Where there's smoke, there's germination.

A wildfire blazes through a chaparral, incinerating the dense brush and charring the scrub oak. The plants that live in this dry western habitat are adapted to the occasional fire, however, and soon re-green the blackened landscape--"almost like magic," says Stanford University ecologist Hal Mooney 

Scientists have long thought that heat is responsible for triggering new growth after a burn, and for some hard-seeded plants that seems to be the case. Heat destroys the outer coat, letting the seed soak up water and germinate.

More recent work shows that chaparral plants respond not to the heat but to the smoke. Out of the cocktail of chemicals gassing from a smoking fire, plant ecologists Jon E. Keeley and C.J. Fotheringham of Occidental College in Los Angeles have found one that has a dramatic effect on germination: nitrogen dioxide.

The researchers, reporting in the May 23 Science, tested the effects of smoke and nitrogen dioxide on seeds of whispering bells (Emmenanthe penduliflora), an annual flowering plant that thrives on newly burned ground.

"What we've shown is that the nitrogen dioxide in smoke, at the same levels that occur in smoke, is sufficient to induce complete germination in the species," says Keeley.

In almost all trials, 100 percent of the seeds germinated in response to the experimental treatments, while the control seeds stayed dormant. "It's the difference between night and day," says Keeley.

The response was strong whether the seeds were exposed directly or indirectly to the trigger. The seeds could be gassed with nitrogen dioxide, smoked with a burning twig, or incubated on sand or filter paper, that Pad been exposed to nitrogen dioxide or to smoke. In the nitrogen dioxide treatments, all the seeds germinated in only half a minute.

Fire ecologist William J. Platt of Louisiana State University in Baton Rouge says he likes the newly found role for nitrogen dioxide. "It may be very general because it is very simple."

The smoke signal seems to work in some other species, but it doesn't explain the germination of all fire-adapted plants, says Keeley. Although fire plays a part in many different plant communities, smoke-induced germination seems to be restricted to plants from climates with hot, dry summers, like those of the Mediterranean. Smoke-induced germination has been found in chaparral-like communities in parts of South Africa and Australia. "It's likely that the nitrogen dioxide mechanism will work in some of those systems as well," says Keeley.

The species in chaparral communities are closely related to desert plants and demand a lot of sunlight, says Keeley. It is to their advantage for seeds to lie dormant until a fire burns away the brush, allowing the seedlings to get enough light, he explains.

Exactly how nitrogen dioxide in smoke triggers germination isn't clear. The researchers found that although water can enter a seed whether it is dormant or not, smoke induces a change in the permeability of a barrier under the seed coat, opening it up to certain other molecules.

Seeds lying dormant in the chaparral soil may be affected by nitrogen dioxide from sources other than wildfires, Keeley and Fotheringham point out. Nitrogen oxides are a by-product of fossil fuel combustion, and nitrogen concentrations in polluted areas like Los Angeles are high enough to trigger seed germination, the researchers suggest. Without a fire to clear the brush, the germinating seeds wouldn't survive, potentially threatening the population.

Although ecological and other impacts from increasing amounts of nitrogen in the environment loom large (SN: 2/15/97, p.100), Keeley says there is as yet no evidence that nitrogen pollution is initiating seed germination.