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Decomposition of Leaf Litter of Paper Birch Trees in Ambient and CO2- and O3-Enriched Air
Parsons, W.F.J., Lindroth, R.L. and Bockheim, J.G.  2004.  Decomposition of Betula papyrifera leaf litter under the independent and interactive effects of elevated CO2 and O3Global Change Biology 10: 1666-1677.

The authors state that "accumulation of fixed C [carbon] in the vegetation and soils of northern temperate forests has been proposed as an important global sink for rising atmospheric CO2 concentrations."  Hence, it is important to determine how elevated CO2 concentrations impact the decay rates of the senesced leaves of such trees; for the slower their decay rate, the longer more carbon will remain sequestered in the soils of the forests they comprise.

What was done
Leaf litter samples from paper birch (Betula papyrifera Marsh.) trees were collected from early September to mid-October 1998 from ambient and CO2-enriched (to 200 ppm above ambient) FACE plots in northern Wisconsin, USA, which were also maintained under ambient and O3-enriched (to 19 ppb above ambient) conditions, after which their mass was determined and they were assessed for a number of chemical constituents (including nitrogen, which hastens leaf decay, and condensed tannins, which retard decay) and placed in 1-mm-aperture litterbags made of fiberglass cloth and left to decay upon the ground for the next twelve months under the same atmospheric conditions.  Then, at the conclusion of the one-year litter-exposure period, the mass of remaining litter was measured and the time required to achieve 95% mass loss determined.

What was learned
Under ambient O3 conditions, the nitrogen concentrations of the leaves in the CO2-enriched plots at the time of litterfall were 31% less than those of the leaves in the ambient-CO2 plots, while condensed tannin concentrations were 64% greater.  Under O3-enriched conditions, the corresponding values of these leaf constituents were 32% less and 99% greater, respectively, although neither result was significantly different from its ambient-O3 counterpart.  These observations suggest that leaf decay rates in the CO2-enriched plots should be lower than those in the ambient-CO2 plots; and the mass-loss rates determined at the end of the one-year exposure period bore out this expectation, with the authors reporting that "for control litter, 5% of mass remained after 3.6 years, while CO2-enriched litter took ~4.5 years to turn over 95% of its mass."

What it means
These results suggest that it could well take 25% more time (4.5 years / 3.6 years) to lose an equivalent percentage of paper birch leaf litter from CO2-enriched forests, independent of the air's O3 concentration.  Combining this fact with the facts that the CO2-enriched trees, in the words of the authors, "attained greater size, and a greater degree of canopy closure, and contributed more litterfall to the development of [the] forest floor than did trees in the control rings," it is clear that the ongoing rise in the air's CO2 content should greatly augment the sequestration of carbon by paper birch tree stands as the air's CO2 content climbs ever higher.

Reviewed 19 January 2005