London air: clean enough for lichen
For a number of years Londoners have been heartened by the return of fish to the Thames, signaling at least some decline in water pollution. Now there's similar encouragement for London's air. The colorful patches of smog-sensitive lichens are returning to rocks and trees around the city.
This implies more than the spectacular reduction of smoke and of the famous oldtime acrid fogs that has visibly transformed the city over the past couple of decades. It means that the level of sulfur dioxide (SO) had dropped to a point where one of the most sensitive of biological indicators -- a variety of species of the foliose lichens -- now find the London atmosphere livable again.
Such are the findings of the first detailed survey of the return of lichens to trees in a large urban area in Britain. It has more than botanical significance, for it shows how effectively a city famous for its air pollution can be cleansed. At the same time it also points up the fact that there are limits to what air quality control can accomplish.
In reporting their survey in Nature, C. I. Rose of the University of London's Chelsea College and D. L. Hawksworth of the Commonwealth Mycological Institute point out that further improvements in the lichen population are likely over the next few years if current trends continue. But, given the amount and type of fuels that still have to be burned in the area, "it is unlikely that London will regain in the foreseeable future many of the species los during the past two centuries."