Consequences of climate change for biotic disturbances in North American forests

About one third of North America is forested. These forests are of incalculably high value to human society in terms of harvested resources and ecosystem services. Epidemics of forest insects and pathogens are the dominant source of disturbance to North American forests (e.g., far exceeding wildfires and storms). We identified 23 insects (6 nonindigenous) and 22 diseases (9 nonindigenous) that are notable agents of disturbance in North American forests.  Growing understanding has reinforced the hypothesis that population dynamics of forest insects and pathogens are frequently connected to climatic variation.  Insect populations are highly responsive to climatic variation due to their physiological sensitivity to temperature, short generation times, and explosive reproductive potential. Similarly, pathogens are highly sensitive to temperature and moisture. Deficiencies of water or mineral nutrients and increases in gaseous CO₂ can have broad influences on tree health that can affect defenses against and tolerance to herbivores and pathogens. Clear examples are offered by recent expansions of mountain pine beetle into high elevation five-needle pine forests of the Rocky Mountains, of southern pine beetle into the NJ Pinelands, regional shifts of outbreak ranges of defoliators, and drought stress increasing susceptibility of trees to pathogens and bark beetle epidemics. Changes in biotic disturbances have broad consequences for forest ecosystems and the services they provide to society. There is a strong need to better understand and predict the interactions among climate, forest productivity, forest disturbance, and the socioeconomic relations between forests and people.