Week #1: Fire

07 Jan 2018

Paper: Influence of tree species on continental differences in boreal fires and climate feedbacks, by Brendan M. Rogers et al. (Nature Geoscience, 2015) [pdf].

The first week paper is about fire—specifically, forest fires. Most of the recent papers on wildfire I found were about fire ecology, meaning the relationship between wildfires and the surrounding environment. As you might imagine, many of these papers are also about climate. It also turns out you can learn a lot about wildfires using satellite imagery, which surprised me at first but makes a lot of sense in retrospect.

Wildfire and climate

The focus of this paper is on fires in the boreal forest (aka taiga or snow forest), a biome typified by coniferous trees such as pines, spruces, and larches. In North America, think Canada and Alaska. In Europe and Asia, think Norway, Sweden, Finland, and Siberia.

Why should we care about fires in the boreal forest? Well, it turns out that the boreal forest is actually the world’s largest biome next to the oceans, and boreal forests comprise about a third of the total global forested area. Many studies predict an increase in the amount of boreal forest burned during the next century, so it will likely be very important for us to understand the drivers and effects of boreal fires and how they interact with climate.

To give an example of how forests interact with climate, we should talk about albedo, a measurement of how much sunlight an object reflects rather than absorbs. If an object has lower albedo, it absorbs more radiation, and the object will get hotter. This is why a black car will get hotter than a white car in the summer—the black car has lower albedo. Forests have relatively low levels of albedo, which means they absorb a lot of heat. For this reason, North American boreal fires are thought to have a net cooling effect on their environment, because the albedo of the area will be higher (i.e., less heat absorption) once the trees are gone. As a quick aside, one can measure the Earth’s albedo by measuring the amount of “earthshine”—this is a great word—which is sunlight that has been reflected off the Earth, onto the moon, and then back to the Earth again.

The impacts of fire

The impacts of a wildfire depend on what timescale you’re considering. In this paper, the authors focus on the impacts of wildfires on three different timescales:

instantaneous impact: “fire intensity”
immediate impact: “fire severity”
longer-term ecosystem change: “burn severity”

Long-story short, and all details aside for now, the authors found that all three of these measurements of fire impact were lower for wildfires in Eurasian boreal forests than in North American boreal forests.

Measuring fires with satellites

Before I get into why boreal wildfires in Eurasia were less intense than those in North America, I want to talk about where the data came from. Most of the data was obtained via satellite imagery from the LP DAAC, a partnership between the U.S. Geological Survey (USGS) and NASA. Isn’t it funny how you can read something like “satellite imagery” and it can sound sort of commonplace? But really, think about it: There is a single satellite, named Terra, that launched in 1999, and has been orbiting Earth once a day ever since. It has five sensors, one of which is called MODIS, that takes pictures of the entire surface of Earth (like this one) at a resolution ranging between 250m to 1km. And all this data is shared free and publicly, so that scientists around the world can learn about vegetation, snow and ice. Data sources like these are really the only way we can even study the effects of wildfires and other natural disasters on such a large spatial scale. That’s a pretty huge payoff for a single sensor on a single satellite.

How trees cope with wildfire

Anyway, back to the paper. Why are the impacts of boreal wildfires so different in Eurasia than in North America? The authors suggest that the driving difference comes from the different tree species in the two continents.

Wildfires in boreal forests are frequent, but different species of trees have adapted to these wildfires in different ways. For example, coniferous trees (which make up the boreal forest) can be put into one of the following categories:

fire embracer: promote high-intensity fires, e.g., by keeping their lower branches; often killed by fire, but regenerate from the seeds they release when burned
fire resister: suppress high-intensity fires, e.g., thick bark increases the chances of surviving a fire
fire avoider: lack fire-adapting traits, and so tend to live in wetter environments where fires are less frequent

The key claim of this paper is that North American boreal forests are dominated by fire embracers, while the boreal forests in Eurasia are dominated by fire resisters. This, they suggest, results in the different impacts of fire between the two continents.

For example, black spruce and jack pine, both fire embracers, accounted for 76% of the burned area in North American boreal forests. There was only one species of fire resister tree in this area, which accounted for only 0.01% of the burned area. By contrast, there are no fire embracers in the boreal forests of Eurasia, which the authors point out is especially surprising given that the same genera (genii? genuses?) of tree species are represented in both continents.

The claim that the traits of individual species may account for differences in fire impact at the level of an entire continent may seem surprising, but the authors point out that high-latitude ecosystems tend to have much lower species diversity compared to those in lower-latitudes. Thus, “species-level influences on ecological processes are evident at large spatial scales.”

The authors also find that, in contrast to North American boreal wildfires, the Eurasian wildfires were either climate-neutral or had a net warming effect. Remember in the paragraph about albedo, I mentioned that North American boreal fires have a net cooling effect. This difference is presumably also due to the different tree species on the two continents, which suggests the importance of considering the traits of different species when modeling the effects of wildfire on climate.

Summary

Topic: Wildfires in the boreal forest, the largest biome next to the oceans
Why we care: Wildfires influence ecosystems and climate (e.g., by changing the albedo)
Key observation: Different species of trees enable different types of fires, which explains why North American boreal fires are often more intense than those in Eurasia
Conclusion: The traits of different tree species comprising boreal forests should be considered when modeling the effects of fires on emissions and climate

Intentional fire-spreading by “firehawk” raptors in Northern Australia [abstract] [pdf] (WTF. Thanks Diego!)