“When a forest burns in a wildfire, should we expect it to return as it was before?” Research scientist, Jonathan Coop, and his team pose this question. It addresses a critical conundrum in ecology, how do ecosystems recover from disturbance and why?
Historically, forest scientists developed successional models, a timeline of when certain species return to an environment after a disturbance, such as a wildfire. We expect the sun-loving, fast-growing plants to come back first followed by the shade-tolerant, slow-growing species. But with a changing climate and thus unpredictable disturbances, the successional models may no longer be accurate. Scientists will need new models to accurately predict ecological recovery after disasters.
Many of the tree species in Western North American forests are adapted to fire, meaning they have certain traits that allow them to either survive fire or recover very quickly after the fire. In general, these communities are wildfire resilient. However, this resiliency may only work when the fires occur with a specific intensity and frequency, called the fire regime. With climate change, fire regimes are shifting and these fire adapted trees may not be adapted for what’s to come.
Thus, when a fire does occur the community that grows back after the fire is different than the community before the fire. This is called conversion. There are two steps that lead to forest conversion by fire. The first is that the forest must burn in a high-severity fire that removes large areas of vegetation. Then, the recovery mechanisms must be inhibited either by a lack of available seed, another burn, an unfavorable climate, or other factors that prevent seeds from growing.
When and when fires occur matters
The first mechanism is a change in the fire regime, meaning the timing and severity of the fires is altered from the historical norm. In the modern era, forests across western North America are burning more area, there are more fires, and the average fire size has increased. These changes are likely driven by climate change, increased human ignitions, and fuel accumulation in forests where fire was actively suppressed for decades. This increase in area burned creates a barrier for seeds because seeds from unburned areas would have to travel much farther to recolonize. When the burn area is smaller, it is similar to shooting a basket from the free-throw line, difficult but manageable (if you are good at basketball). Increase the fire burn area and the seeds are suddenly trying to make a shot from half court and the probability of success decreases substantially.
Larger burn areas also increase the chance that the fire burns are areas that have been burned recently, this process is called a re-burn. In 2020, re-burn is occurring during the wildfires in Northern California. Much of the area where the current fires are burning has already been burnt within the last few years. This prevents the late successional species (shade loving, slow growing) plants to establish and doesn’t allow the landscape to reach the pre-fire state resulting in conversion.
Climate change added fuel to the fire
Climate change is also a major mechanism of forest conversion by changing the environment of the pre and post fire landscape. With a warmer and drier climate the trees become parched and dry and are thus more vulnerable to fire and more likely to not survive the fire. A cascading effect occurs because when the trees die, they no longer produce seed and cannot maintain the tree population after the fire. Additionally, the seeds that are available after the fire may not be able to grow with the current climate.
You may have seen this effect personally if you have ever tried to replant houseplants. I grow mine in a nice indoor environment where they thrive at the right temperature with right moisture. Then, I try to replant them outside in the scorching summer sun. The poor plant isn’t adapted to this new environment and promptly withers and dies. The same thing may happen to trees whose seeds are not adapted to the current climate conditions.
Larger and more frequent fires lead to a change in the plants that grow, and often the plants that now grow in the forest, are more prone to fires! Such changes create a cycle of continued fires and highly flammable vegetation. If there was a condition in the fire that would limit future burning such as a shift to less flammable species, this may temporarily stop the cycle of continuing fires. However, this is usually only temporary and the cycle can repeat if the climate is right for a fire to start.
The factors that determine how forests recover from fire are complex, but the author’s of this study think many North American forests are at risk of conversion to another habitat type. There are ways to help the forests recover to their pre-fire state by addressing the causes of their conversion. The first is to help plants regrow after a fire. Communities may bring in plants from local nurseries or scatter seeds on the landscape after the fire. Unfortunately, the seed and young plants will still die if the area burns again. Therefore, we must develop ways to prevent fires from burning the same area multiple times in a short period of time. Although no full proof methods have been developed yet, research is ongoing.
At the core, we need to address climate change. By lowering the average temperature of the planet, we reduce the chance of severe weather that promotes fires. As an individual, you can do your best to ensure fires don’t start in the first place: extinguish your campfire, don’t drive on dead grass, and avoid unnecessary sparks. Another mechanism to prevent conversion is prescribed burns. These are fires started by professionals in a specific area. They are highly controlled and are often used to reduce the amount of fuel that weakens future wildfires.
Forests need to burn. Fire is a critical ecosystem process and many species rely on the fires to create the habitat they need to find food and shelter. The issue at stake here is that modern fires are too large and too frequent and this does not allow the forest to recover and create the habitat that many plants and animals rely on.
Source: Coop et al. 2020. Wildfire Driven Forest Conversion in Western North American Landscapes. Bioscience 70(8):659-673. https://doi.org/10.1093/biosci/biaa061