Objective Our paper aims to investigate the genetic variation in growth traits of the open-pollinated offsprings (i.e., the second generation) from the 1.5th generation seed orchard of Pinus taeda through multi-site test, with objectives to select the superior families and individuals for validating breeding achievements in Guangdong Province of southern China and promoting improved varieties of P. taeda.

Method 50 half-sib families (including one control) of Pinus taeda were measured across three sites (Dushan, Anfu and Yingde) for 5-year growth traits. Genetic parameters were estimated using R and ASReml software based on the BLUP model. According to breeding values, superior materials were selected with 10% family and 1% individual selection rates and their genetic gains were estimated.

Result (1) The coefficient of variation of individual volume at three sites followed the order of Dushan > Anfu > Yingde, and Yingde exhibited the greatest mean DBH and mean individual volume. There were significant differences in tree height, DBH and individual volume between sites and families (P < 0.01). The interaction between site and family was significant. (2) Single-site heritability estimates for volume, tree height, and DBH varied between 0.120−0.181, 0.079−0.237, and 0.121−0.124, respectively. The range of family heritability of volume, tree height and DBH were 0.402−0.556, 0.383−0.544, and 0.365−0.537, respectively. Based on multi-site joint analysis, the individual heritability estimates for volume, tree height, and DBH were 0.110 ± 0.033, 0.160 ± 0.041, and 0.104 ± 0.031, respectively. The family heritability estimates for these traits were 0.659 ± 0.083, 0.750 ± 0.051, and 0.663 ± 0.081, respectively. The family has high heritability and is suitable for breeding excellent families. The level of individual plant heritability is not significantly different from existing literature reports, and individual plant selection can be carried out. (3) The five superior families at Dushan (P034, W14, P023, W28 and P043), Anfu (W14, W11, 11, 288 and W08), and Yingde (W05, W07, 287, W11 and W14) achieved volume genetic gains of 0.7369, 3.2784 and 3.1157 dm³, respectively. The family W14 performed well in all three test sites. Family W11 performed well in both Anfu and Yingde, and although it was not selected in Dushan, its performance there was still in the upper range, making it a candidate for multi-site deployment. The average genetic gain of volume of selected individuals in Dushan, Anfu and Yingde was 1.6956, 7.1338 and 5.9533 dm³, respectively.

Conclusion The growth traits of 5-year-old half-sib progeny of Pinus taeda have abundant genetic variations among families. Five local superior families and twenty superior individuals were selected from each of the three sites. The W14 family exhibites stable and superior performance across all sites, demonstrating broad adaptability. The results of this study support the broader application and regional deployment of superior loblolly pine families bred in Guangdong Province to surrounding provinces.