Using lichen communities as indicators of forest stand age and conservation value

Evaluating the conservation value of ecological communities is critical for forest management but can be challenging because it is difficult to survey all taxonomic groups of conservation concern. Lichens have long been used as indicators of late successional habitats with particularly high conservation value because lichens are ubiquitous, sensitive to fine-scale environmental variation, and some species require old substrates. However, the efficacy of such lichen indicator systems has rarely been tested beyond narrow geographic areas, and their reliability has not been established with well-replicated quantitative research. Here, we develop a continuous lichen conservation index representing epiphytic macrolichen species affinities for late successional forests in the Pacific Northwest, USA. This index classifies species based on expert field experience and is similar to the "coefficient of conservatism" that is widely used for evaluating vascular plant communities in the central and eastern USA. We then use a large forest survey dataset to test whether the community-level lichen conservation index is related to forest stand age. We find that the lichen conservation index has a positive, linear relationship with forest stand age. In contrast, lichen species richness has only a weak, unimodal relationship with forest stand age, and a binary indicator approach (where species are assigned as either old growth forest indicators or not) has a substantially weaker relationship with forest stand age than the continuous lichen conservation index. Our findings highlight that lichen communities can be useful indicators of late successional habitats of conservation concern at a regional scale. Quantitative lichen indicator systems provide unique information about habitat conservation value that is not captured by traditional community metrics such as lichen species richness. More broadly, indicator systems based on expert experience can have strong biological relevance.