1. The vascular flora of 65 raised bogs was determined across climatic gradients in eastern North America. The major geographic patterns in species richness and floristic composition were analysed using linear regression, detrended correspondence analysis (DCA), and canonical correspondence analysis (CCA). 2. The vascular bog flora consists of 81 species in this region. Four major floristic regions distinguished by DCA correspond to the geographic zonation of bog land-forms. Each of these floristic regions is characterized by a different mean value for species richness. 3. Raised bogs in the southern-continental floristic region (I) contain fewer than 20 species and have the most impoverished vascular floras in eastern North America. In the transitional-continental (II) and northern-continental (III) regions, the bog flora is slightly larger, although the mean number of species remains below 26. 4. The richest bog floras are found in the maritime region (IV), where species richness declines along a latitudinal gradient from 50 species in the south (subregion IV.1) to 32 in the north (subregion IV.2). 5. The relationship of species richness to various environmental factors was analysed using linear regression. The most important factors are mean annual precipitation (MAP) and annual freezing degree-days (FDD) with a base temperature of 0-degrees-C. Factors that are also significantly related to species richness are mean annual temperature (MA T), the number of wet-to-dry habitats, and the concentration of magnesium and sodium in the surface water. No significant relationship was found for growing degree-days (base temperature = 5-degrees-C.), mean annual snow depth, or spatial dimensions of bogs. 6. The regressions indicate that a climatic threshold of 1000 mm of annual precipitation and 1000 freezing degree-days separates the floristically rich maritime bogs from the floristically impoverished bogs of northern and continental regions. 7. The relative importance of these environmental factors is supported by CCA. In addition, the CCA ordination identifies two groups of species on the basis of their response to these environmental factors. Species that are generally restricted to the maritime region (IV) have weighted averages that are above average for MAP and MA T, but below average for FDD. Species that are common to all bogs or that are more continental in their distribution have the reverse relationship. 8. The tolerance of the regional bog flora to the acidic, anoxic rooting zone changes across this climatic gradient. One third of the species that grow on bogs in the southern maritime region are restricted to fens or mineral uplands in continental or northern regions. This shift in ecological tolerance may be related to (i) slower rates of peat accumulation on the maritime bogs, (ii) longer time available for the evolution of local tolerant populations on maritime bogs, or (iii) a physiological adaptation to the maritime climate.
