Is nature sending a smoke signal from the wildfires?

What if I told you that smoke is the alarm siren among trees?

The human toll of the current California wildfires has been catastrophic. The blaze has claimed the lives of more than two dozen people and a devastating amount of property has been lost. The air quality index throughout the Bay Area and Los Angeles has been registering above 150 — an alarming level above which the Environmental Protection Agency warns “everyone may begin to experience health effects.”

But this is a story — one that never ends up on the newspapers made from them — from the perspective of trees. For trees, and the plant kingdom in general, there is no greater existential threat than wildfires. Now imagine that threat being communicated through the for- est as smoke signals.

Smoke is particulate matter emanating from partial combustion and pyrolysis that occur during forest fires. The pyrolysis of organic hydrocarbons releases numerous products including ethylene, which, in my opinion, is one of the most important molecules in the world that barely gets noticed.

Ethylene gas is the simplest alkene in nature. It is not only a product of fire but is also itself flammable and catalyzes feed-forward combustion in the presence of fuel. Ethylene is both synthesized and sensed as a stress hormone by virtually the entire plant kingdom. Its contagious nature is what makes one bad apple spoil the whole bushel. Ethylene-mediated functions include life cycle acceleration of maturation, flowering, reproduction, fruition, ripening, abscission and senescence. Put more simply, ethylene is to plants what cortisol is to animals.

Fire regimes impose a significant disturbance on ecosystems, and evolution has selected a range of adaptive strategies to deal with wildfires. These strategies include not only those that increase survivorship, but also those that promote recolonization during ecological succession. Typically, animals cannot survive the direct effects of fire but possess mobility systems to evade fire. Plants, by contrast, possess limited capacity to evade fire.

Instead, like the alarm call of a white-tailed deer under attack, ethylene emanating from a burning tree serves as a distress signal — like a canary in a coal mine — that activates the alarm stress response among the larger botanical community, especially by those in the downwind direction of the fire.

The forewarned plants accelerate their ethylene-mediated biologic programs and promote serotiny: on- demand flowering, reproduction, seed production, release from seed dormancy, fruition, ripening, senescence, fruit abscission and seed release. Some ethylene-mediated responses promote plant and germ-line protection from fire, while other responses position the lineage more aggressively for the less- competitive, post-fire environment. Another adaptive trait of ethylene for the plant kingdom is its sweetening effect on fruit, which entices consumption by animals who help transport the seeds to distant locations. In addition to fire, other biotic (infection) and abiotic (drought, physical injury, oxidation) disturbances to plants trigger their ethylene-mediated stress responses.

But if stress signaling through ethylene in smoke is adaptive, what are we to make of the fact that ethylene is the most human- synthesized organic compound in the world (over 150 million tons in 2016) for industrial use?

Ethylene is also a major byproduct of fuel combustion such as automobile exhaust. To what degree does the largescale industrial production of ethylene present an illegitimate signal of stress to the plant kingdom? Given the flammable nature of ethylene, are we contributing to inflammation of the ecosystem? Is our largescale production of ethylene a contributor to wildfires and climate change?

Should we be heeding the alarm call of trees?