Healthy forests depend on balancing fire and water

Los Alamos scientists used data collected before and after the 2011 Las Conchas fire to study how water content in the soil changed and how it influenced the ecosystem after the blaze. LANL photo

Chinese philosophy tells of yin and yang, two timeless forces that oppose yet complement each other to maintain the universe’s harmony, influencing everything within it.

Nature has various examples of the concept, including fire and water, both of which can be destructive and beneficial. Like yin and yang, how fire and water interact — and which one reigns supreme, even for a short time — influences the destinies of plants, animals and people.

Fire at first might seem exclusively destructive. According to the National Interagency Fire Center, in 2018 more than 50,000 wildfires scorched more than 8.5 million acres across the United States. Verisk’s 2017 Wildfire Risk Analysis reported losses from wildfires exceeded $5 billion during the past 10 years.

But wildfires aren’t always a bad thing for the environment. Indeed, using fire to manage land goes back thousands of years, with Native American peoples using fire to alter the structure and composition of forest and grassland ecosystems.

Fire-management processes continue today, including prescribed burns. These intentionally set fires eliminate excessive vegetation, but don’t burn too hot. Water and other nutrients remain in the soil. Not only do prescribed burns reduce the risk of wildfires, they also can influence the growth of desirable plants.

Yet there’s still a lot scientists don’t know about the yin-yang interaction between fire and water.

Of particular interest is better understanding how the heat intensity of wildfires changes the the water content of burned soil. The science behind such work is known as hydrology, which studies the properties, distribution and circulation of water on or below the earth’s surface.

Wildfires affect soil moisture either by cutting off a process known as evapotranspiration, when water vapor escapes through tiny pores in plant leaves, or by increasing surface runoff — or both. When a wildfire destroys plant life, that evaporation route known as transpiration is removed. The moisture can’t easily transpire through the vegetation from the soil, so the soil stays wet. But also, after a fire, more water runs off the surface.

Particularly hot wildfires not only destroy living vegetation, but they also change soil properties and consume the water-absorbing layer of leaves, needles, twigs and pieces of bark — called litter and duff — that have fallen to the forest floor. Without litter and duff, the soil absorbs less water. When the rain comes, the resulting surface runoff tears through the soil, and that erosion can cause natural disasters like mudslides (common in places such as California) and the locally familiar flash floods that scour through canyons and arroyos.

To unravel exactly how fire influences moisture in burned soil, hydrologists and other environmental scientists at Los Alamos National Laboratory have developed a sophisticated computer model.

The initial experiment using the Los Alamos model indicated that that soil moisture generally increases following fire with the exception of high-severity-burn areas that experience greater surface runoff — the soil is much drier after a really intense wildfire. If heavy or frequent rainstorms follow a fire, the chance of flooding increases, but the soil winds up drier than it was before the fire, despite all that flowing water.

This experiment also verified the competition between evapotranspiration, which makes soil wetter, and surface runoff, which makes soil drier. Future work using this model and others will help explain and more specifically measure how these two competing processes change the water content in soil based on the severity of the wildfire’s heat.

Wildfires will continue to be unpredictable, but advanced modeling software may one day predict water content in soil across a range of burn severities. That will give decision makers in government and industry better information to understand the risks to water resources. It will also enable planners to develop more effective processes to reduce the risk of wildfires and influence the growth of more-desirable ecosystems.

And it will help scientists deepen their understanding of the timeless link between fire and water and how they influence the ecosystems they affect every day.

Adam Atchley is an ecohydrologist in the Computational Earth Science group Los Alamos National Laboratory. He studies multiple process that are tied together by the flow of water into, over and through the Earth’s surface.