Early–Late Correlations of Growth Traits of Eucalyptus urophylla S.T. Blake Clones over a Rotation

<i>Eucalyptus urophylla</i> is a core tree species for short-rotation industrial timber plantations in South and Southwest China. However, the dynamic correlation rules of its growth traits during the full rotation period remain unclear, and the theoretical research on early selection is insufficient. In this study, 12 pure <i>E. urophylla</i> clones (including U6 and MLA as controls) were used as plant materials. Based on the data of tree height (H), diameter at breast height (DBH, D), and individual tree volume (V) from 0.5 to 7.5 years old, the correlation rules of early and late growth traits were explored, core predictive traits were screened, and the optimal selection age was determined through rank correlation, phenotypic and genetic correlation analyses, combined with regression modeling and selection efficiency calculation. Early selection of <i>E. urophylla</i> clones was feasible: after 3.5 years, the early-late phenotypic and genetic correlation coefficients of H, D, and V all reached significant or highly significant levels, and the genetic correlation coefficients were greater than the phenotypic ones, indicating that genetic factors dominated trait correlations with little environmental interference. All five established early selection regression models passed the highly significant test. Among them, the models of D-early versus D-late, V-early versus V-late, and D-early versus V-late had the highest coefficients of determination (0.9293-0.9385), making them the optimal selection traits; the models of H-early versus H-late and H-early versus V-late had lower coefficients of determination (0.8010-0.8364) due to errors in height measurement. The best selection effect was achieved within 1/2-2/3 of the rotation period: for a 6-year rotation period (pulpwood), the optimal selection age was 3.5 years old (annual efficiency 1.318); for an 8-year rotation period (medium-diameter timber), it was 4.5 years old (annual efficiency 1.345); and for a 12-year rotation period (large-diameter timber), it was 6.5 years old (annual efficiency 1.379). This study not only fills the theoretical gap in early selection of <i>E. urophylla</i> during the full rotation period but also constructs an integrated early selection technology system of "trait screening-model prediction-age determination". It provides key support for shortening the breeding cycle of <i>E. urophylla</i> and achieving precise control of breeding costs and offers important references for early selection research on fast-growing broad-leaved tree species worldwide.