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."
In other words, it would unrealistic to expect a return to the air quality of an unpolluted countryside; although this does not detract from the fact that dirty urban air can be, and has been, cleaned to a considerable extent.
Over Britain as a whole, smoke pollution has dropped 80 percent in urban areas while SO pollution has been cut in half in urban areas since 1960. This is the sort of cleanup that the return of the lichens reflects in London. Rose and Hawksworth note that, while other factors might be involved, the growth rate has correlated so well with improved air quality that there is little doubt that this is the dominant reason for the recolonization. Indeed, much of the lichen growth has come within the past three to seven years.
There should be still more lichen growth ahead, since SO levels in London air are expected to fall for another four years. After that, though, anticipated changes in the fuel mix of the area, which will result in more sulfur being released, are expected to reduce air quality to that prevailing around 1975. Because of this, "only a limited rather than a substantial improvement in the lichen flora of the area seems likely," the scientists say.
Lichens are valuable for such a study. Instrument readings can determine whether or not air quality meets prescribed human standards. But the lichens provided a direct biological test. Either the air is livable for them or not. And many of them are far more intolerant of sulfur pollution than are people. Thus careful surveys of these plants can be an important adjunct of air-pollution monitoring -- a kind of early-warning system for city dwellers.