The objective of this study was to better understand the underlying gene action in eucalyptus, under different plantation densities, for a different set of traits: growth, bark thickness, ecophysiological, and wood chemical property traits. We estimated the magnitude and relative proportion of the various genetic variance components using a eucalyptus genotype by spacing (G x S) interaction experiment. A clonally replicated progeny test including 888 clones belonging to 64 full-sib families of Eucalyptus urophylla x Eucalyptus grandis hybrid was used to estimate genetic parameters using genomic information to assess relationship matrix. Two densities (833 and 2500 trees/ha) were used representing contrasted environments in terms of individual tree available resource. Results showed that for height and circumference, additive-by-spacing (A x S) interaction variance increased from 18 to 55 months old, while dominance-by-spacing (D x S) interaction variance decreased. For bark thickness, specific leaf area, nitrogen, calcium, and magnesium, A x S interaction variance was preponderant. For wood chemical properties, except with Klason lignin, genetic additive effects strongly interacted with spacing compared to non-additive effects.