Department of Environmental Science, Policy and Management, University of California, Berkeley 94720, USA

Background and objectives
Traditionally, tree diseases have been presented as threats to timber production, plantation success and the integrity of urban forests. When forests are viewed not as resources to be harvested or shaped to human expectations, but as dynamic natural systems, forest diseases assume essential roles in shaping the structure and spatial distribution of host tree populations, regulating host population dynamics, affecting the composition of the vegetational community, and providing habitat for forest animals [1]. Indeed, everything from nutrient cycling to maintenance of biological diversity in forest ecosystems depends on the actions of tree diseases.

On the other hand, introduced pathogens, human-induced changes in forest structure or composition and climatic changes may all lead to the development of catastrophic epidemics. Depending on the ecological importance of the host species, such epidemics may result in minor shifts in community composition or in fundamental changes in ecosystem properties.

Natural forest ecosystems are being lost and fragmented throughout the world, particularly in the tropics. Beyond their intrinsic value, the remaining tropical forests represent important reserves of genetic resources. Wild relatives of numerous important fruit crops are found in tropical forests, as are natural populations of important timber trees. Additionally, the diversity of chemical pest defences found in tropical forest plants may contain keys to novel disease control methods.

In this presentation, I will use examples from tropical and temperate forests to explore the role of tree diseases in regulating host tree spatial distributions and population dynamics, and the influence of diseases on community diversity and forest structure. Then I will look at the importance of incorporating disease processes into strategies for forest conservation.

Results and conclusions
Several studies of diseases of wild avocado relatives [2] and important forest species in tropical forests indicate that density-dependent disease development and differential host susceptibility can be important in determining host distribution and dynamics, and in maintaining community diversity. Human-induced disturbances can be associated with increased disease pressures, and may represent serious threats to unique, endemic forest species or even forest types. Finally, important conservation tools such as biological corridors to connect forest fragments may unwittingly increase the risks of catastrophic disease development.

Conservation of unique and diverse forest ecosystems is imperative for myriad reasons; understanding the dynamics and impacts of the diversity of forest diseases is essential to successful conservation efforts.

1. Gilbert GS, Hubbell SP, 1996. BioScience 46, 98-106.
2. Gilbert GS, Hubbell SP, Foster RB, 1994. Oecologia 98, 100-108.