Patterns of North American Fern and Lycophyte Richness at Three Taxonomic Levels

North American monilophyte (fern) and lycophyte richness patterns are examined at three taxonomic levels (species, genus, and family). We determine: (1) if fern richness patterns are associated with water and energy variables that are predicted by the productivity-diversity hypothesis and (2) whether the pattern or strength of the relationship varies with taxonomic level. We present species richness maps for individual families of ferns and lycophytes allowing us to identify taxa with unique distributional patterns and taxa with patterns comparable to ferns in general. To accomplish these goals, we use data from the Flora of North America project for continental North America north of Mexico plus Greenland. We construct 479 GIS fern species range maps and tabulate fern and lycophyte richness in a gridded map with 2500km(2) squares. We perform regressions of fern richness on water and energy climate variables (with squares as data points) in order to identify which variables most influence fern richness. We find that fern richness correlates with water and energy variables in ways consistent with the productivity-diversity hypothesis. A multiple regression model that includes mean annual temperature (MAT) and annual rainfall (RAN) explains 78.1% of the variation in fern family richness. The relationship between fern family richness and climate is stronger than the relationship between fern species richness and climate.