Description

From the text...'The present study examines geographic patterns of allozyme variation in longleaf and loblolly pines to provide evidence for the location of Pleistocene refugia for the two species.... It is proposed that the continuous linear decrease in allozyme variation in longleaf pine from west to east is a result of migration from a single refugium in the west (south Texas or northeast Mexico) after the Pleistocene, with a loss of variability due to stochastic events during migration. The lack of such a trend in allozymes in loblolly pine, coupled with the distinct east versus west variation in fusiform rust resistance and other adaptive traits, suggest that loblolly pine was located in two refugia during the Pleistocene: in Texas/Mexico and Florida/Caribbean, as proposed by Wells et al. (1991). The dashed line east of the Mississippi, in southeast Louisiana and west Mississippi (Figure 1), shows the location of a very steep gradient in fusiform rust resistance which can best be explained by assuming the confluence of two populations (Wells et al., 1991). This, as well as the steep gradient in terpene concentration in the same location, suggests the merging and mixing of the two populations after the retreat of the Wisconsin glaciation. Seed movement guidelines should take into account the differences among the two species. It does not appear that east-west movement of longleaf seed sources need be restricted, especially if seed of the more diverse western populations are moved eastward. More caution should be used in east-west movement of loblolly pine, since there appear to be two different populations.'