Hybridization in Naturally Regenerated Shortleaf Pine as Affected by the Distance to Nearby Artificially Regenerated Stands of Loblolly Pine

The natural range of shortleaf pine encompasses 22 states from New York to Texas, second only to eastern white pine in the eastern United States. It is a species of minor and varying occurrence in most of these states usually found in association with other pines, but it is the only naturally occurring pine in the northwestern part of its range in Oklahoma, Arkansas, and Missouri. Over the whole of this area, it is apparent that shortleaf pine hybridizes with loblolly pine, including areas of species overlap (sympatry) and nonoverlap (allopatry). In this study, we used 25 DNA markers to measure the levels of hybridization and introgression in populations of juvenile and mature shortleaf pine from a wilderness area within the allopatric range of shortleaf pine that bordered loblolly pine plantings. Frequency of hybrids ranged from 9.2 to 24.0% among the populations sampled, and we found that levels of hybridization correlates with distances to the nearest loblolly pine plantations. However, the rates of hybridization and introgression apparently remained the same among the parent and juvenile populations.