Abstract Loblolly pine (Pinus taeda L.) seedlings growing with no interfering vegetation were consistently greater in diameter and biomass than those growing with only herbaceous vegetation. Seedling diameter and biomass were more responsive than height to reduction in interfering vegetation. During the first growing season in the field, pine growth appeared to be influenced more by the presence of herbaceous than woody vegetation. For a given seedling age, the proportion of biomass in branches increased as interfering vegetation decreased. Seedling age and the absence of interfering vegetation each increased the percentage of foliage biomass in four-needled fascicles, fascicle diameter, total length, sheath length, dry weight, and surface area and decreased specific leaf area. Four-needled fascicles from two-year-old seedlings, growing with or without herbaceous vegetation, tended to be greater in diameter, total length, sheath length, dry weight, and surface area and were approximately equal in density to three-needled fascicles. Leaf area (total needle surface area) was strongly related to groundline basal area and was quite responsive to changes in herbaceous vegetation. The strong relationship between stem diameter and leaf area at several levels of interference suggests that either measure might be used as an indicator of loblolly pine response to interference from other plants. Forest Sci. 32:1016-1031.