Dear Sudden Oak Death: What have you done for me lately?

Article reviewed: Interacting disturbances: wildfire severity affected by stage of forest disease invasion

By M.R. Metz, K.M. Frangioso, R.K. Meentemeyer, and D.M. Rizzo, published in the journal Ecological Applications, vol. 21: 313-320

The plot line: These researchers evaluated the influence of Sudden Oak Death (SOD) on wildfire severity. They were able to do so because they had measured forests that were infected to various degrees (ranging from no infection to very advanced infection) by SOD prior to a wildfire (the Basin Complex fire) occurring. They found that, where SOD had recently infected forests and caused lots of standing dead trees, fire severity was greater but SOD infection was not the primary determinant of fire severity. Burn severity was very patchy and influenced by many other factors besides whether or not the area had been infested with SOD. In areas where SOD infection was advanced (i.e. several years since first infections), there was greater burn severity at the forest floor but again SOD infestation was not a major determinant of fire severity. They suggest that management efforts may be more effective if targeted in areas where SOD is still in the initial stages of infestation (i.e. where there are lots of standing dead trees with dead leaves and branches).

Relevant quote: “Our results indicate that the timing of fire relative to disease progression is an important predictor of burn severity in infested areas because differences among fuel types were more important indicators of damage than pathogen presence alone.”

Relevance to landowners and stakeholders:

Wildfire risk is often grossly over-simplified. During this time of year, for example, we begin to hear reports on the news that this year’s fire danger will be “especially high.” If it is a wet spring, they say that fire danger will be “especially high” because of all of the growth of fuels (vegetation) that is occurring. If it is a dry spring, they say that fire danger will be “especially high” because of the dry fuel conditions. And if it is an average year, they usually wait until we have a hot and dry week and then claim that it is actually a very dry year and, you guessed it, fire danger will be “especially high.” The reality is that fire behavior is a result of a huge complexity of factors that include how much fuel is available to burn, the structure of the fuel, the local topography, and the weather conditions at the time of the fire.

Historically, fires in central coast forests of California have not occurred as frequently as they have up in the Sierra Nevada. But as a landowner, I am actually more comfortable in the Sierra Nevada than in central coastal forests when it comes to altering the behavior of fire when occurs. Fires in the coastal forests appear to be more weather-driven and less fuel-driven than those in the Sierra Nevada. And as a forester, I know that I can alter fuels on my land to manage fire behavior. Weather? Not so much. This research suggests that treating areas of recent SOD infestation might lower fire severity a little, but that other factors will accumulate to have more of an influence on fire behavior. For forestland owners in the central coastal forests of California, you should hope that insurance companies don’t read this article.

Relevance to managers:

While infestation occurs as a gradual process, there appears to be 4 logical stages of SOD infection:

1.     Initial infection- trees lose vigor and slowly decline over about 6 years.

2.     Crown mortality- over 1 or 2 more years, leaves and small branches die and slowly shed off of the dead trees

3.     Snag decomposition- snags either gradually crumble apart of fall over

4.     Log buildup- Logs are on the ground and gradually decompose

As discussed in a previous post, Sudden Oak Death is anything but sudden. As the authors of this article point out, SOD truly is a “chronic and progressive stress” rather than a sudden one.

It is during stage 2 above where the authors of this research seem to be recommending that managers focus on in terms of reducing fire risk. This could mean prioritizing fuel-reduction treatments to occur in areas that have high densities of standing dead trees with lots of dead biomass still in the crowns.

Fire severity may be just as high or higher in stage 3, but this study did not measure fine surface fuels so it is unknown. But it would make some intuitive sense to me that a buildup of litter and debris from SOD may increase fire severity. As the authors mention, this needs further study. See a related post on the interaction of bark beetles and fire severity in lodgepole pine forests.

Of course, the most effective management would be to try to stop SOD in the first place, but this is obviously difficult. I heard one of the authors of this article give a talk about management options with respect to lowering SOD infection. He mentioned two things that I recall:

1.     Thinning + burning might be effective (presumably by increasing individual tree vigor and by reducing future fire severity)

2.     A no-host buffer around critical areas (e.g. removing host species around a park core area, for example) would be very difficult because most hosts sprout.

Critique (I always have one, no matter how good the article is):

The primary limitation is the fact that only large logs were measured as surface fuel prior to the fires. Obviously if the researchers had known that a fire was going to happen, they would have been more comprehensive in measuring surface fuels. But just measuring “1000 hour fuels” leaves a lot of the surface fuel equation unaccounted for.