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Young Forest Stands Exposed to Elevated CO2 and Ozone: The Importance of Long-Term Studies
Volume 11, Number 51: 17 December 2008

In an experiment initiated in May of 1998 at the free-air CO2 enrichment (FACE) facility near Rhinelander, Wisconsin (USA), researchers have been growing stands of young trembling aspen (Populus tremuloides Michx.) trees, comprised of five genotypes having differing responses to CO2 and ozone (O3), plus aspen/paper birch (Betula papyrifera Marsh.) mixes and aspen/sugar maple (Acer saccharum Marsh.) mixes. Over the first five years of this study, the positive effects of their 50% increase in CO2 on above- and below-ground tree biomass were essentially nullified by the negative effects of their 39% increase in O3, as reported by King et al. (2005). Now, however, a very different result is beginning to manifest itself.

As described by Pregitzer et al. (2008), "all root biomass sampling previous to 2002 showed that O3 exposure, alone or in combination with elevated CO2, consistently resulted in lower coarse root biomass for all plant communities and lower fine-root biomass for the birch/aspen and birch/maple communities." In their analysis of more recent data, however, they report that "+O3 significantly increased fine-root biomass in the aspen community, and, in combination with +CO2, increased coarse root biomass in both the aspen and birch/aspen communities." Hence, they conclude that "the amount of carbon being allocated to aspen fine-root biomass under elevated O3 is increasing over time [our italics] relative to the control, especially in the +CO2 +O3 treatment, in contrast with most shorter term results, including our own (King et al., 2001)." As a result, they thus further conclude that "the positive effects of elevated CO2 on belowground net primary productivity may not be offset by negative effects of O3."

So what could be the cause of this increasingly positive root biomass response to simultaneous increases in the air's CO2 and O3 concentrations that has gradually over-powered the initial negative impact of the O3 increase?

Pregitzer et al. speculate that the "demise of O3-sensitive aspen genotypes and better overall survival of birch and maple, which are less sensitive to O3 than aspen, may drive a 'stand dynamic' that results in dominance by genotypes and species more tolerant of prolonged exposure to elevated O3." In support of this hypothesis, they note that "in the field, single-tree mortality is apparent in all three of the community types," and they say that "the tolerant aspen genotype that now dominates under +O3 exposure actually grows faster in the +O3 treatment than in the control treatment," citing the work of Kubiske et al. (2007). In addition, they say that "exposure to O3 has also increased the rate at which birch is becoming dominant in the birch/aspen community," citing the work of Zak et al. (2007).

Whatever is ultimately proved to be the cause of their welcome observations, Pregitzer et al.'s study clearly demonstrates the importance of long-term experiments, since stopping at the five-year point of their study would have resulted in the drawing of incorrect long-term conclusions, just as stopping at the ten-year point of the sour orange tree study of Idso and Kimball (2001) -- which ultimately ran for 17 years (Kimball et al., 2007) -- would have also resulted in incorrect long-term conclusions.

Clearly, long-term studies, such as the one we highlight in this Editorial, should be maintained as long as they possibly can be funded, for one never truly knows the long-term consequences until the "long-term" has been reached. Deity may know the end from the beginning, but mortal researchers have to slog along until they get there.

Sherwood, Keith and Craig Idso

Idso, S.B. and Kimball, B.A. 2001. CO2 enrichment of sour orange trees: 13 years and counting. Environmental and Experimental Botany 46: 147-153.

Kimball, B.A., Idso, S.B., Johnson, S. and Rillig, M.C. 2007. Seventeen years of carbon dioxide enrichment of sour orange trees: final results. Global Change Biology 13: 2171-2183.

King, J.S., Kubiske, M.E., Pregitzer, K.S., Hendrey, G.R., McDonald, E.P., Giardina, C.P., Quinn, V.S. and Karnosky, D.F. 2005. Tropospheric O3 compromises net primary production in young stands of trembling aspen, paper birch and sugar maple in response to elevated atmospheric CO2. New Phytologist 168: 623-636.

King, J.S., Pregitzer, K.S., Zak, D.R., Sober, J., Isebrands, J.G., Dickson, R.E., Hendrey, G.R. and Karnosky, D.F. 2001. Fine root biomass and fluxes of soil carbon in young stands of paper birch and trembling aspen as affected by elevated atmospheric CO2 and tropospheric O3. Oecologia 128: 237-250.

Kubiske, M.E., Quinn, V.S., Marquardt, P.E. and Karnosky, D.F. 2007. Effects of elevated atmospheric CO2 and/or O3 on intra- and inter-specific competitive ability of aspen. Plant Biology 9: 342-355.

Pregitzer, K.S., Burton, A.J., King, J.S. and Zak, D.R. 2008. Soil respiration, root biomass, and root turnover following long-term exposure of northern forests to elevated atmospheric CO2 and tropospheric O3. New Phytologist 180: 153-161.

Zak, D.R., Holmes, W.E., Pregitzer, K.S., King, J.S., Ellsworth, D.S. and Kubiske, M.E. 2007. Belowground competition and the response of developing forest communities to atmospheric CO2 and O3. Global Change Biology 13: 2230-2238.