Ecosystems with increased carbon dioxide

Abstract

No one doubts that carbon dioxide (CO2) levels in the atmosphere increased in the past century. But a great source of controversy is what happens to ecological systems when CO2 levels increase. A popular hypothesis, the greenhouse effect, posits that increased CO2 will act as a blanket, increase atmospheric temperatures and damage current ecosystem dynamics (www.greenhousenet.org). Another hypothesis, the greening theory, predicts that more atmospheric CO2 will lead to increased plant growth and increased ecosystem productivity (www.greeningearthsociety.org). Two new reports add important insight to the controversial question: what happens when atmospheric CO2 levels increase?The first study focuses on carbon sequestration when atmospheric carbon increases. Pascal A. Niklaus and colleagues1xCarbon allocation in calcareous grassland under elevated CO2: a combined 13C pulse-labelling/soil physical fractionation study. Niklaus, P.A. et al. Ecology. 2001; 15: 43–50See all References1 examined the effects of nearly doubling atmospheric CO2 levels in biotic and abiotic components of a grassland ecosystem. The group cut out sections of grassland from Switzerland in the spring, and grew them in greenhouses for the rest of the season where the researchers manipulated the atmospheric conditions. Although at elevated CO2 levels, the below-ground carbon in plants increases by nearly a third at the end of the season, the total carbon within the ecosystem does not substantially increase. Above-ground total biomass increased by 10% in the CO2 treatment, whereas the root biomass did not substantially change. The experiment illustrates that increasing carbon levels do not necessarily increase plant biomass. The group also examined soil microorganism, earthworms and soil aggregates as ecological markers. Elevated levels of CO2 results in smaller soil aggregates but does not result in any change in microbial carbon levels or earthworm composition. The experiment shows that increasing CO2 in a grassland ecosystem results in no significant increases in ecosystem carbon uptake.The second group focuses on how increases in atmospheric CO2 affect tree fecundity. Shannon LaDeau and James Clark2xRising CO2 levels and the fecundity of forest trees. LaDeau, S.L. and Clark, J.S. Science. 2001; 292: 95–98Crossref | PubMed | Scopus (107)See all References2 fumigated enriched CO2 into whole sections of loblolly pine forest (Pinus taeda) in North Carolina (USA), thereby manipulating CO2 levels in the entire ecosystem. The group examined cone and seed production as markers of tree fecundity in a forest that was nearing reproductive maturity. After four years of treatment, CO2 fumigation results in a threefold increase in mature cone production per tree, and the cones are smaller in diameter compared with nonfumigated controls. The group also collected seeds that fell around the trees. The treated trees produce three times as many seeds as non-treated controls. Also, by the third year of treatment, twice as many treated trees reached maturity compared with untreated controls.The two studies point out a surprising hypothesis. Elevation of atmospheric CO2 can lead to increased turnover of biomass but not necessarily to an increase in the level of standing biomass or carbon assimilation into the ecosystem. To test this possibility, Niklaus and colleagues could measure fecundity in their grasslands, and possibly manipulate CO2 levels in the field. LaDeau and Clark could further examine the biomass and carbon assimilation in their fumigated forests. The studies complicate the simple models of how atmospheric levels of CO2 affect plant development and ecosystems.