Background
Based on the work of Roderick et al. (2001), Law et al. (2002) and Gu et al. (2003), a powerful case has been made for the proposition that volcanic eruptions tend to increase forest productivity via an enhanced-aerosol-enhanced increase in the reception of diffuse solar radiation at the surface of the earth that increases the net primary productivity (NPP) of forests even in the face of a decrease in total solar radiation reception.  Now, however, equally powerful evidence is provided by Krakauer and Randerson that some other effect of volcanic eruptions may actually be dong just the opposite, i.e., decreasing forest productivity, at least in mid to high northern latitudes.

What was done
Krakauer and Randerson 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 NPP (derived from more than 40,000 cores from more than 1000 sites) to test the hypothesis of Roderick et al.

What was learned
The two scientists say they "found no increase in NPP in our data set 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 60°N, these decreases "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 45°N and 60°N, 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.

What it means
Clearly, these findings present a significant challenge to the hypothesis of Roderick et al., but they are not without problems of their own, one of the greatest being there is as yet no widely-accepted explanation for the significant time lag between volcanic eruptions and the maximum decrease in tree ring width.  In addition, Reichenau and Esser (2003) have found that periods of significant volcanism tend to coincide with periods of declining atmospheric CO2 concentration, in harmony with the hypothesis of Roderick et al.  This confusing situation should serve as a powerful stimulus for further research into the many intriguing questions raised by these disparate observations.

References
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.

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.