COS 192-4 - Emerging infectious disease in a frequently burned ecosystem: Modeling epidemiological interactions between an invasive forest pathogen and wildfire

Background/Question/Methods

Global trade and other anthropogenic activities have caused the introduction of countless non-native pathogens to new ecosystems. However, other global change drivers, including changing disturbance regimes, may shape the transmission and severity of emerging diseases via their effects on the occurrence of hosts, pathogens and microclimatic conditions. Though disturbances, such as wildfire, may be pivotal in determining future severity of these emerging infectious diseases in their introduced ranges, disease-fire interactions have rarely been studied. The disease “sudden oak death,” associated with the introduced oomycete pathogen Phytophthora ramorum, causes landscape-scale tree mortality in California’s fire-prone coastal forests. Sudden oak death has widely varying impacts on susceptible host species, and past research has illustrated that wildfire disturbances in this ecosystem can locally eradicate P. ramorum, shift host composition, and alter vegetation structure in ways that reduce the severity of infections and the mortality impacts of this pathogen. Together, these past studies suggest that rapidly changing fire regimes in these forests may be pivotal in determining the future spread and mortality impacts of this emerging disease.

Results/Conclusions

In this study, we leverage a long-term dataset recording the impacts of wildfire and SOD on forest mortality, regeneration, and growth over 15 years to derive demographic and epidemiological parameters to predict the impacts of changing fire regimes on future disease dynamics. We employ a combination of epidemiological, population, and hierarchical generalized linear models to examine how variation in fire frequency and severity impact patterns of post-fire recolonization by this introduced pathogen, P. ramorum infection, and disease-related mortality. Our results suggest that intermediate fire frequencies may reduce disease impacts and promote persistence of susceptible host trees in areas where, under other conditions, they may be more easily eradicated by P. ramorum. In contrast, more frequent fire may promote disease-free stands, albeit with fundamental shifts in forest demography and composition. In certain stand compositions, longer-fire return intervals may partially maintain existing forest structure, albeit with the loss of susceptible host species. Overall, these results indicate that, in this fire-prone system, SOD may exhibit cyclical behavior as host vegetation recovers, rather than occurring as a singular wave of mortality, constituting a novel biotic disturbance regime with its own "return intervals."