Sweetgum trees thrive under diverse conditions, grow as fast as pine trees and provide the type of fiber needed for specialty papers, and they’ve long been desired by paper and bioenergy producers.
But there’s a hitch: Harvesting mature sweetgums can often be too costly or even ill-advised because they typically grow best on the edges of swamps and in river bottoms, which are often inaccessible during wet, winter months. UGA researchers may have solved this problem.
They’ve crossed American sweetgums with their Chinese cousins, creating hybrid sweetgum trees that have a better growth rate and denser wood than natives, and can produce fiber year-round.
The hybrid sweetgum trees have enormous potential for the production of bioenergy and paper, said Scott Merkle, a professor in UGA’s Warnell School of Forestry and Natural Resources.
“The very best hybrid sweetgums are being propagated to produce elite varieties so that landowners will be able to plant trees that produce more biomass in a shorter time than the top-rated native sweetgum trees,” he said.
The new hybrid varieties already are being sold commercially by ArborGen Inc., which began offering the new varieties in 2015. This important step came after ArborGen’s extensive field testing of the trees produced from tissue cultures in Merkle’s lab in Georgia, South Carolina and Alabama.
The tests showed these new hybrids outperform each of the parent species by 20 percent in both growth rate and wood density. The hybrid varieties are co-owned by ArborGen, a forest seedlings company headquartered in South Carolina, and the University of Georgia Research Foundation.
The new hybrid has been well accepted by forest landowners, particularly in the western areas of the Southeast, including Texas, Arkansas and Oklahoma. ArborGen officials say they produced 300,000 seedlings this year and sold out before the hybrid could even be marketed.
“We were testing the viability of this tree in the marketplace and are pleased to say it was received even better than expected,” said John Pait, vice president of sales and marketing for ArborGen. “We are now taking orders for next year with anticipation that we will see the same, if not greater demand, than this year. There will be a limited supply so we are asking customers who are interested to contact us as soon as possible.”
A number of other hybrid trees have been produced for forest crops, including hybrid poplars, eucalyptus and pines. But they haven’t been that popular in the Southeast’s forestry industry, said Merkle, who is also Warnell’s associate dean for research. Hybrid poplars haven’t performed well in the Southeast because they don’t tolerate drought well, he said, and hybrid eucalyptus can’t grow north of Florida because they’re not cold tolerant enough to survive freeze damage. And the only commercially available hybrid pine trees, he added, are crosses between loblolly and pitch pines, which are really best suited to grow in the mountains of West Virginia.
But sweetgums, including hybrids, already have adapted to grow in the region in a variety of sites.
Merkle decided to combine hybrid breeding with somatic embryogenesis, the process of mass propagating trees from somatic cells—essentially propagating the embryos from seeds produced from crossing the two sweetgum species.
Combining the hybridizing and propagating techniques sped up testing that could have taken much longer, Merkle said. It also allowed them to more accurately test the quality of the new trees than by growing one tree from one seed and then waiting.
After planting the seedlings, researchers simply watched to see which ones fared the best, ultimately narrowing it down to four varieties that showed the best growth rate and the densest wood. ArborGen then picked out those trees with the best traits, produced thousands more by rooted cuttings and began selling them last year.