Environmental optimality, not heterogeneity, drives regional and local species richness in lichen epiphytes

AIM: We evaluate the scale dependence of species richness–environment relationships with a continent‐wide analysis of lichen epiphyte communities. Specifically, our goals are to assess: (1) the dependence of local richness on regional processes, (2) whether species richness is primarily influenced by heterogeneity in environmental conditions or the central tendency of those conditions, and (3) whether the relative influence of these different aspects of the environment differs between local communities and regional species pools. LOCATION: Forests of the contiguous United States. METHODS: We used variation partitioning and model averaging of linear models to relate macrolichen richness at 1923 forest inventory plots (c. 4000 m²) to measures of environmental heterogeneity and mean conditions at local and regional scales. Data included 17 local environmental variables and 11 regional‐scale variables which were obtained from a national forest inventory, herbarium records and several climate data sources. RESULTS: Regional‐scale variables explained more unique variation in local species richness and generally had stronger effects than variables measured locally. However, most variation in local richness was explained jointly by local and regional variables. At both local and regional scales, variables measuring environmental heterogeneity explained little variation in species richness and had weaker effects than variables characterizing mean environmental conditions. MAIN CONCLUSIONS: Species richness of epiphytic macrolichens is not regulated by environmental heterogeneity locally or regionally and instead tracks large‐scale climate gradients of water availability and temperature. Richness in local communities is influenced by processes operating at both regional and local scales, highlighting the importance of determining large‐scale drivers of lichen richness across the North American continent. This research demonstrates a general method for comparing the influence of different aspects of the environment on species richness across scales and should be applicable to many different taxonomic groups.