We have compared lichen communities in three layers (canopy, trunk, ground) of 15 old growth and managed Northern Hardwood and Hemlock-Hardwood forest plots in Michigan and Wisconsin, to develop an old-growth forest indicator system and to assess the impact of forest management. Old-growth stands of both forest types have more lichen species than managed stands. Equivalent stands of the two forest types have about equal numbers of species and of nitrogen-fixing cyanolichens. Hemlock-Hardwood stands have species evenly distributed among forest layers, while Northern Hardwood stands have more species in taller layers. All managed stands of both forest types have fewer and less abundant cyanolichen species than do old-growth stands. Species loss occurs most in canopy and trunk layers of managed Northern Hardwood stands, but trunk and ground layers of managed Hemlock-Hardwood stands. Large, long-dead standing snags and large partially-decayed coarse woody debris (CWD: especially mossy logs) are particularly important substrates fostering increased species diversity of forest lichen communities. Bark-living lichen species tend to be substrate-selective both with respect to tree species and to tree age, so a wide range of tree sizes as well as a variety of tree species fosters increased lichen community diversity. Lack of snags and CWD is more critical in Hemlock-Hardwood stands because trunks and bases of living hemlocks are too shady for most lichens. Management of forest stands which leaves abundant snags and old-tree islands, and leaves or enhances large CWD should deter loss of lichen community diversity. Nitrogen-fixing lichens and ground-layer lichens are two identifiable lichen "guilds" whose diversity is negatively impacted by logging.

Key words: forest, lichen communities, lichens, management, old-growth