Research Forester USDA Forest Service; Southern Research Station Knoxville, Tennessee, United States
Background/Question/Methods
In general, moving north or south from the equator shows a decline in species (taxon) diversity for most life forms. The latitudinal diversity gradient is, arguably, one of the famous patterns in species distributions and occurrence. Actual empirical tests of this hypothesis are few. I used data from the USDA Forest Service, Forest Inventory and Analysis (FIA) program to empirically test for differences in diversity across latitudes in the eastern USA. Three north-south transects, approximately 1200 km long and 300 km apart, were established in GIS software. Two of the transects were east and one was west of the Mississippi River. The northern-most latitude was 41.500, the southern-most latitude was 30.500. Along each transect a sample unit (SU) collection point was established to lasso all the FIA SUs inside a 40-km radius circle. Each transect had 5 SU collection points approximately 240 km apart. Approximately 250 FIA SUs were inside each circular polygon. The species diversity metric applied was tree species richness (S) and the alpha diversity (point diversity) approach was defined by the number of tree species ≥2.54 cm dbh on each SU. The size of each SU was approximately 0.067 ha.
Results/Conclusions
All three transects showed increases in alpha (S) between the southern- and northern-most SU capture locations. The eastern-most transect showed a 4.30 and 5.82 average (S) SU-1, south vs. north, respectively. The middle transect showed a 3.52 and 5.97 average (S) SU-1; the western transect showed a 3.99 and 6.84 average (S) SU-1. The highest number of species recorded on any SU was 14. The study results do not corroborate the latitudinal gradient diversity decline hypothesis. Possible reasons for this are: 1) the alpha (S) measure in a SU does not capture the full complement of species (SU size too small), 2) the change in and number of habitats, 3) the amount/degree of disturbance, and 4) various stages of succession may be present. Because the study is based upon a random sample without stratification on any parameters, any of the conditions listed above may impact the results. This study demonstrates one way to assess tree species (S) over a large geographic area. Future studies need to incorporate beta and gamma diversity measures. Challenging is the development of empirical methods and means to collect data that help in studying the species diversity latitudinal gradient, a good application of FIA data.
