As Pollen Clouds Drift in, Native Pine Trees Hybridize
The natural range of shortleaf pine is vast, spanning 22 states. The tree grows as far south as Texas and as far north as New York, often in association with other native pines such as loblolly. Shortleaf and loblolly pines’ natural ranges overlap, although they tend to prefer different site conditions. Loblolly usually grows on moister sites than shortleaf, but mixed stands that include both species are widespread, especially across the mid-South.
In the Ouachita and Ozark Mountains of Arkansas, Oklahoma, and Missouri, shortleaf is the only naturally-occurring pine, and it forms extensive stands, especially on drier, south-facing slopes.
Increasingly, shortleaf and loblolly pine are cross-pollinating, forming hybrids at a rate that threatens the genetic identity of shortleaf pine. Although hybrids may look like their parents, they do not have the unique genetic traits of their parent species, nor the ecological attributes that allowed their parents to thrive. Scientists from the U.S. Forest Service Southern Research Station (SRS) and Oklahoma State University recently collaborated to review decades of research about the causes and implications of shortleaf-loblolly hybridization. For more information about their initial study, read Shortleaf Pine: a Species Slipping Away?
Another collaborative study between SRS scientists and Oklahoma State University researchers found that the distance between loblolly pine plantations and shortleaf pine stands was a major factor in the amount of hybridization between the two species. The study, recently published in the Southern Journal of Applied Forestry, was led by University researchers John F. Stewart and Charles G. Tauer with SRS co-authors James M. Guldin, project leader of the SRS Southern Pine Ecology Unit, and C. Dana Nelson, project leader of the SRS Forest Genetics and Ecosystems Biology Unit.
“The timberland surrounding the Caney Creek Wildness Area of the Ouachita National Forest in Arkansas was an ideal outdoor laboratory for this study, and a microcosm for the potential of loblolly pine pollen to fertilize naturally regenerating shortleaf pine stands,” said Guldin.
Shortleaf pine is the only naturally occurring pine in the study plots, but over the last 40 years, nearby private lands had been converted to intensively managed loblolly pine plantations. Plantations can produce enormous clouds of pollen, which drift into neighboring stands of pure shortleaf.
After gathering pine needles in the study plots and analyzing their DNA, the researchers found that both young and older stands had similar numbers of hybrids in their midst. However, study plots that were closer to loblolly plantations had more hybrids, probably because they faced a denser cloud of loblolly pine pollen each spring.
“We found that even a mile or so is somewhat effective in keeping these species from hybridizing,” says Guldin. This means that managers who want to preserve pure shortleaf pine stands and minimize hybridization should maintain a buffer between loblolly plantations and shortleaf stands. In some cases, such as when a small loblolly pine plantation is surrounded by pure shortleaf stands, converting the loblolly stand back to shortleaf may be better even if the timber in the loblolly stand might otherwise be considered premature to cut.
Prescribed fire may also be an effective tool to kill hybrid seedlings, which do not have shortleaf pine’s fire tolerance and do not sprout as well. First generation hybrids also have somewhat distinctive needles and cones, so they can be recognized in the field and targeted for removal during early thinning treatments. “Anything that can be done to maintain physical or ecological distance between naturally-regenerated shortleaf stands and planted loblolly stands in the Ozarks and Ouachitas is probably a good idea for land managers to consider, especially on Federal lands,” said Guldin.
For more information, contact James M. Guldin at jguldin@fs.fed.us