The aim of this study was to evaluate morpho-physiological variables responsible for differential growth of Pinus taeda seedlings under subtropical natural environmental conditions in order to determine key eco-physiological characters for selection of better adapted genotypes in the context of climate change. The following variables were evaluated in half-sibling individuals from two mothers, a fast-growing (CR) and a slow-growing (CL) genotype: height and basal diameter (BD) increment, stomatal conductance (g_s), minimum and maximum daily water potential (ψ), photosynthesis at light saturation (A_sat), branch specific hydraulic conductivity (k_s), hydraulic conductance (K_h), leaf specific hydraulic conductivity (b and aerial biomass allocation. The CR genotypes had higher height and BD increment and increased biomass production. Daily g_s and ψ patterns and A_sat were similar among genotypes. In contrast, CR genotypes presented a higher water conduction efficiency per leaf area unit under conditions of high soil water availability, although their k_s wbas more affected under water deficit conditions than in CL genotypes. The observed differences in biomass allocation within aerial compartments, leading to changes in the whole hydraulic architecture of the plant, would be responsible for differential growth rates. In addition, the observed trend of a lower stomatal regulation in CR, reflected in a higher degree of anisohydrism and higher k_s losses in a dry period, suggests that these genotypes could be more vulnerable to water deficits than CL, requiring further studies to test this hypothesis.