What was done
Among other things, the authors evaluate the health effects of sulfur dioxide pollution on Russian forests.

What was learned
The authors note that "sulfur dioxide (SO2) causes widespread damage to plants, because it can spread through large distances, and its emissions into the atmosphere are large."  In 1996, for example, they say that "total SO2 emission from the industrial areas of Russia comprised 5866.76 thousand tons, or 42.2% of the total emission of liquid and gaseous pollutants."  In quantifying the extent and severity of this phenomenon, they find that 1.3 million hectares of Russian forest land have been adversely affected by SO2 pollution.  They further note that total forest destruction occurs on 2-5% of the above area, and that heavy, moderate and slight damage occur on 10-15%, 30-40% and 40-50% of this area, respectively.

What it means
We report these results to indicate the seriousness of SO2 pollution for forest health and to highlight the fact that atmospheric CO2 enrichment can significantly alleviate sulfur dioxide's adverse biological consequences.  Hallgren (1984), for example, demonstrated that a 300-ppm increase in the air's CO2 concentration stimulated the photosynthetic rate of Scots pine trees by 64% in unpolluted air and by 77% in air with abnormally high SO2 concentrations, while a 600-ppm increase in atmospheric CO2 stimulated photosynthetic rates in this important forest species by 85% in unpolluted air and by 110% in air of high SO2 concentration.  With respect to agricultural species, Carlson (1983) found that a 900-ppm increase in the air's CO2 concentration boosted photosynthetic rates of soybeans by 87% in unpolluted air but by whopping 715% in high-SO2 air.  Hence, the ongoing rise in the air's CO2 content can do much to either totally prevent or significantly alleviate the adverse consequences of anthropogenic SO2 pollution.

References
Carlson, R.W.  1983.  The effect of SO2 on photosynthesis and leaf resistance at varying concentrations of CO2.  Environmental Pollution Series A 30: 309-321.

Hallgren, J.-E.  1984.  Photosynthetic gas exchange in leaves affected by air pollutants.  In: Koziol, M.J. and Whatley, F.R. (Eds.).  Gaseous Air Pollutants and Plant Metabolism.  Butterworths, London, UK, pp. 147-159.