Snowy winter highways may be forgotten. But midsummer is a season that highlights the effects of de-icing salt. Weak or dying roadside plants may suffer from salt pollution. With them, however, there may be some vigorous newcomers whose normal habitat is by the sea.
Botanist Peter D. Moore of King's College, University of London, says that some maritime plants have become widespread along Britain's road verges. It is a change that British botanists have been watching for a number of years.
For them, it is an opportunity to study how plant life adapts to relatively sudden environmental change, as when alien species spread along Britain's new railways in Victorian times. It is also, however, another warning of the far-reaching effects of the millions of tons of salt spread on British, and on North American, roads every year.
Britain uses on the order of a million tons of salt in a typical winter. North American roads receive something like six times that amount. In terms of roadside soil pollution, this delivers around three to four kilograms of salt per square meter (5.5 to 7.4 pounds per square yard) per year. Rates of up to six kilograms per square meter (11 pounds per square yard) have been reported for the United States.
As Moore notes in describing the situation in Nature, roadside verges in salted areas receive many times - often 50 times - as much salt input as the most exposed seaside grasslands. ''. . . it is not surprising that the effects of such treatment are being increasingly felt,'' he observes.
Moore first called attention to the spread of the seaside plants six years ago. At that time, he was commenting on a survey of roadside plants around Newcastle-upon-Tyne in Northumberland County. Investigators found several salt-loving species at distances up to 13 kilometers from the sea. These included the sea aster (Aster tripolium), salt marsh grass (Puccinella distans), sea plantain (Plantago maritima), and sea spurrey (Spergularia marina).
Of these, all but the sea aster, which seems to have difficulty establishing itself, are still gradually advancing. The salt marsh grass now has been reported widely throughout northeastern England, the Midlands, East Anglia, and Kent. Moore speculates that part of the aster's trouble may be its dependence on the wind to disperse its seeds. The other plants bear seeds near the ground, where they could be picked up and carried by muddy auto tires.
Whatever the reason for an individual plant's success or failure, the invasion of the halophytes (salt-loving plants) is well under way along the roadsides of England. Extinctions of long-established species may be more abundant than replacement by immigration, Moore says. Also, he suggests that the native species may change their genetic makeup, evolving to cope better with the salt.
The seemingly simple safety practice of de-icing winter roads has already raised concern about ground water pollution. Now it seems to have started an unplanned ecological experiment - one whose progress should be carefully monitored in North America as well as in Britain.