The Consequence of Tree Pests and Diseases for Ecosystem Services

Background Trees are major components of many terrestrial ecosystems and are grown in managed plantations and orchards to provide a variety of economically important products, including timber, pulp, fiber, and food. They are subject to a wide range of pests and diseases, of which the most important causative agents are viruses, bacteria, fungi, oomycetes, and insect herbivores. Research on tree pests and diseases has had a historical focus on trees of direct economic importance. However, some epidemics and infestations have damaged and killed common trees that are integral parts of natural ecosystems. These have harmed valuable landscapes and highlighted the wide-ranging consequences arising from tree pests and diseases. There is also growing concern that aspects of globalization—in particular, higher volumes and new forms of trade—may increase the risk of disease spread. A forest providing numerous ecosystem services is subject to a disease epidemic that reduces the abundance of a dominant native species, resulting in a change in forest structure. Initially, a wide range of ecosystem services ( A to D ) are harmed. But as trees grow to replace lost species, some (perhaps carbon storage or water purification) are regained, whereas others (perhaps the biodiversity supported by the diseased tree species) are permanently disrupted. Policy measures can both help prevent new diseases being introduced (the first stage) or improve recovery through management practices or planting resistant trees. Advances We review the challenges in maintaining tree health in natural and managed ecosystems. It is argued that it is helpful to consider explicitly the consequences of pests and diseases for the full range of ecosystem services provided by trees. In addition to forest and orchard products, tree pests and diseases can affect the ability of forests to sequester and store carbon, reduce flood risk, and purify water. They can affect the biodiversity supported by trees and the recreational and cultural values accorded to woodland by people. Many of these benefits are uncosted and enjoyed by different classes of stakeholders, which raises difficult questions about who should be responsible for measures to protect tree health. Changes in the risk of pest and disease introduction, the increasing prevalence of genetic reassortment leading to novel disease threats, and the potential role of climate change are all highlighted. Outlook Modern pest and disease management is based on an extensive science base that is rapidly developing, spurred in particular by modern molecular technologies. A research priority is to build a better understanding of why certain pathogens and insects become major pests and diseases. This will involve a better understanding of the molecular basis of pathogenicity and herbivory, as will ecological insights into why some species reach epidemic prevalence or abundance. It will also help anticipate which species may become a problem if they are transported to new geographical regions, recombine with other organisms, or experience new climatic conditions. However, identifying all species that may become pests will be impossible, and the Review stresses the importance of risk management at the “pathway of introduction” level, especially when modern trade practices provide potential new routes of entry. Last, when ecosystem services are provided by woods and forests rather than individual tree species, we need to understand better the consequences of pests and diseases that attack or feed on particular species.