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BVOCs: What Are They? ... and How Do They Function Within the Context of Global Climate Change?
Volume 6, Number 35: 27 August 2003

"Plants re-emit a substantial fraction of their assimilated carbon into the atmosphere as biogenic volatile organic compounds (BVOCs) that affect the chemical and physical properties of the atmosphere."  With these words, Peņuelas and Llusia (2003) begin their review of a very significant but largely unappreciated subject: the many important roles of BVOCs in both responding to and forcing global climate change.

The two scientists from the Ecophysiology Unit of the Center for Ecological Research and Forestry Applications of the Universitat Autonoma de Barcelona in Spain first remind us 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 Peņuelas 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 (Peņuelas et al., 1995; Shulaev et al., 1997)."

Of particular importance within the context of global climate change is the scientists' belief 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 protecting earth's plants: first, the aerial fertilization effect of atmospheric CO2 enrichment, which is typically more effective at higher temperatures [see Growth Response to CO2 with Other Variables (Temperature - Agricultural Crops, Grassland Species and Tree Species) in our Subject Index], and second, the tendency for rising temperatures to spur the production of higher concentrations of heat-stress-reducing BVOCs in plants.

There may also be a couple of other phenomena that favor earth's plants within this context.  Peņuelas and Llusia note, for example, that "the increased release of nitrogen into the biosphere by man probably also enhances BVOC emissions by increasing the level of carbon fixation and the activity of the responsible enzymes (Litvak et al., 1996)."  In addition, they point out that the natural conversion of abandoned agricultural lands to forests and the implementation of planned reforestation projects should help the rest of the biosphere too, reporting that additional numbers of "Populus, Eucalyptus or Pinus, which are major emitters, might greatly increase BVOC emissions."

Most intriguing of all, perhaps, is how increased BVOC emissions might impact climate change itself.  The authors say that "BVOCs generate large quantities of organic aerosols (Kavouras et al., 1998) that could affect climate significantly by forming cloud condensation nuclei." [See Clouds (Condensation Nuclei) and Aerosols (Biological - Terrestrial) in our Subject Index.]  "As a result," they continue, "there should be a net cooling of the Earth's surface during the day because of radiation interception," noting that Shallcross and Monks (2000) "have suggested that one of the reasons plants emit the aerosol isoprene might be to cool the surroundings in addition to any physiological or evaporative effects that might cool the plant directly."

To be fair, Peņuelas and Llusia indicate some ways by which BVOC emissions might also produce warming effects, citing the studies of Hayden (1998), Fuentes et al. (2001) and some of their own thinking on the subject.  However, these potential positive feedbacks are much more localized and speculative than are the negative feedbacks they enumerate.

Currently, we are nowhere near being able to write the last word on this subject and will probably not be for several years.  "As is the case with so many environmental issues," write Peņuelas and Llusia, "interactive interdisciplinary research among biologists, physicists and chemists at foliar, ecosystem, regional and global scales is needed to solve these puzzles."  That they do indeed need solving and not sweeping under the rug (as is done when climate-alarmists claim we know enough now to mandate CO2 emissions reductions), is demonstrated by the authors' calculations that "global warming over the past 30 years could have increased the BVOC global emissions by approximately 10%, and a further 2-3°C rise in the mean global temperature ... could increase BVOC global emissions by an additional 30-45%."

The climatic consequences of such sizeable BVOC increases could be huge; and until we know what they are, it is prudent not to get the regulatory cart before the scientific horse in the matter of anthropogenic CO2 emissions, especially when it is noted, as reported by Peņuelas and Llusia, that global warming "increases the emission rates of most BVOCs exponentially."  Clearly, we need to determine the both the signs and the magnitudes of the many phenomena with which BVOCs interact before we can ever pretend to know the grand trajectory of future climate change or its many biological consequences.  Until then, it should be full speed ahead with respect to CO2 and BVOC science, but a long time out with respect to CO2 regulation.

Sherwood, Keith and Craig Idso

References
Fuentes, J.D., Hayden, B.P., Garstang, M., Lerdau, M., Fitzjarrald, D., Baldocchi D.D., Monson, R., Lamb, B. and Geron, C.  2001.  New directions: VOCs and biosphere-atmosphere feedbacks.  Atmospheric Environment 35: 189-191.

Hayden, B.P.  1998.  Ecosystem feedbacks on climate at the landscape scale.  Philosophical Transactions of the Royal Society of London, Series B - Biological Sciences 353: 5-18.

Kavouras, I.G., Mihalopoulos, N. and Stephanou, E.G.  1998.  Formation of atmospheric particles from organic acids produced by forests.  Nature 395: 683-686.

Litvak, M.E., Loreto, F., Harley, P.C., Sharkey, T.D. and Monson, R.K.  1996.  The response of isoprene emission rate and photosynthetic rate to photon flux and nitrogen supply in aspen and white oak trees.  Plant, Cell and Environment 19: 549-559.

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

Peņuelas, 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.

Shallcross, D.E. and Monks, P.S.  2000.  A role for isoprene in biosphere-climate-chemistry feedbacks.  Atmospheric Environment 34: 1659-1660.

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