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Burning States

Wildfires are on the move in North America. Not just sweeping across landscapes in a season, but also changing where they appear over the years.

Why are fires a problem in parts of the United States? A trio of papers recently published in Ecosphere tackles the question from different angles.

Firefighter against burning trees
Image: Canva.

Moloney and colleagues start in the Mojave and Sonoran deserts in the southwestern United States. They note that the deserts haven’t tended to have a wildfire problem. In a desert, there tends to be less biomass to burn than elsewhere. This seems to have changed in recent years. Before 2005 there were a lot of years with above average rainfall. Then came wildfire. Moloney’s team have been looking at what changed with the rainfall and found invasive species. The example they give is Schismus arabicus a grass that can proliferate when rain gives it a chance. By putting on so much mass, some think it can carry fire much better than native plants. The botanists wanted to see if this really were true, or just a good story.

The team tested the idea by setting up experimental plots in the two deserts focussed on the native Larrea tridentata or creosote bush. They applied different scenarios for rainfall and fire treatments to see how they responded.

At the Sonoran site, they found rainfall increased the density of exotic plants – but not at the Mojave site. There’s not a simple solution. So what is going on?

Moloney’s team argue that the desert is complex. For example, while the Mojave site saw the native plants doing well, they did well with the creosote bushes. Out in the open, it was the invasive grasses that grew. That means that the bushes are no longer as isolated, and the grasses can carry the fire from one bush to the next. That would seem like the answer – except the authors also say that S. arabicus doesn’t burn hot enough to set creosote bushes on fire.

While the grass itself doesn’t ignite the creosote bushes, it can ignite the shrubs under the bushes. These shrubs can then pass on the fire to the creosote bushes. But there is another problem on blaming fires on increased biomass. Rainfall is predicted to drop with global warming. So biomass should fall shouldn’t it?

Obviously, not all years are the same. They vary in rainfall, and this variability is the problem. The authors write: “With higher rainfall, biomass due to native annuals growing under shrubs increases, producing more fuels that can ignite and increase the probability that the creosote bush itself will burn. One year of increased rainfall may not be enough to tip the balance, but if one high‐rainfall year is followed by a second year, increased seed production during the first year could give rise to explosive biomass production during a second year of increased rain, further increasing fuel loads and fire risk. This seems to be the situation that led to the historically bad fire season in the desert Southwest during 2005… There were two years of increased rainfall, and very large fires occurred in creosote shrublands across the Sonoran and Mojave deserts…”

While more and stronger wildfires are bad for us, are they bad for the plants in the long term? After all, some plants need fires to remove competitors. It depends where you look, and another paper in Ecosphere recently looks at Wyoming.

The title of Mahood and Balch’s paper gives away what they’ve found: Repeated fires reduce plant diversity in low‐elevation Wyoming big sagebrush ecosystems (1984–2014)

The surprise in the paper is that while repeated fires are bad for the sagebrush ecosystem, at dry and low elevations, even one fire is a problem. The authors say: “In lower elevation A. tridentata ssp. wyomingensis systems, our results show that one fire can convert this shrub‐dominated system to one composed mainly of introduced annual grasses and forbs, and we demonstrate that this new state can persist for decades with little sign of recovery to its prior condition.”

Further fires reduce biodiversity. It suggests that sagebrush that sagebrush does not do well with fire. But is this true? Mahood and Balch say in their article: “Disagreement on the actual historical fire rotation limits our ability to determine whether Wyoming big sagebrush is fire‐sensitive or fire‐resistant. However, this question may be irrelevant given the disruption and interaction between invasive annual grasses and fires. We demonstrate that when both fire and invasive annual grasses operate in conjunction, sagebrush is fire‐sensitive. Moreover, we show that an alternate exotic grass state can persist for 17 yr post‐fire even with only a single burn.”

This is a crucial point. Fires cause damage, but maybe exotic plants are working to prevent repair of ecosystems. Instead, they use the cleared landscape to their own advantage. But while fire may be causing problems in some parts of the United States, a lack of fire may be a problem elsewhere. This is the topic of a third paper in Ecosphere.

Stockdale and colleagues ask: Could restoration of a landscape to a pre‐European historical vegetation condition reduce burn probability? Instead of encroaching grasses, they look at encroaching trees. They write: “Montane regions throughout western North America have experienced increases in forest canopy closure and forest encroachment into grasslands over the past century; this has been attributed to climate change and fire suppression/exclusion. These changes threaten ecological values and potentially increase probabilities of intense wildfire.”

The test site for Stockdale’s team was across the border in Southern Alberta. The group restored Bob Creek Wildland to test three ideas. First, cutting back the tree cover would reduce overall burn probability. This is because the fire would have to cross different vegetations, instead of sweeping through homogeneous habitats.
They also thought it would change what areas were more likely to burn. Removing fuel from some areas should make them much less likely to burn than their neighbours. Finally, they thought it would reduce the probability of high-intensity fire.

Not all fires are the same. For Stockdale and colleagues, a high-intensity fire is one where the output of power is more than 4000 kW/m.* It’s not an arbitrary figure. This is when a surface fire becomes a crown fire, and you need to change your firefighting tactics.

The scientists restored Bob Creek Wildland to its state in 1909. The restoration happened in 2014, so that undid over a century of change. They then looked at how it would burn.

It’s excellent that I was never likely to get within a thousand miles of the study. Besides not being a botanist, I’m also sometimes a simple thinker. If I wanted to test how something burned, the first thing I’d look for is a box of matches. If someone more intelligent could wrest the matches away from me, then next thing I would look at would be the historical records. But from 2014 to now is far too short a timeframe to make sensible conclusions.

Stockdale and colleagues had a different answer: “To model burn probability and fire intensity, we used Burn‐P3, which is a Monte Carlo simulation model based on the Prometheus fire growth engine…, and simulates ignition and spread of fires across the landscape. Burn‐P3 combines deterministic fire growth (influenced by fuels, topography, and weather) with probabilistic fire ignition locations, fire duration, and weather…” Unlike other experiments, where you can control fire regimes, Stockdale’s team were looking at the probability of where fires start and spread. To find sensible patterns you need to have fires begin in different locations, then reset the landscape exactly. You can’t do that in reality, so it has to be done in simulation. You can, however, check your model is sound by seeing if its predictions match the spread of fires that happen naturally in a season.

The simulations found that there was very little difference in overall burn probability. What the team did find is that there was a big difference in the probability of high-intensity fires. They dropped by half. In about a tenth of the landscape, the likelihood of a high-intensity fire dropped to one-tenth of the probability in a modern habitat.

The authors say: “The only explanation for differences between the two scenarios is changes in the speed at which fires moved across the landscape (rate of spread), which is exclusively attributable to changes in the vegetation. This is because we held constant the number, location, and timing of ignitions; duration of burning; and the weather conditions under which the fires burned.”

The model also highlights some potential problems: “However, the shift to increased grassland cover is not without risks too, because while fires in grasslands are generally easier to suppress or manage due to lower intensities, the increased rate of spread in these fuel types in spring and fall can offset this very quickly. Under conditions of extreme winds and low humidity, grassland fires can be virtually impossible to contain and will rapidly spread to more intense‐burning fuels (coniferous forests) or to nearby values at risk. The placement of downwind vegetation brakes that generally reduce rates of spread may not be effective under all weather conditions.” Shifts to extreme weather could add extra unpredictability to fires in the future.

Stockdale and colleagues conclude with a point that applies to all the papers. We talk of conserving landscapes and habitats using a specific moment as the reference point. Yet this conserved habitat will be interacting with a new climate and different pressures from disturbance and invasions. In seeking to conserve something from the past, you could end up creating another distinctive habitat for the future. The authors conclude: “Rather than simply reconstructing a single point in time, an ideal solution would be to determine a range of ecologically sustainable conditions and choose the best reference point within that range that will achieve the landscape objectives…”

The demands of fire management are complex. There are biotic and abiotic factors and on top of that social issues. Like a fire itself, the view of fire management varies depending on the ingredients and the place you watch it from. If you want to learn more, there are plenty of other fire ecology papers in the February 2019 issue of Ecosphere.

* This surprised me. I was expecting intensity to be per square metre. But rainfall is length1and not length2, so I probably need to red more about fire intensity. The source Stockdale et al. use is available online. See page 38.