One of the earlier studies to explore the effects of elevated atmospheric CO2 on seed production - as well as the subsequent phenomena they trigger - was that 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 the 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 the ambient and 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 atmospheric CO2 enrichment 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) employed a FACE experiment where daytime atmospheric CO2 concentrations above a sheep-grazed pasture in New Zealand were increased by 115 ppm, in order 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)."
Wang and Griffin (2003) grew dioecious white campion 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 phenomenon bodes well for the biodiversity of earth's many ecosystems.
Two years later, Wang (2005) grew well watered and fertilized specimens of the same plant (Silene latifolia) from seed to maturity in pots within controlled environment chambers that were maintained at mean CO2 concentrations of 386 and 696 ppm, while documenting various reproductive responses during growth and at final harvest, after which the seeds that had been produced by the plants in this experiment were used to grow a second generation of plants under the same environmental conditions in which the parent plants had been grown.
In the first experiment, the total reproductive biomass of the plants that were grown in CO2-enriched air was 32% greater than that of the plants grown in ambient air, as was the total number of fruit produced. In the second experiment, for seeds from female plants grown in ambient air, 55% of all emergence occurred within six days of sowing, while for seeds from plants grown in CO2-enriched air, 67% of total emergence occurred during the same period. In addition, 87% of the seeds from the elevated-CO2-grown plants ultimately germinated, while only 67% of the seeds from the ambient-CO2-grown plants did so. Last of all, there was a tendency for a greater percentage of female progeny to be produced in the CO2-enriched air than in ambient-air (56.3% vs. 52.7%).
The combined effect of a greater number of seeds being produced per female plant, a higher percentage of seed germination, and more female-biased seed production in CO2-enriched air would seem to suggest that white campion plants will fare well in a high-CO2 world of the future, which might cause some to worry, seeing it is a rather cosmopolitan and somewhat weedy species. It must be remembered, however, that any plants of agricultural value with which it might compete will also be doing better in such a future world. Hence, the greater importance of this study is what it may imply about other dioecious species, especially in light of the fact that Silene latifolia, in the words of Wang, "has become a model system for studying sexual dimorphism and sex-determination mechanisms and is likely the most extensively studied dioecious species."
In one final study that examined the effect of temperature, as opposed to elevated CO2, Kuparinen et al. (2009) investigated the effects of a warming-induced increase in local convective turbulence (caused by a postulated 3°C increase in local temperature) on the long-distance dispersal (LDD) of seeds and pollen based on mechanistic models of wind dispersal (Kuparinen et al., 2007) and population spread (Clark et al., 2001) in a boreal forest of southern Finland. For light-seeded herbs, the group of researchers reported that spread rates increased by 35-42 m/yr (6.3-9.2%), while for heavy-seeded herbs the increase was 0.01-0.06 m/yr (1.9-6.7%). In addition, they note that "climate change driven advancements of flowering and fruiting phenology can increase spread rates of plant populations because wind conditions in spring tend to produce higher spread rates than wind conditions later in the year."
As for the significance of their findings, the four researchers, hailing from France, Germany, Israel and the United States, write that -- in addition to the obvious benefits of greater LDD (being better able to move towards a more hospitable part of the planet) -- the increased wind dispersal of seeds and pollen may "promote geneflow between populations, thus increasing their genetic diversity and decreasing the risk of inbreeding depression," citing the work of Ellstrand (1992) and Aguilar et al. (2008), while further noting that "increased gene flow between neighboring populations can accelerate adaptation to environmental change," citing the work of Davis and Shaw (2001) and Savolainen et al. (2007), which phenomena are all very positive developments. In fact, they report that the "dispersal and spread of populations are widely viewed as a means by which species can buffer negative effects of climate change [italics added]."
References
Aguilar, R., Quesada, M., Ashworth, L., Herrerias-Diego, Y. and Lobo, J. 2008. Genetic consequences of habitat fragmentation in plant populations: susceptible signals in plant traits and methodological approaches. Molecular Ecology 17: 5177-5188.
Baskin, C.C. and Baskin, J.M. 1998. Seeds: Ecology, Biogeography, and Evolution of Dormancy and Germination. Academic Press, San Diego, CA.
Clark, J.S., Lewis, M. and Hovarth, L. 2001. Invasion by extremes; population spread with variation in dispersal and reproduction. American Naturalist 157: 537-544.
Davis, M.B. and Shaw, R.G. 2001. Range shifts and adaptive responses to quaternary climate change. Science 292: 673-679.
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.
Ellstrand, N.C. 1992. Gene flow by pollen: Implications for plant conservation genetics. Oikos 63: 77-86.
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.
Kuparinen, A., Katul, G., Nathan, R. and Schurr, F.M. 2009. Increases in air temperature can promote wind-driven dispersal and spread of plants. Proceedings of the Royal Society B 276: 3081-3087.
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.
Savolainen, O., Pyhajarvi, T. and Knurr, T. 2007. Gene flow and local adaptation in trees. Annual Review of Ecology, Evolution and Systematics 38: 595-619.
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. 2005. Reproduction and progeny of Silene latifolia (Caryophyllaceae) as affected by atmospheric CO2 concentration. American Journal of Botany 92: 826-832.
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.
Last updated 2 May 2012