Linked and compound disturbances: Forest disease and fire
A warming climate alters forest disease dynamics by influencing rates of pathogen transmission and susceptibility of hosts to infection and mortality. Disease-related tree mortality changes the quantity and quality of fuels in fire-prone forests, leading to potential interactions with wildfire, which is likely increasing in frequency and severity under climate warming. Disease-fire interactions can lead to positive or negative feedbacks if one disturbance affects the susceptibility of the forest to a subsequent disturbance. When forest disease emerges as a result of novel host-pathogen interactions, as with the introduction of exotic pathogens, then the disease itself may be a novel disturbance that can interact with existing fire regimes in unknown ways, even in the absence of a changing climate. Understanding how new invasive forest diseases interact with changing fire risk is crucial to the management of forests under global change. Phytophthora ramorumis an example of a non-native pathogen causing widespread tree mortality (sudden oak death) in fire-prone forests of California and Oregon. Large wildfires in California in 2008 provided an opportunity to test disease-fire interactions and understand the characteristics of one disturbance that might lead to feedbacks with another disturbance.
Both disease and fire alter forest composition because tree species differ in their susceptibility to injury or mortality by either disturbance. Wildfire thus changes disease dynamics by changing the relative abundance of hosts with varying competencies to transmit the pathogen. Additionally, a forest disease such as SOD influences the severity of subsequent fires by changing fuel composition and influencing fire behavior. The interaction between SOD and fire has unexpected consequences, including the increased mortality of species that are otherwise resilient to both disturbances, such as coast redwood. Environmental conditions that may change at various spatial and temporal scales under climate warming influence fire-related tree mortality directly as well as indirectly through changes to disease dynamics and the resulting accumulation of fuels. The complexity of disease-fire disturbance interactions increases when considering the role of human systems, which may impose a wide range of management efforts with varying goals across scales ranging from individual behavior to regional, national and international policies. Despite the implicit coupled nature of ecological processes, disease spread, and management actions, investigations of the reciprocal feedbacks between disturbance interactions and human activities remain understudied.