Shortleaf pine: a species at risk?
Since the 1950s the existence of natural hybrids between shortleaf pine and loblolly pine has been recognized and reported in the literature. In a range-wide study of isoenzyme diversity in shortleaf pine. we found 16 percent of the trees from western populations were hybrids. based on the isocitrate dehydrogenase (IDH) locus. In stands thought to be pure shortleaf pine in west central Arkansas (Mt. Ida). we found 15 percent of the trees were hybrid. As a follow-up study to confirm or discount these results. we sampled native stands across Montgomery County, Arkansas, including the Mt. Ida area. These stands were mixed loblolly pine and shortleaf pine in the southeast part of the county and pure shortleaf pine in the northwest corner. In these stands we again found (1) a relatively high percentage of hybrid trees (14 percent); (2) hybrids in shortleaf pine stands beyond the natural range of loblolly pine; (3) introgression occurring in both directions; and (4) t~e IDH locus.a reliable marker for species and hybrid determination. We are now engaged In a range-Wide study of both loblolly pine and shortleaf pine to examine the cause and consequences of natural hybridization between these two species.