Population dynamics of forest tree pathogens

Natural and managed forests are challenged with a multitude of disturbances caused by, e.g., storm, frost, drought, fire, pests, and pathogens. Changed weather, global trade, and new management methods will influence the dynamics of fungal pathogens. In this chapter, we present examples of pathogen population with distinct patterns in population dynamics. A long-lived pathogen with a long common history with its host normally shows a balanced population development with small fluctuations in population size. When conditions change, for example, in forest management methods or climate, the possibility for population expansion appears that can result in local or global increases in disease incidence. Pathogens whose success is highly restricted by weather tend to show a more cyclic dynamic of their populations, and outbreaks events will occur irregularly. The outcome for an invasive fungal pathogen that is introduced into a new environment can differ although the size of the population will increase over time. Through hybridization and introgression of functional genes, the competitive advantage may arise which will further boost the growth of the population. Keeping track of pathogen population changes is of importance for forecasting and risk analyses of forest damage. Understanding factors that influence pathogen population dynamics will help recognition of forest damage patterns and disease outbreaks. This can inform development of management methods to mitigate spread of new diseases.