The following is from a presentation to the Forest Biology Student Association, Oct. 9 2013
The Rim Fire began on August 17, 2013 in the Tuolumne River Canyon. By the time the fire perimeter stopped spreading on September 21, it encompassed 257,135 acres, or 402 square miles (more than twice the area of Lake Tahoe). That makes it the third largest wildfire in California’s recorded history, and the largest wildfire that burned primarily through forest. That list of largest California fires is heavily skewed towards the last ten years, and most scientific studies on the topic suggest that large fires are going to become increasingly common in the state, due both to temperature increases and fuel load increases that make fires more difficult to control.
Here’s a video of the spread of the Rim Fire perimeter:
The Rim Fire has received an enormous amount of attention in the popular press. Among the reasons for this: It burned at very high severity over very large areas, and it burned into a very popular National Park (Yosemite). These two factors that make the Rim Fire interesting to the public also make it interesting to scientists.
Ecologists are interested in fire severity because it can influence the dynamics of regenerating vegetation, as well as properties of the soil and how it responds to subsequent rainstorms. The severity of
two time periods. Here’s a map of the initial estimate of fire severity for the Rim:
The Rim Fire burned at around 40% high severity, meaning that on almost 100,000 acres, the vegetation was completely killed. From the map above, it is obvious that many of these high-severity patches are extremely large, meaning that finding natural sources of tree regeneration is potentially difficult.
Because the Rim Fire burned into Yosemite National Park, ecologists have the opportunity to look at how the fire behaved when it entered an area that had been burned frequently in the recent past. The National Park Service, particularly in Yosemite and Sequioia/Kings Canyon, has the ability to allow wildfires to burn under the right conditions (i.e. when the weather is not too hot and windy). This is important because fire is a natural part of many forests in the Sierra Nevada, and when it is suppressed, fuel loads can build up to levels where when fire eventually does occur and cannot be contained, the fire burns at high severity, as happened with the Rim Fire. Yosemite has kept excellent records of when and where managed wildfires have occurred within its boundaries. Here’s a look at that data, with the Rim Fire perimeter superimposed:
In a number of places along the northern and eastern edge of the Rim Fire perimeter, the fire appears to have gone out or burned at lower severity when it entered stands that had been burned previously. This is precisely what fire ecologists predict should happen in this situation. Some areas along highway 120 were burned in a “back fire” lit intentionally to burn into the main fire, in this case to the northwest of highway 120. In contrast, areas directly west of Hetch Hetchy had limited recent fire activity, despite being within the national park, and were burned at high severity.
Much will be written about the Rim Fire in the years to come, and it promises to yield a trove of valuable scientific information. But there is little doubt that large wildfires like this are increasingly likely in this region, and forest managers and the general public need to confront this fact and devise ways to live with fire at lower intensities to prevent future occurrences of 50,000+ acre high-severity patches.