of modeled stands, and that the difference between the two increased linearly throughout the entire sampled age range.

So what's the explanation for the huge discrepancy?  Cary et al. suggest that long-term recruitment and the periodic appearance of additional late-successional species (increasing biodiversity) may have significant effects on stand productivity, infusing the primary unit of concern, i.e., the ever-evolving forest super-organism, with greater vitality than would have been projected on the basis of characteristics possessed by the unit earlier in its life.  They also note that by not including effects of size- or age-dependent decreases in stem and branch respiration per unit of sapwood volume in models of forest growth, respiration in older stands can be over-estimated by a factor of two to five.

How serious are these model shortcomings?  For the real-world forests studied by Cary et al., they produce predictions of carbon sequestration that are only a little over half as large as what is observed in nature for 200-year-old forests; while for 400-year-old forests they produce results that are only about a third as large as what is characteristic of the real world.  And as the forests grow older still, the difference between reality and model projections grows right along with them.

Yes, old soldiers, as one of the greatest of them once said, may well just fade away, but not old forests.  They may slow down a bit; but with new recruits continually enlisting in the battle against rising atmospheric CO2 concentrations, they just keep on sequestering carbon ... and they do it at a very respectable rate.  Don't let anyone tell you otherwise.

Dr. Sherwood B. Idso Dr. Keith E. Idso

References
Carey, E.V., Sala, A., Keane, R. and Callaway, R.M.  2001.  Are old forests underestimated as global carbon sinks?  Global Change Biology 7: 339-344.

Chambers, J.Q., Higuchi, N. and Schimel, J.P.  1998.  Ancient trees in Amazonia.  Nature 391: 135-136.

Condit, R., Hubbell, S.P. and Foster, R.B.  1995.  Mortality-rates of 205 neotropical tree and shrub species and the impact of a severe drought.  Ecological Monographs 65: 419-439.

Graybill, D.A. and Idso, S.B.  1993.  Detecting the aerial fertilization effect of atmospheric CO2 enrichment in tree-ring chronologies.  Global Biogeochemical Cycles 7: 81-95.