Hybridization Leads to Loss of Genetic Integrity in Shortleaf Pine: Unexpected Consequences of Pine Management and Fire Suppression

hybridization is normally prevented by different flowering times in the two species, but under certain climatic/weather conditions hybridization may occur (Dorman and Barber 1956), and natural hybrids of shortleaf pine and loblolly pine have been reported (Zobel 1953, Hare and Switzer 1969, Edwards-Burke et al. 1997 and Stewart et al. 2010). The questions are where, when, and why do they hybridize, and what are the consequences? A Brief History of Shortleaf Pine and Loblolly Pine Management Since the inception of pine management in the southeastern United States during the 1930s and 1940s, loblolly pine has been the favored species. Schultz (1997, p. 1–3) clearly states why: “loblolly pine is the ideal tree for site restoration and forest management” and, that loblolly pine is “the most hardy and versatile of all of the southern pines, in terms of its ability to reproduce and grow rapidly on diverse sites.” In contrast, shortleaf pine has not been favored because of slow early growth, and in some places its susceptibility to littleleaf disease, caused by a complex of factors including low soil nitrogen, poor soil drainage, and the funguslike Phytophthora cinnamomi Rands (Dorman 1976, Mistretta 1984). However, the primary reason for dominance of loblolly pine in southern forests today is that the species has been the backbone of southern pine tree improvement programs (for the reasons given by Shultz (1997) mentioned previously) for 50 years and has been extensively planted by forest industry. Intensive silvicultural inputs and the success of the tree improvement programs have increased yield of loblolly pine plantations from approximately 90 ft ac 1 per year (6.3 m ha 1 per year) in natural stands to currently over 350 ft ac 1 per year (24.5 m ha 1 per year) in plantations (Fox et al. 2007). These dramatic increases in growth have led to establishment of over 30 million ac of pine plantations in the southeastern United States, composed primarily of loblolly pine and to a lesser extent slash pine (Pinus elliottii Engelm.; Fox et al. 2006). Consequently, loblolly pine plantations have replaced natural stands of shortleaf pine, mixed shortleaf pine–upland hardwood stands, and, in particular, mixed loblolly–shortleaf pine stands in the upper Gulf Coastal Plain. In addition, loblolly pine plantations of both North Carolina and Arkansas–Oklahoma origin have been established, primarily by forest industry, outside its native range into the Interior Highlands of Oklahoma, Arkansas, and Missouri. These areas were traditionally dominated by pure shortleaf pine stands and mixed shortleaf pine–oak and oak–pine stands. As a consequence of the focus on loblolly pine through the last half of the 20th century, shortleaf pine has been largely ignored and unmanaged across much of its range, and it is diminishing in numbers of trees and stands. The exclusion of fire has contributed to the decline of shortleaf pine in natural stands. Young shortleaf pines generally sprout after fire, whereas loblolly pine does not. Without fire on naturally fire-prone sites, loblolly pine survives and will outgrow shortleaf pine, whereas fire selectively eliminates loblolly pine regeneration (Williams 1998). In other areas, fire suppression gives later successional hardwoods an advantage. Accumulation of duff and litter as well as greater understory shading prevents shortleaf pine regeneration. In the Ozark Mountains, with fire excluded, Guyette et al. (2007) predict shortleaf pine numbers will be reduced by 80% and then stabilize within 200 years; Moser et al. (2007) reached a similar conclusion for shortleaf pine across the South. Several recent estimates are that shortleaf pine ecosystems have already been reduced by more than 40% in the last half a century (Guldin et al. 1999, South and