Biologists propose a better way to assess the conservation value of North American old-growth forests — using lichens, sensitive bioindicators of environmental change. Old-growth forests are usually defined by tree age, but the authors argue this overlooks the importance of biodiversity in those habitats. Lichens are the ideal candidates to measure this biodiversity. Scorecards with suites of lichens specific to these forests can be developed for use by conservation biologists and forest managers.