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Volcanic Eruptions (Biological Impact) -- Summary
In our Editorial of 10 Oct 2001, we describe (among several other things) how increases in atmospheric aerosols increase the amount of diffuse solar radiation reaching the earth's surface and how increased diffuse lighting reduces the volume of shade within vegetative canopies, leading to an increase in whole-canopy photosynthesis.  How significant is this process when the aerosols in question are the product of volcanic eruptions?

Roderick et al. (2001) provide a good estimate based on one of our favorite approaches to questions of this type: the utilization of a unique "natural experiment."  Focusing on the eruption of Mt. Pinatubo in June of 1991, they report that 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 describing this phenomenon, they arrived at the conclusion that the Mt. Pinatubo eruption should 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 vegetative productivity in the year following the eruption, which should have reduced the expected increase in the air's CO2 concentration that year by about 1.2 ppm.

Interestingly, this calculated reduction in atmospheric CO2 concentration is of the same sign and magnitude as the perturbation that was actually observed in 1992 (Sarmiento, 1993).  What makes this correspondence even more impressive is the fact that the reduction in atmospheric CO2 was coincident with an El Nio event, for in the words of Roderick et al., "previous and subsequent such events have been associated with increases in atmospheric CO2."  In addition, the reduction in total solar radiation received at the earth's surface during this period would also have had a tendency to cause the air's CO2 content to rise, as it would have tended to lessen global photosynthetic activity.

Shortly thereafter, Gu et al. (2003) reported that they had used "two independent and direct methods to examine the photosynthetic response of a northern hardwood forest (Harvard Forest, 42.5N, 72.2W) 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 [volcanic aerosol] 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 range of applicability of their observations, Gu et al. say 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."

Almost contemporaneously, however, Krakauer and Randerson (2003) came to the opposite conclusion, based on a study wherein they used ice core records of sulfur deposition to identify the timing and magnitude of 23 Pinatubo-scale volcanic eruptions over the past millennium, together with a global database of dated tree-ring widths that correlate with forest net primary production (NPP, derived from more than 40,000 cores from more than 1000 sites), to test the hypothesis of Roderick et al.  These scientists say they found no increase in NPP immediately following eruptions over the past millennium.  In fact, they report they found "a significant decrease in ring width for trees in middle to high northern latitudes following eruption sulfur peaks."  For trees north of 60N, these decreases, in their words, "were in the range of 2-8% and persisted for ~8 years following the sulfur peaks," with "the maximum reduction in ring width (8.1% 2.7%) occurring in year 4 and an average decrease in years 0-8 of 5.0 1.4%."  For trees between 45N and 60N, the response was more muted: "the maximum reduction in ring width (4.3 1.2%) occurred in year 6, and the average decrease in years 0-8 was 1.9 0.8%."  Everywhere else, tree ring width departures for years 0-8 were not significant.

Clearly, these findings present a significant challenge to the hypothesis of Roderick et al. and the findings of Gu et al., but they are not without problems of their own, one of the greatest being that there is no widely-accepted explanation for the significant time lag between volcanic eruptions and the maximum decrease in tree-ring width.  In addition, working with long-term monthly time series of the atmosphere's CO2 concentration, Reichenau and Esser (2003) found that times of anomalous declining atmospheric CO2 concentration coincided with periods of significant volcanism, in harmony with the hypothesis of Roderick et al.; and they thus concluded that "volcanic eruptions with considerable aerosol production may create disturbances of the (biospheric) carbon cycle by increasing the photosynthetic carbon uptake due to the enhanced diffuse fraction of the incoming [solar] radiation," which accords with the work of Roderick et al. and Gu et al., as well as with the complementary studies of Cohan et al. (2002), Law et al. (2002) and Gu et al. (2002).

In conclusion, the bulk of the studies to address the subject have provided evidence for the propensity of volcanic aerosols to significantly enhance the vegetative productivity of plant assemblages such as forests.  The one dissenting study, however, should serve as a stimulus to further research on the topic.

References
Cohan, D.S., Xu, J., Greenwald, R., Bergin, M.H. and Chameides, W.L.  2002.  Impact of atmospheric aerosol light scattering and absorption on terrestrial net primary productivity.  Global Biogeochemical Cycles 16: 10.1029/2001GB001441.

Gu, L., Baldocchi, D., Verma, S.B., Black, T.A., Vesala, T., Falge, E.M. and Dowty, P.R.  2002.  Advantages of diffuse radiation for terrestrial ecosystem productivity.  Journal of Geophysical Research 107: 10.1029/2001JD001242.

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.

Krakauer, N.Y. and Randerson, J.T.  2003.  Do volcanic eruptions enhance or diminish net primary production?  Global Biogeochemical Cycles 17: 10.1029/2003GB002076.

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.

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.

Last updated 25 May 2005