Growth characteristics and early selection in a Bt-transgenic poplar F₁ hybrid population under coppice management

Early selection is a key strategy to accelerate the breeding cycle in poplar, but its reliability depends on the stability of growth traits across different growth cycles. Coppicing, a routine management practice in poplar plantations, can substantially alter growth performance and may affect the efficiency of early selection. In this study, a large F₁ hybrid population derived from ‘Nanlin 895’ ( Populus deltoides × P. euramericana ) as female parent and two Bt -transgenic Euramerican poplar clones (‘B3–44’ and ‘G-252’) as male parents was evaluated for plant height (PH), ground diameter (GD), and first-order branch number (FBN) before and after coppicing. Without accounting for family background, the Bt gene showed a persistent suppressive effect on PH and a lag effect on GD, whereas sex had no significant effect on juvenile growth. However, when family background was included, the Bt effect on PH was reduced to marginal significance (p = 0.06 and 0.07, respectively). Family background was the dominant factor for PH and FBN, while coppicing activated genetic variation in GD between the two families. The Bt × family interaction was not significant for any trait before or after coppicing. A quadrant classification integrating initial stem volume index and its change after coppicing identified two types of superior genotypes: “stable superior” and “high‑response”. The latter represents individuals with low initial growth but strong regrowth after coppicing—a potential category that would have been missed by traditional single‑year selection. These results demonstrate that incorporating regrowth capacity into selection criteria provides a more comprehensive strategy for selecting superior genotypes in coppice‑managed poplar breeding programs. The elite individuals identified in this study serve as valuable genetic resources for breeding new poplar varieties with high insect resistance and high yield.