The fires of Sierra Nevada-past: Low severity, high impact

[originally posted on www.foreststeward.com on Aug. 2, 2010]

Article reviewed: Fire regimes, forest change, and self-organization in an old-growth mixed-conifer forest, Yosemite National Park, USA

By A.E. Scholl and A.H. Taylor, published in Ecological Applications, Vol. 20 pp. 362-380, available for download here.

The plot line: The researchers went to a forest in Yosemite National Park that had no evidence of recent disturbance (what one might refer to as “old growth”). By measuring the annual growth rings of trees and by estimating when dead trees had originated, they reconstructed what the forest looked like prior to 1899 (when Euro-American settlement and fire suppression started changing forests). They confirmed that accuracy was in the right ball-park with survey data that were collected from the same area in 1911. Like many other studies, they found convincing evidence that the forest of the past was a lot different than it is today. The forest-past had far fewer trees, more ponderosa pine, and less white fir and incense cedar. They deduce that patchy, low severity fires burning less than 10 years apart functioned to “maintain” forest structure by killing individual or groups of trees and by creating conditions amenable to seedling establishment of several tree species.

Relevant quote: “Multiple re-burns at relatively short intervals (5–10 yr) will need to be applied for a sustained period to reduce surface fuels and thin the canopy… application of high-severity prescribed fire would create novel conditions compared to fire effects over the last four hundred years.”

Relevance to landowners and stakeholders:

1905 was a dark year in the natural history of the western US. It is when the policy of fire suppression was implemented (and Bambi hadn’t even come out in theaters yet). Since then, forests have marched in a slow and circuitous fashion farther and farther away from their past condition (a condition largely maintained by Native Americans). In recent decades, researchers have been focusing on quantifying what those pre-fire suppression conditions were. How many trees were there? How big were they? What species were there? These are important questions for landowners and stakeholders who have restoration as an objective.

There is growing realization, however, that those pre-settlement conditions can never actually be restored. The environment, both physical and social, is totally different than it was then. Even if we could know exactly what the forest looked like and were then able to reconstruct it, we would not re-create the forest of the past since it would then change under novel environmental and social conditions. Reconstruction studies like this one that quantify past forest structure are critical for land managers because they help inform restoration treatments in a very general way (i.e. they don't provide "hard targets," but rather set the stage or range of possible targets. Some generalities highlighted by this study include:

• Fire suppression has led to homogenization of forest structure. Variability in structure at several scales is a worthwhile restoration objective.

• Fire: what have you done for me lately? Perhaps Janet Jackson sang this because she knew that fire was much more likely to occur in areas that had not recently burned (within one or two decades).

• Low severity fires rule. There is not a consistent definition of what makes a fire low- versus moderate-severity. In this study, they conclude that low severity fires were the norm and that they should be used in restoration treatments. These “low severity” fires, however, would include locally intense flare ups that killed individual or groups of mature trees that would create canopy gaps up to 4 or 5 acres in size (Personally, I would tend to call this type of fire “moderate severity.”)

Relevance to managers:

For managers hoping to use prescribed fire as a restoration tool in forests similar to the one used in this study, there are several applications that are implied from the study: 

• Repeated low-severity fires at high frequency may be preferable over one high-severity fire. Canopy gaps for shade intolerant species can be developed by the repeated burns and patchy tree mortality (your bound to get some hot spots after several burns).

• At the ~5000 acre scale, there is not much evidence from this study to suggest that south facing slopes should be burned more frequently than north facing slopes. Although from a fire hazard or tactical stand point, there might be.

• To get closer to the forest structure that was present before fire suppression, one would reduce density to roughly 1/3 the present density and basal area would be cut roughly in half. Trees of all size classes would be reduced in density, with a more dramatic reduction in smaller trees. Avoid hard-target upper diameter limits (such as, "thou shalt not kill a tree greater than 24" dbh!").

• Species composition could be restored by having higher mortality in shade-tolerant species, although it may be necessary to actively recruit ponderosa pine in order to achieve it’s past composition.

Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:

I do not prefer the term “self organization” because it hints at the misconception that forests somehow come into perfect harmony if they are left alone. Or it suggests that, prior to fire suppression, the forest was in perfect balance. The authors clearly do not have this connotation in mind, since they discuss the fact that climatic conditions in the past were different than they are now. But the term brings to mind an outdated way of thinking about forests as achieving a “steady state” environment, when actually they are constantly changing and interacting with disturbances and climatic trends. Again, I am sure that the authors are not trying to imply this connotation, but perhaps a different phrase could have been used.

There are lots of sources of uncertainty when it comes to reconstruction studies. There are missing data (trees that decomposed away), inaccuracies in decomposition rates, assuming dead trees grew at similar rates as live trees, assuming that all sudden growth releases/suppressions were caused by fire and not insects or other physical damage. The authors discuss these and state the need for caution in interpreting the results. But in this case, the authors had the unique opportunity to use actual data that was collected in the study area in 1911 as a way to judge the accuracy of their reconstruction. 1911 was shortly after fire suppression began, but is still close enough to be a great opportunity to validate the reconstruction methods.

It is therefore puzzling why they did not reconstruct their forest back to the same exact year as the survey (1911). Instead, they compare their 1899 reconstructed forest with the 1911 measured forest. Why not use the same year? The forest could have changed considerably between 1899 and 1911. From a graph in the paper, it appears that the fire with the largest extent in the last 400 years occurred in 1900. This could have changed the structure throughout the study area considerably. They found that the 1899 reconstructed forest was no different 1911 forest, but perhaps it was different in 1911. Using the same year for comparison may have provided useful information on the accuracy of the reconstruction method.