Article: Stevens-Rumann, C. S., Kemp, K. B., Higuera, P. E., Harvey, B. J., Rother, M. T., Donato, D. C., Morgan, P. and Veblen, T. T. (2017), Evidence for declining forest resilience to wildfires under climate change. Ecol Lett. doi:10.1111/ele.12889
It started out with a flame
An unfortunately familiar image in the news in 2017, especially if you live in California like I do, was wildfire. But while these disasters rightly get a lot of attention while they are happening, it is also critical for us to understand what happens to fire-ravaged landscapes in the following years. In the years and decades after a forest fire, the forest has a chance to reestablish and adapt to changes in its environment. In some cases, the forest is able to start re-growing the species that were there before the fire. If enough individual trees grow, the forest will eventually recover and look much like it did originally. In other cases, trees could be replaced with grasses or shrubs, resulting in a different type of environment than the one that was there previously.
Which path a forest takes following a fire is determined in part by environmental conditions in the years after the fire. While this has always been the case, the effects of climate change, including changes in rainfall and temperature, can have an important impact on whether or not a forest is able to recover fires. Understanding these potential changes to our forests is important since forests play a critical role in water supply, wildlife habitat, carbon storage, and natural resource supply.
How did it end up like this?
To understand how climate change has impacted forest recovery, Camille Stevens-Rumann and colleagues assembled a large data set for forests across the Rocky Mountains that experienced fires between 1988-2011. They included data on tree seedling growth following the fire, physical characteristics of the site (such as slope, location, etc.), and climate information (including long-term averages and conditions in the years immediately after the fire). They divided the data based on date into pre-2000, in which conditions were generally cooler and wetter, and post-2000, in which conditions were generally warmer and drier and thus more characteristic of the impacts of climate change.
The researchers used this data set to answer pressing questions about post-wildfire recovery:
-Was there a difference in ability of forests to recover in the post-2000 period compared to the pre-2000 period?
-What forces are responsible for preventing forest recovery?
-Are certain types of forests less able to recover under the impacts of fire and climate change?
Not much of a bright side
In this study, scientists found that fewer of the forests that burned in the early 21st century recovered than compared to those that were burned in the 20th century. Among the sites where fires occurred before 2000, 70% grew enough new tree seedlings to return to their pre-fire state, whereas for fires after 2000, only 46% of sites recovered fully. The percentage of sites in which no trees regrew after the fire also increased from 19 to 32%.
A major factor behind this was the warmer and drier conditions that became more common in the period after 2000. In the fires before 2000, wetter periods appeared to be important helping to reestablish trees in the burned areas. After 2000, such windows of more favorable growing conditions occurred less frequently, making it more difficult for the forest to recover. The researchers also saw a difference in recovery between forest types. Forests in drier areas were more likely to undergo a transition towards a grassland or shrubland. Forests in more moist areas were more likely to remain forested, but experienced a change in the number of trees and species of trees present.
While studies have shown that climate change can increase the frequency and severity of forest fires, this study suggests it can also impact the ability of forests to recover following these fires. We could be losing valuable ecosystems that we rely on. The combined impacts of wildfires and climate change present a serious threat to our forests, one that we must continue to study and understand.