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Fuels, not fire weather, control carbon emissions in boreal forest

Fuels, not fire weather, control carbon emissions in boreal forest
Rockets represent carbon stored in wood, trees, and soil in four main boreal forest regions. Though fire weather helps “ignite” the rockets, the amount of emissions each forest can produce is determined by fuel load (soil layers) and flammability (soil moisture). Credit: Victor Leshyk, Center for Ecosystem Science and Society

As climate warming stokes longer fire seasons and more severe fires in the North American boreal forest, being able to calculate how much carbon each fire burns grows more urgent. New research led by Northern Arizona University and published this week in Nature Climate Change suggests that how much carbon burns depends more on available fuels than on fire weather such as drought conditions, temperature, or rain. In a large retrospective study that stretched across Canada and Alaska, the international team of researchers found that the carbon stored belowground in soil organic matter was the most important predictor of how much carbon a fire will release.


The team surveyed the vast Western Boreal’s diverse forest conditions by analyzing field data collected from 417 burn sites in six ecoregions in Canada and Alaska between 2004-2015. They found that the amount of carbon stored in soils was the biggest predictor of how much carbon would combust, and that soil moisture was also significant in predicting carbon release.

“In these northern forests, soil, not trees, can account for up to 90 percent of carbon emissions, so we expected that these organic soils would be a significant driver,” said lead author Xanthe Walker of the Center for Ecosystem Science and Society at Northern Arizona University. “But we were surprised that fire weather and the time of year a fire starts proved to be poor indicators of carbon combustion. It’s really about the fuels that are there when a fire starts.”

That’s a pivotal finding, since fire weather, as measured by a Fire Weather Index, is one of the main tools scientists and fire managers currently use to model carbon emissions in these boreal forests. This study suggests fuels should be a bigger component of those models. “When we think of climate change and wildfires, we often instinctively think of extreme weather conditions,” said Marc-André Parisien, a research scientist with the Canadian Forest Service and co-author of the study. “But our study shows that vegetation also matters—a lot! Predicting future vegetation is a tough nut to crack, but this study emphasizes the need to keep chipping away at it.”

The vegetation patterns they uncovered were complex—soil moisture, tree species composition, and stand age at the time of fire all interacted to predict combustion amounts. For instance, highly flammable black spruce was generally a predictor of carbon combustion, and the presence of this species increased with site moisture and stand age at the time of fire. But such interactions are likely to change with the climate. For example, as the climate warms and fire intervals shorten, black spruce stands are being replaced by deciduous trees and jack pine, which grow in shallower soils that release less

The weather phenomenon behind yesterday’s storms, explained

At its peak, there were more than a quarter of a million power outages across Massachusetts yesterday caused by a strong line of thunderstorms that moved from Ontario, Canada, all the way across southern New England. As the cleanup continues, you might wonder what caused all the tree damage and resulting power outages.

First of all, what occurred yesterday was not a tornado. Tornadoes are a specific wind phenomenon in which the wind is rotating; we did not have any rotation yesterday. What we did see were microbursts. A microburst is just a small downburst that is usually less than 4 kilometers across. You can contrast this with a derecho, another severe wind phenomenon, which is a line of straight-line winds that lasts a longer time and moves across a wider area. One might even argue that yesterday’s line of microbursts was in a sense a derecho, but I will leave that to the folks at the National Weather Service to decide.

No matter what we call it, the winds yesterday were caused by strong wind coming down from higher levels of the atmosphere, where part of the thunderstorm was occurring, and being brought to the surface.

A typical microburst brings strong winds from inside of a thunderstorm to the surface.
A typical microburst brings strong winds from inside of a thunderstorm to the surface.NOAA

Thunderstorms are created when we have strong updrafts carrying moist air higher in the atmosphere, which is subsequently cooled, and condenses. This process releases latent heat and allows that storm to continue to develop as that warm air rises; sometimes, like yesterday, these storms go on to reach severe criteria. Severe thunderstorms need to have at least one of the following: wind gusts of at least 58 miles per hour, 1 inch of hail, or a tornado.

Thunderstorms are caused when warm and moist air is lifted higher in the atmosphere from the surface by a lifting mechanism such as a cold front, the sun, or even a sea breeze.
Thunderstorms are caused when warm and moist air is lifted higher in the atmosphere from the surface by a lifting mechanism such as a cold front, the sun, or even a sea breeze.NOAA

In these storms hail and lots of rain are created in the upper parts of the thunderstorm. When the updraft is no longer powerful enough to keep the hail and rain from falling, rain and hail will start to rapidly come down to the surface. As this happens, the strong winds that are in the upper part of a thunderstorm get pulled down to the surface with them. Here’s an analogy to help visualize this. Think about how you can feel air rushing by you when a large truck passes as you stand on a sidewalk. The truck is dragging the air in the same way the air is being dragged by rain and hail. In the truck example air moves horizontally by you, but in the thunderstorm that air hits the ground and then rushes outward, creating damage.

Sometimes the winds can be brought to the surface without any rain and the updraft simply collapses on itself. These are called dry microbursts. When we experience heavy rain with strong winds we have a wet microburst. Either