Overview - Yellow-poplar was one of the original four species selected by Professor Thor for improvement research. He conducted an intensive selection program and grafted superior phenotypes into a breeding orchard. Controlled pollinations and progeny testing of the resulting seedlings have been conducted in cooperation with the Tennessee Division of Forestry. Clonal tests have been established in cooperation with the University of Georgia. More recently, materials from Professor Thor's breeding orchard have been used for molecular research. The Tennessee Valley Authority also conducted research on yellow-poplar and have established a clone bank of phenotypic selections, altitudinal variation tests, regional variation tests, and controlled pollinated progeny tests that are now managed by the UT Tree Improvement Program (UT-TIP).

Embling Tests - The yellow-poplar clonal study is a cooperative effort between Professor Scott Merkle, the University of Georgia and the UT-TIP. In 1989, controlled crosses were made in the yellow-poplar orchard at the East Tennessee Research and Education Center near Knoxville. Immature samaras were harvested in July and sent to Professor Merkle. The embryos were placed into a somatic embryogenic culture system developed by Professor Merkle. Germinating embryos were returned, in vitro, to the Tree Culture Laboratory, a cooperative effort between Professor (now emeritus) Otto J. Schwarz and the UT-TIP. The clones were cultured for 9 months and then transplanted into small Cone-tainers™ and grown for one season under shade. In 1992, the clones were outplanted at four locations in Tennessee in a randomized complete block design, 12 blocks per replication, 3-4 replications. Clones from eight different controlled cross lines were planted in 5 clone X 5 clone blocks (one clone per block). Three blocks contained mixed clones, and one block contained seedlings produced at the East Tennessee State Nursery from the above orchard. Currently, three locations are still viable tests.