Interacting Disturbances: Wildfire Severity Affected by Stage of Forest Disease Invasion

The presence of disease in wildfire prone areas has generated the assumption that increased disease outbreaks result in increased fire severity. Studies suggest that this relationship is in fact more complex and empirical data show that fuel loading and disease stage are more indicative than merely the presence of disease alone. One challenge when evaluating the effects of disease on fire severity is that often no pre-fire data exist in infected areas, so mapping post-fire characteristics that are induced by pre-fire forest conditions becomes speculative. Metz et al. (2011) had the unique opportunity to observe the effects of Sudden Oak Death (SOD) on wildfire severity on the California Central Coast. They examined results from the 2008 Basin Complex [fire] that had a perimeter encompassing 98 of the 280 plots established in 2006 and 2007 to monitor the effects of SOD on the forest. They found that there was generally minimal difference in fire severity in SOD infested and non-infested plots, except that the more concentrated dead fuel loading on the ground in SOD areas did increase the effects of fire on soil characteristics. Furthermore, the minor effect of SOD on fire severity was more observable in areas that were in the early stages of SOD infections because of the high presence of dead leaves and small diameter branches in the canopy that had not yet fallen to the understory. These “light flashy fuels” can be very volatile when ignited and can increase fire severity. –Lindon Pronto

Metz, Margaret R., Frangioso, Kerri M., Meentemeyer, Ross K., Rizzo, David M., 2011. Interacting disturbances: wildfire severity affected by stage of forest disease invasion. Ecological Applications 21, 313–320.

Sudden Oak Death is an infectious pathogen that is increasingly affecting California coastal forests with high rates of tree mortality, resulting in increased dead fuel loading and altering overall fuel type characteristics; this has varied implications for forest management practices and wildland fire suppression tactics. An extensive network of forest monitoring plots in Big Sur California was able to provide more clarity to these causal uncertainties. The 280 forest monitoring plots were selected as 500-m<!–[if gte msEquation 12]>²<![endif]–> areas, wherein a variety of measurements and classifications were made such as vegetation basal areas (from diameter at breast height) to determine fuel loading, and estimated time since death and whether infection induced or not. The network of plots was established in 2006 and 2007. In 2008 the Basin Complex burned through 35% of the study area, and 61 of the plots were measured immediately after the fire in order to compile accurate data for the pre and post-fire stages. The purpose of the comparison was to answer the following questions: (1) Did pre-fire fuel loads vary among areas that differ in pathogen presence or impacts? (2) Was burn severity higher in areas that had previously experienced higher SOD mortality? (3)Does the stage of disease progression influence burn severity because of changes in fuels through time?
The forest plots were defined under two fuel types (redwood–tanoak and mixed evergreen) and were measured on pre and post-fire occasions to determine disease incidence, mortality, amount of coarse woody debris, and other physical and biological fuel characteristics. First, Mann-Whitney U tests were used on both fuel types to determine the composite burn index (CBI) in both pathogen infested and non infested plots. Secondly, further sequences of tests were performed to find, for example, if CBI increased in areas with higher basal areas of standing dead trees in infected plots; through this process the authors set about to determine some of the relationships of SOD induced fuel characteristics on wildfire severity. The 2006–2007 plot mortality data were used as a proxy for observing an increase of new host mortality, as well as to observe the progression of longer term SOD effects (downed logs) on fuel characteristics and subsequent fire severity.
The results indicated a variety of both anticipated general assumptions as well as slightly more unforeseen overall causal relationships to fire severity. First, Metz et al. found that standing basal area and downed log volume were significantly higher in pathogen infested plots; however, there was no significant difference in abundance of live or dead non-host species between infested and non-infested plot areas. Second, despite the increase in dead woody material of SOD-associated species, no significant increase in burn severity between infested and uninfested plots was observed. Third, the authors found that it was more constructive to evaluate the relationships between burn severity and fuel abundance when two categories were created; one for recent host mortality and one representing an older SOD presence. Recent host mortality is characterized by more fine dead fuels, while longer term infection is observed by more downed heavy fuels (logs). Overall burn severity had a positive linear correlation to higher fuel loadings, as opposed to the presence of the pathogen alone; presence of the pathogen was not indicative of the overall fuel loading. However, one positive linear correlation was found—that higher standing basal areas and downed log volumes in infested plots resulted in increased soil burn severity. Furthermore, it can be assumed that elevated soil burn severity has other implications such as post-fire soil and ash erosion effects on watersheds; however these were not explored in this study. Although SOD mortality does affect a portion of these forests, a significant importance for host fuel abundance in determining burn severity, was not found when infested and uninfested plots were compared.
A similar phenomena to SOD, is the bark beetle outbreaks in many other western forests that also results in areas of high tree mortality. For both these instances it has been widely assumed that there is a positive correlation between tree mortality and fire severity. For both these instances, we now have empirical results that suggest otherwise. This study is consistent with similar studies that suggest perhaps the most significant contributing trait of tree mortality to fire severity is the relationship of time from disease or pest outbreak to time of fire occurrence. In conclusion, Metz et al. cautioned that fire severity was not consistent throughout, due to the large variance in terrain, fuel availability, and weather characteristics at the time of the fire across all of their plots. Concerns that tree mortality significantly influences fire severity are still valid, as geographic areas not explicitly covered by this study can contain a wide variety of species, topographical feature, temperature ranges, and humidity gradients. Nevertheless, this study provides a rare data set for pre and post-fire forest characteristics in SOD infested areas. Further research in this area may be helpful in guiding management and policy decisions for addressing SOD and fire hazards in California forests.

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