Hybrids in the Seed Orchards

Keeping Loblolly Pine Genes out of Shortleaf Pine Seed Orchards

seed orchard
Shortleaf pine in Mount Ida Seed Orchard, Arkansas. Photo by Barbara Crane, USFS.

Shortleaf pine is under siege, and one of the threats has emerged in seed orchards. “In some shortleaf seed orchards, 10 percent of trees are hybrids,” says U.S. Forest Service research geneticist Dana Nelson. “Although the majority we’d consider shortleaf pine, even a few hybrids is enough to raise concern.”

To some extent, shortleaf and loblolly pine have always hybridized. However, hybrids have become so common that they threaten shortleaf pine’s genetic integrity. Hybrid trees can thrive when life is easy, but they do not tolerate fire, ice, or drought like pure shortleaf pine.

“We worry that back in the 1970s and 1980s, natural shortleaf-loblolly hybrids were occasionally mistaken for pure shortleaf pine,” says Nelson. “The hybrids may have been incorporated into the seed orchards then.” Seed orchards exist to provide land managers with large quantities of seeds for restoration projects.

After a prescribed fire, shortleaf pine seedlings resprout vigorously. Fire is likely to kill hybrid trees. Photo by Holly Campbell, courtesy of the Shortleaf Pine Initiative.

Unlike loblolly pine, most shortleaf pine seeds are supplied by Forest Service National Forest System seed orchards. “Loblolly pine seeds are often supplied by private companies through university-led tree improvement programs,” says Forest Service regional geneticist Barbara Crane. “However, the Forest Service owns 95 percent of all shortleaf pine seed orchards.”

The scientists tested shortleaf pine trees and seedlings from eight seed orchards in the southern U.S. The National Forest System operates two of the seed orchards, and state forestry agencies run the remaining six.

In addition to Nelson and Crane, the research team included John Stewart, a post-doctoral fellow at Oklahoma State University and lead author, as well as his university colleague Rodney Will. The study was published in the journal Forest Science.

The scientists found that some of the hybrids were 87 to 93 percent shortleaf pine. “These trees may have some loblolly pine character, but most of their genomic material appears to be shortleaf pine,” says Nelson. If these trees only crossed with pure shortleaf pine, the loblolly component would become even less consequential.

However, as many as 10 percent of trees in some seed orchards were hybrids with a much higher percentage of loblolly pine genes – some as high as 50 percent. “Plans are underway to cut down and remove these hybrids from the seed orchards,” says Crane.

basal crook
Shortleaf pine’s crooked root collar (right) protects it from fire. Loblolly doesn’t have a crook (left), and shortleaf-loblolly hybrids have an inconspicuous crook that doesn’t confer fire tolerance. Photo by Curtis Lilly, Oklahoma State University.

Seed orchards with many hybrid trees also had many hybrid seedlings. The nursery environment makes it more difficult to tell whether a seedling is pure shortleaf pine or a hybrid – when pure shortleaf seedlings grow in the wild, the base of their stem is crooked. Loblolly pine and hybrids do not have this basal crook, which is an adaptation to fire.

However, pure shortleaf pine seedlings grown in nurseries often lack a basal crook. “Nursery managers should not cull seedlings without basal crooks,” says Will. “This would remove far too many non-hybrid shortleaf pine seedlings.”

Most of the shortleaf pine seedlings from nurseries are not hybrids. However, phasing out hybrids from seed orchards is critical. The scientists also recommend keeping loblolly pines from growing within 400 feet of shortleaf pine seed orchards. The distance will help prevent loblolly pollen from fertilizing shortleaf pine seeds.

“Shortleaf pine is an important component of southeastern forests and woodlands,” says Nelson. “Keeping shortleaf pine healthy will keep forests resilient.”

Read the full text of the article.

For more information, email Dana Nelson at dananelson@fs.fed.us.

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