Just as marine phytoplankton that are exposed to rising temperatures give off greater quantities of gases that lead to the production of greater quantities of cloud condensation nuclei, which create more and brighter clouds that reflect more incoming solar radiation back to space and thereby either reverse, stop or slow the warming that initiated this negative feedback phenomenon (see Aerosols (Biological - Aquatic) in our Subject Index), so too do terrestrial plants respond in like manner, thereby enhancing their ability to survive in a changing global environment. What is more, earth's terrestrial plants have a tendency to operate in this manner ever more effectively as the air's CO2 content rises ever higher.
A good introduction to this subject is provided by the review paper of Peñuelas and Llusia (2003), who say that biogenic volatile organic compounds -- or 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 processes occurring in many plant tissues. Some of the functions of these substances, according to the two scientists, include acting as "deterrents against pathogens and herbivores, or to aid wound sealing after damage (Pichersky and Gershenzon, 2002)." They also say that BVOCs 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, in the opinion of Peñuelas and Llusia, is the growing realization 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 claimed ill effects of CO2-induced global warming on earth's vegetation, there are likely to be two strong ameliorative phenomena that act to protect the planet's plants: (1) the aerial fertilization effect of atmospheric CO2 enrichment, which is typically more strongly expressed at higher temperatures [see Growth Response to CO2 with Other Variables (Temperature - Agricultural Crops, Grassland Species and Tree Species) in our Subject Index], and (2) the tendency for rising air temperatures and CO2 concentrations to spur the production of higher concentrations of heat-stress-reducing BVOCs. With respect to temperature, for example, Peñuelas and Llusia calculate 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%."
There may also be a couple of other phenomena that favor earth's plants within this context. Peñuelas 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 indicate that the 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. Peñuelas and Llusia say that "BVOCs generate large quantities of organic aerosols that could affect climate significantly by forming cloud condensation nuclei." As a result, they say "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."
However, not all experiments have reported increases in plant BVOC emissions with increasing atmospheric CO2 concentrations, one example being that of Constable et al. (1999), who found no effect of elevated CO2 on monoterpene emissions from Ponderosa pine and Douglas fir trees. Some studies, in fact, have even reported decreases in BVOC emissions, such as those of Vuorinen et al. (2004), who worked with cabbage plants, and Loreto et al. (2001), who studied monoterpene emissions from oak seedlings. On the other hand, Staudt et al. (2001) observed CO2-induced increases in BVOC emissions in the identical species of oak. An explanation for this wide range of results comes from Baraldi et al. (2004), who -- after exposing sections of a southern California chaparral ecosystem to atmospheric CO2 concentrations ranging from 250 to 750 ppm in 100-ppm increments for a period of four years -- concluded that "BVOC emission can remain nearly constant as rising CO2 reduces emission per unit leaf area while stimulating biomass growth and leaf area per unit ground area." In most of the cases investigated, however, BVOC emissions tend to increase with atmospheric CO2 enrichment; and the increases are often huge.
A case in point is provided by the study of Jasoni et al. (2003), who grew onions from seed for 30 days in individual cylindrical flow-through growth chambers under controlled environmental conditions at atmospheric CO2 concentrations of either 400 or 1000 ppm. At the end of the study, the plants in the CO2-enriched chambers had 40% more biomass than the plants grown in ambient air, and their photosynthetic rates were 22% greater. In addition, the CO2-enriched plants exhibited 17-fold and 38-fold increases in emissions of the BVOC hydrocarbons 2-undecanone and 2-tridecanone, respectively, which Jasoni et al. make a point of noting "confer insect resistance against a major agricultural pest, spider mites." More generally, they conclude that "plants grown under elevated CO2 will accumulate excess carbon and that at least a portion of this excess carbon is funneled into an increased production of BVOCs," which have many positive implications in the realms of both biology and climate, as noted above.
A number of studies suggest that the phenomena discussed in the preceding paragraphs do indeed operate in the real world. Kavouras et al. (1998), for example, measured a number of atmospheric gases and particles in a eucalyptus forest in Portugal and analyzed their observations to see if there was any evidence of biologically-produced gases being converted to particles that could function as cloud condensation nuclei. Their work demonstrated that certain hydrocarbons emitted by vegetation (isoprene and terpenes, in particular) do indeed experience gas-to-particle transformations. In fact, aerosols (or biosols) produced from two of these organic acids (cis- and trans-pinonic acid) comprised as much as 40% of the fine particle atmospheric mass during daytime hours.
These findings clearly demonstrate that the biology of the earth can indeed influence the climate of the earth. Specifically, they reveal a direct connection between the metabolic activity of trees and the propensity for the atmosphere to produce clouds. What is more, the relationship is one that is self-protecting of the biosphere: as the air's CO2 content rises, plant productivity rises, which leads to an enhanced evolution of biogenic gases, which leads to the production of more cloud condensation nuclei, which leads to the creation of more clouds that reflect more solar radiation back to space, which tends to counter any increase in the strength of the atmosphere's greenhouse effect that may have been produced by the initial rise in the air's CO2 content.
A similar study was conducted by O'Dowd et al. (2002), who measured aerosol electrical-mobility size-distributions before and during the initial stage of an atmospheric nucleation event over a boreal forest in Finland. Simultaneously, organic vapor growth rate measurements were made of particles that nucleated into organic cloud-droplets in the flow-tube cloud chamber of a modified condensation-particle counter. This work demonstrated, in their words, that newly-formed aerosol particles over forested areas "are composed primarily of organic species, such as cis-pinonic acid and pinonic acid, produced by oxidation of terpenes in organic vapours released from the canopy."
Commenting on this finding, O'Dowd et al. note that "aerosol particles produced over forested areas may affect climate by acting as nuclei for cloud condensation," but they say there remain numerous uncertainties involving complex feedback processes "that must be determined if we are to predict future changes in global climate." This being the case, we wonder how anyone can presume to decide what should or should not be done about anthropogenic CO2 emissions; for if we can't predict future changes in global climate without the knowledge just specified, how do we know if we even need to be worried about the matter?
Shifting from trees to a much tinier plant, Kuhn and Kesselmeier (2000) collected lichens from an open oak woodland in central California, USA, and studied their uptake of carbonyl sulfide or COS in a dynamic cuvette system under controlled conditions in the laboratory. When optimally hydrated, COS was absorbed from the atmosphere by the lichens at a rate that gradually doubled as air temperature rose from approximately 3 to 25°C, whereupon the rate of COS absorption dropped precipitously, reaching a value of zero at 35°C. Why is this significant?
COS is the most stable and abundant reduced sulfur gas in the atmosphere and is thus a major player in determining earth's radiation budget. After making its way into the stratosphere, it can be photo-dissociated, as well as oxidized, to form SO2, which is typically converted to sulfate aerosol particles that are highly reflective of incoming solar radiation and, therefore, have the capacity to significantly cool the earth as more and more of them collect above the tropopause. This being the case, it is only natural to suspect that biologically-modulated COS concentrations may play a role in keeping earth's surface air temperature within bounds conducive to the continued existence of life; and that is exactly what is implied by the observations of Kuhn and Kesselmeier. Once air temperature rises above 25°C, the rate of removal of COS from the air by this particular species of lichen declines dramatically; and when this happens, more COS remains in the air, which increases the potential for more COS to make its way into the stratosphere, where it can be converted into sulfate aerosol particles that can reflect more incoming solar radiation back to space and thereby cool the earth. And since the consumption of COS by lichens is under the physiological control of carbonic anhydrase -- which is the key enzyme for COS uptake in all higher plants, algae and soil organisms -- we could expect this phenomenon to be generally operative throughout much of the plant kingdom. Hence, this biological "thermostat" may well be powerful enough to define an upper limit above which the surface air temperature of the planet may be restricted from rising, even when changes in other forcing factors, such as greenhouse gases, produce an impetus for it to do so.
Although BVOCs emitted from terrestrial plants both small and large are thus important to earth's climate, trees tend to dominate in this regard; and recent research suggests yet another way in which their response to atmospheric CO2 enrichment may provide an effective counterbalance to the greenhouse properties of CO2.
We have described various aspects of this intriguing CO2-induced cooling effect in our Editorials of 10 Oct 2001, 6 Aug 2003 and 22 Dec 2004. The phenomenon begins with the propensity for CO2-induced increases in BVOCs, together with the cloud particles they spawn, to enhance the amount of diffuse solar radiation reaching the earth's surface (Suraqui et al., 1974; Abakumova et al., 1996), which is followed by the ability of enhanced diffuse lighting to reduce the volume of shade within vegetative canopies (Roderick et al., 2001), which is followed by the tendency for less internal canopy shading to enhance whole-canopy photosynthesis (Healey et al., 1998), which finally produces the end result: a greater photosynthetic extraction of CO2 from the air and the subsequent reduction of the strength of the atmosphere's greenhouse effect.
How significant is this process? Roderick et al. provide a good estimate based on one of our favorite approaches to questions of this type: the utilization of a unique "natural experiment," a technique that has been used extensively by Idso (1998) to evaluate the overall climatic sensitivity of the planet. Specifically, Roderick and his colleagues consider the volcanic eruption of Mt. Pinatubo in June of 1991. This event ejected enough gases and fine materials into the atmosphere that it produced sufficient aerosol particles to greatly increase the diffuse component of the solar radiation reaching the surface of the earth from that point in time through much of 1993, while only slightly reducing the receipt of total solar radiation.
Based on a set of lengthy calculations, Roderick et al. concluded that the Mt. Pinatubo eruption may well 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 photosynthesis in the year following the eruption, which would have reduced the ongoing rise in the air's CO2 concentration that year by about 1.2 ppm. Interestingly, this reduction is about the size of the real-world reduction that was measured that year (Sarmiento, 1993). What makes this observation even more impressive is the fact that the CO2 reduction was coincident with an El Niño event; because, in the words of Roderick et al., "previous and subsequent such events have been associated with increases in atmospheric CO2." In addition, the observed reduction in total solar radiation received at the earth's surface 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.
Additional real-world evidence for the existence of this phenomenon was provided by Gu et al. (2003), who reported using "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." They found 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." In reflecting on the significance of these observations, Gu et al. stated 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."
As impressive as these findings are, Gu et al. did not stop there. They went on to document a powerful propensity for the extra diffuse light created by increased cloud cover to further enhance forest photosynthesis, and to do so even though the total flux of solar radiation received at the earth's surface is typically significantly reduced under such conditions (Stanhill and Cohen, 2001). Based on still more real-world data, for example, they reported finding that "Harvard Forest photosynthesis also increases with cloud cover, with a peak at about 50% cloud cover."
In all of the original investigations of this phenomenon, which also include the studies of Law et al. (2002), Farquhar and Roderick (2003) and Reichenau and Esser (2003), the source of the enhanced aerosol concentration, i.e., the key natural experiment, was a massive volcanic eruption. So what happens under more normal conditions? This is the question that was asked by Niyogi et al. (2004): "can we detect the effect of relatively routine aerosol variability on field measurements of CO2 fluxes, and if so, how does the variability in aerosol loading affect CO2 fluxes over different landscapes?"
To answer this question, the group of sixteen researchers used CO2 flux data from the AmeriFlux network (Baldocchi et al., 2001) together with cloud-free aerosol optical depth data from the NASA Robotic Network (AERONET; Holben et al., 2001) to assess the effect of aerosol loading on the net assimilation of CO2 by three types of vegetation: trees (broadleaf deciduous forest and mixed forest), crops (winter wheat, soybeans and corn) and grasslands. Their work revealed that an aerosol-induced increase in diffuse radiative-flux fraction [DRF = ratio of diffuse (Rd) to total or global (Rg) solar irradiance] increased the net CO2 assimilation of trees and crops, making them larger carbon sinks, but that it decreased the net CO2 assimilation of grasslands, making them smaller carbon sinks.
How significant were the effects observed by Niyogi et al.? For a summer mid-range Rg flux of 500 W m-2, going from the set of all DRF values between 0.0 and 0.4 to the set of all DRF values between 0.6 and 1.0 resulted in an approximate 50% increase in net CO2 assimilation by a broadleaf deciduous forest located in Tennessee, USA. Averaged over the entire daylight period, they further determined that the shift from the lower to the higher set of DRF values "enhances photosynthetic fluxes by about 30% at this study site." Similar results were obtained for the mixed forest and the conglomerate of crops studied. Hence, they concluded that natural variability among commonly-present aerosols can "routinely influence surface irradiance and hence the terrestrial CO2 flux and regional carbon cycle." For these types of land-cover (forests and agricultural crops), that influence is to significantly increase the assimilation of CO2 from the atmosphere; and this effect greatly overpowers the opposite effect that occurs over grasslands, primarily because earth's trees and shrubs are responsible for fully two thirds of the planet's net primary production.
What is especially exciting about these real-world observations is that much of the commonly-present aerosol burden of the atmosphere is plant-derived. Hence, it can be appreciated that earth's woody plants are themselves responsible for emitting to the air that which ultimately enhances their photosynthetic prowess. In other words, earth's trees significantly control their own destiny, i.e., they alter the atmospheric environment in a way that directly enhances their opportunities for greater growth.
Humanity also helps in this regard; for as we pump ever more CO2 into the atmosphere, the globe's woody plants quickly respond to its aerial fertilization effect, becoming ever more productive, which leads to even more plant-derived aerosols being released to the atmosphere, which stimulates this positive feedback cycle to a still greater degree. Stated another way, earth's trees use some of the CO2 emitted to the atmosphere by man to alter the aerial environment so as to enable them to remove even more CO2 from the air. The end result is that earth's trees and humanity are working hand-in-hand to significantly increase the productivity of the biosphere; and it is happening in spite of all of the true anthropogenic insults to the environment that work in opposition to enhanced biological activity.
In light of these several observations, it should be painfully obvious, as we originally suggested in our Editorial of 10 October 2001, that the historical and still-ongoing CO2-induced increase in atmospheric BVOCs should have had, and should be continuing to have, a significant cooling effect on the planet that exerts itself by both slowing the rate of rise of the air's CO2 content and reducing the receipt of solar radiation at the earth's surface, neither of which effects is included in any general circulation model of the atmosphere of which we are aware. Hence, it should be equally obvious that climate-alarmist predictions of catastrophic CO2-induced global warming are simply catastrophic exaggerations.
One final beneficial effect of CO2-induced increases in BVOC emissions is described by Goldstein et al. (2004), and that is the propensity of BVOCs to destroy tropospheric ozone.
As a bit of a background, earth's vegetation is responsible for the production of vast amounts of ozone (O3; Chameides et al., 1988; Harley et al., 1999), but it is also responsible for destroying a lot of O3. With respect to the latter phenomenon, Goldstein et al. mention three major routes by which O3 exits the air near the earth's surface: leaf stomatal uptake, surface deposition, and within-canopy gas-phase chemical reactions with BVOCs. The first of these exit routes, according to them, accounts for 30-90% of total ecosystem O3 uptake from the atmosphere (= O3 destruction), while the remainder has typically been attributed to deposition on non-stomatal surfaces. However, they note that "Kurpius and Goldstein (2003) recently showed that the non-stomatal flux [from the atmosphere to oblivion] increased exponentially as a function of temperature at a coniferous forest site," and that "the exponential increase with temperature was consistent with the temperature dependence of monoterpene emissions from the same ecosystem, suggesting O3 was lost via gas phase reactions with biogenically emitted terpenes before they could escape the forest canopy."
In a study designed to take the next step towards turning the implication of this observation into something stronger than a mere suggestion, Schade and Goldstein (2003) demonstrated that forest thinning dramatically enhances monoterpene emissions. In the current study, Goldstein et al. take another important step towards clarifying the issue by measuring the effect of forest thinning on O3 destruction in an attempt to see if it is enhanced in parallel fashion to the thinning-induced increase in monoterpene emissions.
In a ponderosa pine plantation in the Sierra Nevada Mountains of California, USA, a management procedure to improve forest health and optimize tree growth was initiated on 11 May and continued through 15 June 2000. This procedure involved the use of a masticator to mechanically "chew up" smaller unwanted trees and leave their debris on site, which operation reduced plantation green leaf biomass by just over half. Simultaneously, monoterpene mixing ratios and fluxes were measured hourly within the plantation canopy, while total ecosystem O3 destruction was "partitioned to differentiate loss due to gas-phase chemistry from stomatal uptake and deposition."
Goldstein et al. report that both the destruction of ozone due to gas-phase chemistry and emissions of monoterpenes increased dramatically with the onset of thinning, and that these phenomena continued in phase with each other thereafter. Hence, they "infer that the massive increase of O3 flux [from the atmosphere to oblivion] during and following mastication is driven by loss of O3 through chemical reactions with unmeasured terpenes or closely related BVOCs whose emissions were enhanced due to wounding [by the masticator]." Indeed, they say that "considered together, these observations provide a conclusive picture that the chemical loss of O3 is due to reactions with BVOCs emitted in a similar manner as terpenes," and that "we can conceive no other possible explanation for this behavior other than chemical O3 destruction in and above the forest canopy by reactions with BVOCs."
Goldstein et al. say their results "suggest that total reactive terpene emissions might be roughly a factor of 10 higher than the typically measured and modeled monoterpene emissions, making them larger than isoprene emissions on a global scale." If this proves to be the case, it will be a most important finding, for it would mean that vegetative emissions of terpenes, which lead to the destruction of ozone, are significantly greater than vegetative emissions of isoprene, which lead to the creation of ozone (Poisson et al., 2000). In addition, there is substantial evidence to suggest that the ongoing rise in the air's CO2 content may well lead to an overall reduction in vegetative isoprene emissions (see Isoprene in our Subject Index), while at the same time enhancing vegetative productivity, which may well lead to an overall increase in vegetative terpene emissions. As a result, there is reason to believe that the ongoing rise in the air's CO2 content will help to reduce the ongoing rise in the air's O3 concentration, which should be a boon to the entire biosphere.
In conclusion, a wealth of real-world evidence is beginning to suggest that both rising air temperatures and CO2 concentrations significantly increase desirable vegetative BVOC emissions, particularly from trees, which constitute the most prominent photosynthetic force on the planet, and that this phenomenon has a large number of extremely important and highly beneficial biospheric consequences.
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.
Baldocchi, D., Falge, E., Gu, L.H., Olson, R., Hollinger, D., Running, S., Anthoni, P., Bernhofer, C., Davis, K., Evans, R., Fuentes, J., Goldstein, A., Katul, G., Law B., Lee, X.H., Malhi, Y., Meyers, T., Munger, W., Oechel, W., Paw U, K.T., Pilegaard, K., Schmid, H.P., Valentini, R., Verma, S., Vesala, T., Wilson, K. and Wofsy, S. 2001. FLUXNET: A new tool to study the temporal and spatial variability of ecosystem-scale carbon dioxide, water vapor, and energy flux densities. Bulletin of the American Meteorological Society 82: 2415-2434.
Baraldi, R., Rapparini, F., Oechel, W.C., Hastings, S.J., Bryant, P., Cheng, Y. and Miglietta, F. 2004. Monoterpene emission responses to elevated CO2 in a Mediterranean-type ecosystem. New Phytologist 161: 17-21.
Chameides, W.L., Lindsay, R.W., Richardson, J. and Kiang, C.S. 1988. The role of biogenic hydrocarbons in urban photochemical smog: Atlanta as a case study. Science 241: 1473-1475.
Farquhar, G.D. and Roderick, M.L. 2003. Pinatubo, diffuse light, and the carbon cycle. Science 299: 1997-1998.
Goldstein, A.H., McKay, M., Kurpius, M.R., Schade, G.W., Lee, A., Holzinger, R. and Rasmussen, R.A. 2004. Forest thinning experiment confirms ozone deposition to forest canopy is dominated by reaction with biogenic VOCs. Geophysical Research Letters 31: 10.1029/2004GL021259.
Harley, P.C., Monson, R.K. and Lerdau, M.T. 1999. Ecological and evolutionary aspects of isoprene emission from plants. Oecologia 118: 109-123.
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.
Holben, B.N., Tanré, D., Smirnov, A., Eck, T.F., Slutsker, I., Abuhassan, N., Newcomb, W.W., Schafer, J.S., Chatenet, B., Lavenu, F., Kaufman, Y.J., Castle, J.V., Setzer, A., Markham, B., Clark, D., Frouin, R., Halthore, R., Karneli, A., O'Neill, N.T., Pietras, C., Pinker, R.T., Voss, K. and Zibordi, G. 2001. An emerging ground-based aerosol climatology: Aerosol Optical Depth from AERONET. Journal of Geophysical Research 106: 12,067-12,097.
Idso, S.B. 1998. CO2-induced global warming: a skeptic's view of potential climate change. Climate Research 10: 69-82.
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.
Kavouras, I.G., Mihalopoulos, N. and Stephanou, E.G. 1998. Formation of atmospheric particles from organic acids produced by forests. Nature 395: 683-686.
Kuhn, U. and Kesselmeier, J. 2000. Environmental variables controlling the uptake of carbonyl sulfide by lichens. Journal of Geophysical Research 105: 26,783-26,792.
Kurpius, M.R. and Goldstein, A.H. 2003. Gas-phase chemistry dominates O3 loss to a forest, implying a source of aerosols and hydroxyl radicals to the atmosphere. Geophysical Research Letters 30: 10.1029/2002GL016785.
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.
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.
Niyogi, D., Chang, H.-I., Saxena, V.K., Holt, T., Alapaty, K., Booker, F., Chen, F., Davis, K.J., Holben, B., Matsui, T., Meyers, T., Oechel, W.C., Pielke Sr., R.A., Wells, R., Wilson, K. and Xue, Y. 2004. Direct observations of the effects of aerosol loading on net ecosystem CO2 exchanges over different landscapes. Geophysical Research Letters 31: 10.1029/2004GL020915.
O'Dowd, C.D., Aalto, P., Hameri, K., Kulmala, M. and Hoffmann, T. 2002. Atmospheric particles from organic vapours. Nature 416: 497-498.
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.
Poisson, N., Kanakidou, M. and Crutzen, P.J. 2000. Impact of non-methane hydrocarbons on tropospheric chemistry and the oxidizing power of the global troposphere: 3-dimensional modeling results. Journal of Atmospheric Chemistry 36: 157-230.
Reichenau, T.G. and Esser, G. 2003. Is interannual fluctuation of atmospheric CO2 dominated by combined effects of ENSO and volcanic aerosols? Global Biogeochemical Cycles 17: 10.1029/2002GB002025.
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.
Sarmiento, J.L. 1993. Atmospheric CO2 stalled. Nature 365: 697-698.
Schade, G.W. and Goldstein, A.H. 2003. Increase of monoterpene emissions from a pine plantation as a result of mechanical disturbances. Geophysical Research Letters 30: 10.1029/2002GL016138.
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.
Stanhill, G. and Cohen, S. 2001. Global dimming: a review of the evidence for a widespread and significant reduction in global radiation with discussion of its probable causes and possible agricultural consequences. Agricultural and Forest Meteorology 107: 255-278.
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.
Vuorinen, T., Reddy, G.V.P., Nerg, A.-M. and Holopainen, J.K. 2004. Monoterpene and herbivore-induced emissions from cabbage plants grown at elevated atmospheric CO2 concentration. Atmospheric Environment 38: 675-682.Last updated 6 July 2005