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Cloud Condensation Nuclei (Condensation Nuclei: Biological Effects) – Summary
In the other two sub-headings of the Cloud Condensation Nuclei section of our Subject Index, we describe various climatic effects of anthropogenic- and biologically-produced aerosols and gases, concluding that these substances, thankfully, tend to cool the planet when there is an independent impetus for warming.  Under this sub-heading we discuss various biological effects of these same atmospheric constituents; and we find that these effects, too, are beneficial.

We begin by noting that while reducing the total amount of solar radiation received at the surface of the earth, increases in both anthropogenic- and biologically-produced aerosols tend to enhance the diffuse component of solar radiation received there (Suraqui et al., 1974; Abakumova et al., 1996), which phenomenon tends to reduce the volume of shade within vegetative canopies (Roderick et al., 2001), which enhances whole-canopy photosynthesis (Healey et al., 1998), which leads to a greater extraction of CO2 from the air and enhanced sequestration of its carbon in plant tissues and soil.  How significant is this multi-linked chain of events?

Roderick et al. (2001) provide an illuminating answer to this question based on an analysis of a unique "natural experiment."  Focusing on the volcanic eruption of Mt. Pinatubo in June of 1991 - which ejected enough fine materials and aerosol-producing gases into the atmosphere that it greatly increased the diffuse component of the solar radiation reaching the surface of the earth from that point in time through much of 1993 - they calculated that the eruption may have resulted in the removal of an extra 2.5 Gt of carbon from the atmosphere due to its diffuse-light-enhancing stimulation of terrestrial vegetation the following year, which would have reduced the ongoing rise in the air's CO2 concentration by about 1.2 ppm.  Interestingly, this reduction is of the same magnitude as what was directly measured that year (Sarmiento, 1993).

What makes this comparison even more impressive is the fact that the CO2 reduction was coincident with an El Niño event.  Why is that significant?  Because, in the words of Roderick et al., "previous and subsequent such events have been associated with increases in atmospheric CO2."  Also, the observed reduction in total solar radiation received at the surface of the earth during this period would have had a tendency to reduce the amount of photosynthetically-active radiation incident upon earth's plants, which would also have had a tendency to cause the air's CO2 content to rise, as it would tend to lessen global photosynthetic activity.  Nevertheless, the increase in the diffuse component of the flux of solar radiation received at the earth's surface was great enough to overcome these competing influences and boost global net photosynthesis to such an extent that it significantly slowed the rate of rise of the air's CO2 concentration.

What other evidence is there for this intriguing phenomenon?

Gu et al. (2003) report that they "used two independent and direct methods to examine the photosynthetic response of a northern hardwood forest (Harvard Forest, 42.5°N, 72.2°W) to changes in diffuse radiation caused by Mount Pinatubo's volcanic aerosols," finding that "around noontime in the midgrowing season, the gross photosynthetic rate under the perturbed cloudless solar radiation regime was 23, 8, and 4% higher than that under the normal cloudless solar radiation regime in 1992, 1993, and 1994, respectively," and that "integrated over a day, the enhancement for canopy gross photosynthesis by the volcanic aerosols was 21% in 1992, 6% in 1993 and 3% in 1994."  Commenting on the significance of these observations, Gu et al. note that "because of substantial increases in diffuse radiation world-wide after the eruption and strong positive effects of diffuse radiation for a variety of vegetation types, it is likely that our findings at Harvard Forest represent a global phenomenon."  Based on still more real-world data, Gu et al. additionally note that "Harvard Forest photosynthesis also increases with cloud cover, with a peak at about 50% cloud cover."

In an equally impressive study, Law et al. (2002) compared seasonal and annual values of CO2 and water vapor exchange across several forest, grassland, crop and tundra sites that are part of the international FLUXNET program that is investigating the responses of these exchanges to variations in a number of environmental factors, including direct and diffuse solar radiation.  They report that "cloud-cover results in a greater proportion of diffuse radiation and constitutes a higher fraction of light penetrating to lower depths of the canopy (Oechel and Lawrence, 1985)," and that "net carbon uptake (net ecosystem exchange, the net of photosynthesis and respiration) was greater under diffuse than under direct radiation conditions."  More importantly, they say that "Goulden et al. (1997), Fitzjarrald et al. (1995), and Sakai et al. (1996) showed that net carbon uptake was consistently higher during cloudy periods in a boreal coniferous forest than during sunny periods with the same PPFD [photosynthetic photon flux density]."  In fact, they say that "Hollinger et al. (1994) found that daily net CO2 uptake was greater on cloudy days, even though total PPFD was 21-45% lower on cloudy days than on clear days."

In addition to enhancing the photosynthetic uptake of atmospheric CO2 by enhancing the amount of diffuse solar radiation received at the surface of the earth, the biogenic volatile organic compounds or BVOCs that are emitted from plants exert a number of other important effects on earth's vegetation. Peñuelas and Llusia (2003), for example, say that BVOCs constitute "one of nature's biodiversity treasures."  Comprised of isoprene, terpenes, alkanes, alkenes, alcohols, esters, carbonyls and acids, this diverse group of substances is produced by a variety of physiological processes operating in many different plant tissues.  Some of the functions of these substances, according to Peñuelas and Llusia, include acting as "deterrents against pathogens and herbivores, or to aid wound sealing after damage (Pichersky and Gershenzon, 2002)."  They also say they provide a means "to attract pollinators and herbivore predators, and to communicate with other plants and organisms (Peñuelas et al., 1995; Shulaev et al., 1997)."

Of particular importance within the context of global climate change is the feeling among many scientists, in the words of Peñuelas and Llusia, that "isoprene and monoterpenes, which constitute a major fraction of BVOCs, might confer protection against high temperatures" by acting "as scavengers of reactive oxygen species produced [within plants] under high temperatures."  If this is indeed the case, it can be appreciated that with respect to the presumed ill effects of CO2-induced global warming on earth's vegetation, there are likely to be two strong ameliorative phenomena helping earth's plants: first, the aerial fertilization effect of atmospheric CO2 enrichment, which is typically more effective at higher temperatures, and second, the tendency for rising temperatures to spur the production of higher concentrations of heat-stress-reducing BVOCs.

A study to determine the effects of atmospheric CO2 enrichment on one of these other beneficial impacts of BVOCs was conducted by Jasoni et al. (2003), who grew onions from seed for 30 days in individual cylindrical flow-through growth chambers at atmospheric CO2 concentrations of either 400 or 1000 ppm.  By the end of the experiment, the plants in the CO2-enriched air had produced 40% more biomass than the plants grown in ambient air, and they exhibited 17-fold and 38-fold increases, respectively, in gaseous emissions of two particular hydrocarbons: 2-undecanone and 2-tridecanone.  Why is this important?  Because Jasoni et al. report that these two odd-chain ketones "confer insect resistance against a major agricultural pest, spider mites (Fery and Kennedy, 1987; Chatzivasileiadis and Sabelis, 1997; Chatzivasileiadis et al., 1999)."

In conclusion, it is clear that both anthropogenic- and biologically-produced aerosols that act as cloud condensation nuclei ultimately end up promoting photosynthesis in plants via the enhanced shading produced by the extra and longer-lasting clouds they create, which allows greater photosynthetic rates to be maintained in the interiors of plant canopies.  Also, the enhanced emissions of BVOCs produced by atmospheric CO2 enrichment are important for the many positive biological impacts they have on the plants that produced them.

Abakumova, G.M., Feigelson, E.M., Russak, V. and Stadnik, V.V.  1996.  Evaluation of long-term changes in radiation, cloudiness, and surface temperature on the territory of the former Soviet Union.  Journal of Climatology 9: 1319-1327.

Chatzivasileiadis, E.A., Boon, J.J. and Sabelis, M.W.  1999.  Accumulation and turnover of 2-tridecanone in Tetranychus urticae and its consequences for resistance of wild and cultivated tomatoes.  Experimental and Applied Acarology 23: 1011-1021.

Chatzivasileiadis, E.A. and Sabelis, M.W.  1997.  Toxicity of methyl ketones from tomato trichomes to Tetranychu urticea Koch.  Experimental and Applied Acarology 21: 473-484.

Fery, R.L. and Kennedy, G.G.  1987.  Genetic analysis of 2-tridecanone concentration, leaf tricome characteristics, and tobacco hornworm resistance in tomato.  Journal of the American Society of Horticultural Science 11: 886-891.

Fitzjarrald, D.R., Moore, K.E., Sakai, R.K. and Freedman, J.M.  1995.  Assessing the impact of cloud cover on carbon uptake in the northern boreal forest.  In: Proceedings of the American Geophysical Union Meeting, Spring 1995, EOS Supplement, p. S125.

Goulden, M.L., Daube, B.C., Fan, S.-M., Sutton, D.J., Bazzaz, A., Munger, J.W. and Wofsy, S.C.  1997.  Physiological responses of a black spruce forest to weather.  Journal of Geophysical Research 102: 28,987-28,996.

Gu, L., Baldocchi, D.D., Wofsy, S.C., Munger, J.W., Michalsky, J.J., Urbanski, S.P. and Boden, T.A.  2003.  Response of a deciduous forest to the Mount Pinatubo eruption: Enhanced photosynthesis.  Science 299: 2035-2038.

Healey, K.D., Rickert, K.G., Hammer, G.L. and Bange, M.P.  1998.  Radiation use efficiency increases when the diffuse component of incident radiation is enhanced under shade.  Australian Journal of Agricultural Research 49: 665-672.

Hollinger, D.Y., Kelliher, F.M., Byers, J.N. and Hunt, J.E.  1994.  Carbon dioxide exchange between an undisturbed old-growth temperate forest and the atmosphere.  Ecology 75: 134-150.

Jasoni, R., Kane, C., Green, C., Peffley, E., Tissue, D., Thompson, L., Payton, P. and Pare, P.W.  2003.  Altered leaf and root emissions from onion (Allium cepa L.) grown under elevated CO2 conditions.  Environmental and Experimental Botany 51: 273-280.

Law, B.E., Falge, E., Gu,. L., Baldocchi, D.D., Bakwin, P., Berbigier, P., Davis, K., Dolman, A.J., Falk, M., Fuentes, J.D., Goldstein, A., Granier, A., Grelle, A., Hollinger, D., Janssens, I.A., Jarvis, P., Jensen, N.O., Katul, G., Mahli, Y., Matteucci, G., Meyers, T., Monson, R., Munger, W., Oechel, W., Olson, R., Pilegaard, K., Paw U, K.T., Thorgeirsson, H., Valentini, R., Verma, S., Vesala, T., Wilson, K. and Wofsy, S.  2002.  Environmental controls over carbon dioxide and water vapor exchange of terrestrial vegetation.  Agricultural and Forest Meteorology 113: 97-120.

Oechel, W.C. and Lawrence, W.T.  1985.  Tiaga.  In: Chabot, B.F. and Mooney, H.A. (Eds.), Physiological Ecology of North American Plant Communities.  Chapman & Hall, New York, NY, pp. 66-94.

Peñuelas, J. and Llusia, J.  2003.  BVOCs: plant defense against climate warming?  Trends in Plant Science 8: 105-109.

Peñuelas, J., Llusia, J. and Estiarte, M.  1995.  Terpenoids: a plant language.  Trends in Ecology and Evolution 10: 289.

Pichersky, E. and Gershenzon, J.  2002.  The formation and function of plant volatiles: perfumes for pollinator attraction and defense.  Current Opinion in Plant Biology 5: 237-243.

Roderick, M.L., Farquhar, G.D., Berry, S.L. and Noble, I.R.  2001.  On the direct effect of clouds and atmospheric particles on the productivity and structure of vegetation.  Oecologia 129: 21-30.

Sakai, R.K., Fitzjarrald, D.R., Moore, K.E. and Freedman, J.M.  1996.  How do forest surface fluxes depend on fluctuating light level?  In: Proceedings of the 22nd Conference on Agricultural and Forest Meteorology with Symposium on Fire and Forest Meteorology, Vol. 22, American Meteorological Society, pp. 90-93.

Sarmiento, J.L.  1993.  Atmospheric CO2 stalled.  Nature 365: 697-698.

Shulaev, V., Silverman, P. and Raskin, I.  1997.  Airborne signaling by methyl salicylate in plant pathogen resistance.  Nature 385: 718-721.

Suraqui, S., Tabor, H., Klein, W.H. and Goldberg, B.  1974.  Solar radiation changes at Mt. St. Katherine after forty years.  Solar Energy 16: 155-158.

Last updated 17 August 2005