We begin our mini-review of elevated atmospheric CO2 effects on seed production -- as well as the subsequent phenomena they trigger -- with the study of Steinger et al. (2000), who collected seeds from Bromus erectus plants that had been grown at atmospheric CO2 concentrations of 360 and 650 ppm, and who germinated some of both groups of seeds under those same two sets of conditions. In the first part of their study, they found that the elevated CO2 treatment (1) increased individual seed mass by about 9% and (2) increased seed carbon-to-nitrogen ratio by almost 10%. However, they also learned that these changes in seed properties had little impact on subsequent seedling growth. In fact, when the seeds produced by ambient or CO2-enriched plants were germinated and grown in ambient air, there was no significant size difference between the two groups of resultant seedlings after a period of 19 days. Likewise, when the seeds produced from ambient or CO2-enriched plants were germinated and grown in the high CO2 treatment, there was also no significant difference between the sizes of the seedlings derived from the two groups of seeds. However, the CO2-enriched seedlings produced from both groups of seeds were almost 20% larger than the seedlings produced from both groups of seeds grown in ambient air, demonstrating that the direct effects of elevated atmospheric CO2 concentration on seedling growth and development were more important than the differences in seed characteristics produced by the elevated atmospheric CO2 concentration in which their parent plants grew.
In another study conducted about the same time, Edwards et al. (2001) utilized a FACE experiment where daytime atmospheric CO2 concentrations above a sheep-grazed pasture in New Zealand were increased by 115 ppm to study the effects of elevated CO2 on seed production, seedling recruitment and species compositional changes. In the two years of their study, the extra daytime CO2 increased seed production and dispersal in seven of the eight most abundant species, including the grasses Anthoxanthum odoratum, Lolium perenne and Poa pratensis, the legumes Trifolium repens and T. subterranean, and the herbs Hypochaeris radicata and Leontodon saxatilis. In some of these plants, elevated CO2 increased the number of seeds per reproductive structure, while all of them exhibited CO2-induced increases in the number of reproductive structures per unit of ground area. In addition, they determined that the CO2-induced increases in seed production contributed in a major way to the increase in the numbers of species found within the CO2-enriched plots.
In a five-year study of a nutrient-poor calcareous grassland in Switzerland, Thurig et al. (2003) used screen-aided CO2 control (SACC) technology (Leadley et al., 1997) to enrich the air over half of their experimental plots with an extra 300 ppm of CO2, finding that "the effect of elevated CO2 on the number of flowering shoots (+24%) and seeds (+29%) at the community level was similar to above ground biomass response." In terms of species functional groups, there was a 42% increase in the mean seed number of graminoids and a 33% increase in the mean seed number of forbs, but no change in legume seed numbers. In most species, mean seed weight also tended to be greater in plants grown in CO2-enriched air (+12%); and Thurig et al. say it is known from many studies that heavier seeds result in seedlings that "are more robust than seedlings from lighter seeds (Baskin and Baskin, 1998)."
Rounding out the papers we have reviewed on this subject, Wang and Griffin (2003) grew dioecious white cockle plants from seed to maturity in sand-filled pots maintained at optimum moisture and fertility conditions in environmentally-controlled growth chambers in which the air was continuously maintained at CO2 concentrations of either 365 or 730 ppm. In response to this doubling of the air's CO2 content, the vegetative mass of both male and female plants rose by approximately 39%. Reproductive mass, on the other hand, rose by 82% in male plants and by 97% in females. In the female plants, this feat was accomplished, in part, by increases of 36% and 44% in the number and mass of seeds per plant, and by a 15% increase in the mass of individual seeds, in harmony with the findings of Jablonski et al. (2002), which they derived from a meta-analysis of the results of 159 CO2 enrichment experiments conducted on 79 species of agricultural and wild plants. Hence, because dioecious plants comprise nearly half of all angiosperm families, we may expect to see a greater proportion of plant biomass allocated to reproduction in a high-CO2 world of the future, which ultimate result should bode well indeed for the biodiversity of earth's many ecosystems.
References
Baskin, C.C. and Baskin, J.M. 1998. Seeds: Ecology, Biogeography, and Evolution of Dormancy and Germination. Academic Press, San Diego, CA.
Edwards, G.R., Clark, H. and Newton, P.C.D. 2001. The effects of elevated CO2 on seed production and seedling recruitment in a sheep-grazed pasture. Oecologia 127: 383-394.
Jablonski, L.M., Wang, X. and Curtis, P.S. 2002. Plant reproduction under elevated CO2 conditions: a meta-analysis of reports on 79 crop and wild species. New Phytologist 156: 9-26.
Leadley, P.W., Niklaus, P.A., Stocker, R. et al. 1997. Screen-aided CO2 control (SACC): a middle ground between FACE and open-top chambers. Acta Oecologica 18: 39-49.
Steinger, T., Gall, R. and Schmid, B. 2000. Maternal and direct effects of elevated CO2 on seed provisioning, germination and seedling growth in Bromus erectus. Oecologia 123: 475-480.
Thurig, B., Korner, C. and Stocklin, J. 2003. Seed production and seed quality in a calcareous grassland in elevated CO2. Global Change Biology 9: 873-884.
Wang, X. and Griffin, K.L. 2003. Sex-specific physiological and growth responses to elevated atmospheric CO2 in Silene latifolia Poiret. Global Change Biology 9: 612-618.