Genetic Evaluation and Selection of Eucalyptus urophylla Families and Hybrids for Cold Tolerance in Northern Guangdong, South China

To identify superior eucalyptus genotypes adapted to the cold-prone northern margin of Guangdong Province and to elucidate the genetic basis underlying growth, cold tolerance, and heterosis, this study evaluated 51 eucalyptus genetic materials with diverse genetic backgrounds, including Eucalyptus urophylla families and E. urophylla-derived interspecific hybrid combinations. Growth and cold tolerance traits were measured at 4.0 and 6.5 years after planting to systematically assess genetic variation, combining ability, and selection potential among different genetic materials. Significant genetic variation was observed among the tested materials for both growth and cold tolerance traits. Several families and hybrid combinations exhibited superior performance in stem volume production and cold tolerance, indicating substantial potential for genetic improvement. BLUP analysis revealed a genetic trade-off between growth and cold tolerance in 31 families; however, several elite genotypes exhibited a favorable combination of rapid growth and enhanced cold tolerance, making them promising candidates for future breeding programs. Combining ability analysis demonstrated that both additive and non-additive genetic effects contributed to growth performance, and superior parental combinations with high general combining ability and specific combining ability effects were identified. Furthermore, interspecific hybrid evaluation showed that E. urophylla × Eucalyptus grandis hybrids generally exhibited greater growth potential and more stable productivity, whereas E. urophylla × Eucalyptus camaldulensis hybrids displayed stronger adaptation to low-temperature stress. These findings provide valuable genetic resources and theoretical support for developing fast-growing, cold-tolerant eucalyptus cultivars and improving the stability and productivity of subtropical plantation forests.