Neighborhood Competition Correlates with Height–Diameter Growth Allocation and Gene Expression in Clonal Eucalyptus urophylla × E. grandis Plantation

Height–diameter (H–D) growth patterns describe the relative growth of tree height and stem diameter and may vary with local competitive conditions. We examined a Eucalyptus urophylla × E. grandis clone DH32-29 plantation across 12 stand units. Phenotypic comparisons included 364 interior trees and 254 true stand-edge trees; RNA-seq was conducted on a balanced subset of 72 trees (36 interior and 36 edge trees), not on the full phenotypic sample. Neighborhood competition around interior trees was characterized using the Hegyi index and basal area in larger trees (BAL), and growth traits were compared between interior and edge trees. Hegyi and BAL were negatively associated with growth, with stronger associations for diameter increment than for height increment. Unadjusted comparisons suggested different growth patterns, but plot-adjusted sensitivity analysis supported differences only in diameter increment and H/D. WGCNA identified modules associated with individual allometric traits, and XGBoost prioritized expression features with moderate model performance (R2 = 0.404). qRT-PCR supported expression trends for selected genes but did not test gene function. These observational results identify associations among neighborhood structure, growth patterns, and gene-expression variation; edge contrasts may also reflect differences in light, wind exposure, microclimate, or soil conditions. The candidate modules and genes require independent and functional validation.