The Floras of North America project is an attempt to summarize data from all of the vascular floras written for North America north of Mexico.  In the analyses presented here, I utilize species counts from almost 3000 published floras to assess the latitudinal gradient of decreasing species richness from south to north.  This gradient has been considered since the time of von Humboldt to be one of the strongest and most repeatable patterns of biodiversity.

Results/Conclusions

The latitudinal gradient in vascular biodiversity is remarkably weak south of 50 degrees N.  Furthermore, the gradient is highly modified by elevation, spatial grain, and other factors.  Exotic plant diversity may peak at intermediate latitudes.  The species-latitude relationship differs from the genus- and family-latitude relationships.  Future research, in collaboration with the EPSCoR Ecological Forecasting Project, will assess whether climatic or energetic factors are stronger predictors of diversity than are purely geographic factors.