In this chapter we review strategies for monitoring lichen biodiversity and for monitoring ecosystem function with lichens. Concern about threats to lichen biodiversity worldwide has led to increased interest in monitoring the status of lichens and assessing threats (e.g. reviews [1, 32]). The word "health" referring to the degree of maintenance of normal, natural ecosystem function, as in "forest health" or "ecosystem health", is widely used and accepted in some parts of the world (e.g. [17]), but not in others, so we use the phrase  "ecosystem function" for the concept in this chapter. For monitoring of ecosystem function, we focus on methods that can be applied on broad scales and/or to detect responses to multiple causes other than point-source pollution. This chapter is intended to complement the detailed discussion by Ferretti and Erhardt (chapter 9, this volume) of issues in the design of monitoring programs to assess pollution response of lichens. Most of that discussion is relevant to our focus, and our discussion builds on that chapter.
We define monitoring as studies designed to assess trends of change over time as well as variation in status across an area for lichen communities or indicators of ecosystem function. We consider any study which is designed to be repeated over time to assess trends a monitoring study, even before repeat sampling has been completed. We emphasise quantitative methods for monitoring lichens.
The definition of "community" used in this chapter is very broad, referring to a group of lichen species growing together, but allowing broad interpretation of the size of a community. Our usage thus includes both very fine-scale definitions of a community (for instance the lichen community of vertical faces of granite tombstones in a local area) and much broader definitions (for instance the lichen community of oak forests in eastern North America). We do this to emphasise that similar questions about the relationship of lichen community composition to environmental and management variables can be addressed at different spatial scales.