Both shortleaf and loblolly pine are native to the southeastern United States, where the two species have coexisted and occasionally hybridized for millennia. Historically, hybrids were rare. In the 1950s hybrids made up just 3 percent of the pines in shortleaf stands, but since then their numbers have skyrocketed. Today, just two or so generations later, shortleaf stands contain on average 45 percent hybrids.
Scientists from the U.S. Forest Service and Oklahoma State University recently collaborated to review decades of research about the causes and implications of shortleaf-loblolly hybridization. University researchers Charles Tauer, John Stewart, Rodney Will and Curtis Lilly led the study, which was partially funded by the Forest Service Southern Research Station (SRS). SRS scientists James M. Guldin and C. Dana Nelson also contributed to the study and to the article reporting the results published in The Journal of Forestry.
The researchers found that hybridization is threatening both species, but especially shortleaf pine. When hybrid offspring cross with their parents over generations, the genetic identity of one or both original species begins to change. Hybrids may look similar to a parental species, and under some environmental conditions may thrive, but they may also compromise the unique ecological attributes that allowed the parental species to survive. In essence, hybridization averages the traits of the parent species in the hybrid progeny, which means some of the genetic uniqueness of the parent species is lost across generations.
For example, shortleaf-loblolly pine hybrids may not be as tolerant of fire as pure shortleaf pines. Shortleaf pine seedlings have a crooked root collar that grows below ground and protects dormant buds from fire, enabling young trees to re-sprout even if the top part is killed. Loblolly pine seedlings, which are vulnerable to fire, do not have this crook at all; loblolly-shortleaf hybrids have inconspicuous crooks, and are not able to resprout after fire like pure shortleaf.
On the other hand, loblolly has more rapid early growth rates than shortleaf, as do hybrids. Loblolly pine’s fast growth and versatility have made it the backbone of the South’s timber industry since the 1930s and the focus of southern pine tree improvement programs for over 50 years. Loblolly pine has been intensively managed and widely planted, and is now far more common than shortleaf pine. There was a time when shortleaf pine was actually more abundant than loblolly pine, but those days are gone. By some estimates, the acreage of shortleaf pine in southern forests fell by more than 40 percent in the 20th century. The shrinking shortleaf population may have already lost genetic diversity, making further study of genetic diversity and population structure an urgent priority.
Fortunately, managers who want to maintain the original genetics of shortleaf pine have several options. Prescribed fire can reconnect isolated shortleaf pine stands, increase regeneration success, and under certain conditions kill young shortleaf-loblolly pine hybrids. Additionally, a distance of just a few miles between shortleaf pine stands and loblolly stands or plantations can drastically lower hybridization. “Shortleaf pine is a species worth protecting,” says Guldin. It’s a beautiful native tree, and we anticipate that it will be more resistant and resilient than loblolly pine under the hot, dry weather that tomorrow’s climate may bring us.”
For more information, contact James M. Guldin at firstname.lastname@example.org