Forest lichen communities and environment–How consistent are relationships across scales?

Question: How consistent are relationships of forest lichen community composition with environmental variables across geographic scales within region and across regions? Location: Northwestern continental USA and east central continental USA. Method: Four macrolichen data sets were compiled using identical plot sample protocol: species abundance estimated in 0.4-ha permanent plots on a systematic grid, as part of government (USDA-FS) forest inventory programs. One data set in each region represented a large area; the other represented part of the large area. We used global NMS ordination of plots based on species abundance to extract major axes of variation in community composition. Correlations of species, guilds, and environmental variables with ordination axes were compared between geographic scales for the two regions. Results: Primary axes of community variation at larger scales were correlated with climate variables and related geographic variables such as latitude and elevation, and with pollution. Forest vegetation variables such as stand age and tree species composition became more important at small scales. Community variation unexplained by macro-environment variables also became more important at small scales. Of several hundred species tested, ten lichen species showed consistent behaviour between scales within region (one also across regions) and are thus potential general indicators of ecological conditions in forests. Of six lichen guilds tested, several show strong patterns not consistently related to environmental conditions Conclusions: Interpretation of lichen species and community composition as indicating particular environmental conditions is context-dependent in most cases. Observed relationships should not be generalized beyond the geographic and ecological scale of observation. Abbreviations: NMS = Global Non-metric Multidimensional Scaling ordination. Nomenclature: Esslinger (2005) for lichens; Mitchell & More (2002) for tree species.