This series of maps shows the relative similarity of the native vascular flora between U.S. counties and selected individual U.S. states and/or regions. Although not exhaustive, we have selected various states to represent each region or the country, and ones that best reflect unusual patterns of floristic similarities. For some states, profound similarities (or dissimilarities) become obvious once their floras are compared. Note, for example, how dissimilar the floras of Alaska and Hawaii (maps #2 and #15) are from the rest of the U.S. For Hawaii, this is quite predictable, with 85% of its native flora being endemic. However, Alaska’s dissimilarity with other US states is far less predictable, since less than 3% of its flora is endemic. Although a high percentage of the circumboreal flora of Alaska is shared with other northern areas in Canada, Greenland, and throughout vast areas of tundra in the arctic and subarctic, only a relatively low percentage of Alaskan species is shared with the flora of other U.S. states (see the seventeen northernmost tier counties of Washington, Idaho, and Montana).

Other patterns of floristic similarities exist between state floras of certain states of the Great Plains (map #13) and the rest of the U.S. flora (specifically the states of Nebraska and Kansas [maps #24 and #17]). For its relatively small size (1,500 native species), map #24 shows that Nebraska has one of the most ubiquitously similar floras of any U.S. state or Canadian province. It has zero endemic species, and there is not a single U.S. county that shares less than 6% of its flora with Nebraska’s. Even more unusual is that only one county in California, and thirteen counties in Georgia shares 5-10% of their floras with Nebraska's; all other U.S. counties share 10% or more. Therefore, it can be reasonably stated that species comprising Nebraska’s flora are most widely shared of any in North America. One might argue that given Texas’s large flora and geographic location spanning both eastern and western floristic regions, it should earn this dubious distinction of having a most shared flora. However, since Texas, unlike Nebraska, is not centrally located with respect to northern and southern floristic regions and has such a large native flora (compared with that of most other U.S. states), many being endemic or near endemic, it is disqualified from consideration.

It may surprise some to see the great floristic similarity that exists between the floras of Tennessee and Virginia, (maps #34 and #38), regarding their shared similarities with the floras of the Great Plains and Texas in these latter two geographic areas.

It has long been known that physical geographic barriers such as mountain ranges, hot and cold deserts, dry plains, or even great rivers can play significant roles in restricting or limiting the migration or emigration of plants into new areas. In certain cases, the advancement of entire regional floras can be stultified or halted by such barriers. The effects of these barriers can be seen via the reduction of floristic similarities between state and regional floras across the North American landscape. For example, notice how floristic similarities of the floras of the Northeastern U.S. (map # 28) and Great Plains (map #13) drop, once confronted by the Rocky Mountains and its flora. Notice too how the dryness of the Great Plains serves as a barrier, thus reducing floristic similarities between its flora and state floras eastward and westward of it. Also notice how the Great Plains serves as a more effective barrier to the floras of the Southeast U.S. and to the Blue Ridge, (maps # 33 and #5) than it does to the flora of the Northeastern U.S. Thus, west of the Great Plains, there is a greater sharing of species with Northeastern flora than there are with the floras of Southeast or Blue Ridge.

There are many suppositions and theories that can be developed by studying these maps. Although I have presented a few of the more obvious ones, please take time to analyze the ones in your geographic area, to see if other, more cryptic patterns or peculiarities can be identified. We would greatly appreciate learning of any theories or suggestions that you might have regarding these maps.  -JTK