Wildfire alters the disturbance impacts of an emerging infectious disease via changes to host occurrence and demographic structure

1. Anthropogenic activities have altered historical disturbance regimes,
and understanding the mechanisms by which these shifting perturbations
interact is essential to predicting where they may erode ecosystem
resilience. Emerging infectious plant diseases, caused by human
translocation of nonnative pathogens, can generate ecologically-damaging
forms of novel biotic disturbance. Further, abiotic disturbances, such as
wildfire, may influence the severity and extent of disease-related
perturbations via their effects on the occurrence of hosts, pathogens, and
microclimates; however, these interactions have rarely been examined. 2.
The disease “sudden oak death” (SOD), associated with the introduced
pathogen Phytophthora ramorum, causes acute, landscape-scale tree
mortality in California’s fire-prone coastal forests. Here, we examined
interactions between wildfire and the biotic disturbance impacts of this
emerging infectious disease. Leveraging long-term datasets that describe
wildfire occurrence and P. ramorum dynamics across the Big Sur region, we
modeled the influence of recent and historical fires on epidemiological
parameters, including pathogen presence, infestation intensity,
reinvasion, and host mortality. 3. Past wildfire altered disease dynamics
and reduced SOD-related mortality, indicating a negative interaction
between these abiotic and biotic disturbances. Frequently-burned forests
were less likely to be invaded by P. ramorum, had lower incidence of host
infection, and exhibited decreased disease-related biotic disturbance,
which was associated with reduced occurrence and density of
epidemiologically-significant hosts. Following a recent wildfire, survival
of mature bay laurel, a key sporulating host, was the primary driver of P.
ramorum infestation and reinvasion, but younger, rapidly regenerating host
vegetation capable of sporulation did not measurably influence disease
dynamics. Notably, the effect of P. ramorum infection on host mortality
was reduced in recently-burned areas, indicating that the loss of mature
host canopies may temporarily dampen pathogen transmission and “release”
susceptible species from significant inoculum pressure. 4.Synthesis:
Cumulatively, our findings indicate that fire history has contributed to
heterogeneous patterns of biotic disturbance and disease-related decline
across this landscape, via changes to the both the occurrence of available
hosts and the demography of epidemiologically-important host populations.
These results highlight that human-altered abiotic disturbances may play a
foundational role in structuring infectious disease dynamics, contributing
to future outbreaks, and driving biotic disturbance regimes.