Green Alert

November 28, 2001 Vol. 1, No. 11

Negotiators of the Kyoto Protocol – the document intended to spell-out how nations can achieve the goals enshrined by the Framework Convention on Climate Change – provide a role for forests to play in mitigating the rising atmospheric concentration of carbon dioxide (CO2) caused by use of fossil fuels and deforestation. At its most simplistic, the idea is to provide some sort of credit for planting and/or preserving forests. Working out the details has proven to be difficult but, conceptually, the idea rests on two well-established and straightforward pillars: (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 in the air’s CO2 content. In this way, terrestrial carbon sequestration becomes a partial solution to the "global warming problem."

The concept is not altogether popular with those who prefer a "wrenching transformation of society" through forced reductions in anthropogenic CO2 emissions. They construct a logical daisy chain intended to dismiss the idea as environmentally unsound. First, they assert that carbon sequestration by forests is only viable when forests are young and growing vigorously. As forests age, they claim, the trees gradually lose their carbon sequestering prowess. Forests older than a hundred years, they will assert, essentially become useless for removing CO2 from the air; they annually lose as much CO2 via respiration as they take in via photosynthesis. Closing the loop, they argue this places additional pressure on old-growth forests to be logged and converted into carbon sequestering tree plantations (dooming biodiversity and spotted owls).

Although demonstrably erroneous, with enough repetition a twisted scenario such as this begins to sound reasonable. Doesn’t the metabolism of every living thing slow as it ages? We grudgingly admit that this is so, even for trees. But some species of trees live a remarkably long time. By way of example, research in Panama (Condit et al., 1995), Brazil (Chambers et al., 1998), and several areas of the southwestern United States (Graybill and Idso, 1993) demonstrates how a number of different trees live for nearly one and a half millennia.

At a hundred years of age, these CO2 super-slurpers are mere youngsters. Yet, even in their really old age, their appetite for the vital gas, though diminished, is not lost. 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. Protracted, slow growth (evident in annual increases in trunk diameter) by very old, very large trees means they still can absorb a heck of a lot of CO2 in a given year. This is especially true when "about 50% of the above-ground biomass [of the Brazilian forests Chambers et al studied in the central Amazon region] is contained in less than the largest 10% of the trees."

But the carbon sequestering potential is not only in the trees, important as they are.

Think of a forest as a huge super-organism, if you will. It is the forest – not an individual tree – that is the unit of primary importance when it comes to determining the ultimate amount of carbon that can be sequestered on a unit of land area. Cary et al. (2001), 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 such model predictions against real-world data gathered from northern Rocky Mountain sub-alpine forests ranging in age from 67 to 458 years, they found that aboveground net primary productivity in 200-year-old natural stands was almost twice as great as that of modeled stands. The difference between a modeled forest and a real one increased linearly throughout the entire sampled age range.

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

How serious could such model shortcomings be? Compared against the kind of real-world forests studied by Cary et al., they produce predictions of carbon sequestration little more than half as large as what is observed in nature for 200-year-old forests. For 400-year-old forests they produce results only about a third as large as what is characteristic of the real world. As the forests’ age increases, so does the discrepancy.

Old soldiers, as a great one once said, fade away – but not old forests. They slow down a bit and attract new recruits enlisted in the effort to rid the world of rising atmospheric CO2 concentrations.

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.

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This continues a series of "greening alerts" that Greening Earth Society periodically publishes online in response to research concerning the impact of carbon dioxide emissions on earth’s biosphere, especially as it relates to plant life’s ability to sequester carbon. The alerts are prepared by Drs. Sherwood B. Idso and Keith E. Idso of the Center for the Study of Carbon Dioxide and Global Change in Tempe, Arizona (www.co2science.org).