Three common associates on secondary-successional pine sites (Andropogon virginicus, Liquidambar styraciflua, and Pinus taeda) were established in a field study in which a wide array of plant densities and species proportions were established using an additive series design. To mimic a specific competitive scenario (i.e., a managed earlysuccessional Pinus stand), Andropogon and Liquidambar were established a year prior to the establishment of Pinus. Competitive effect (the attenuation of resources) and competitive response (the growth of each species as a function of resource availability) were determined.Effect on soil water varied among species, depth of soil, and time. In the surface soil, soil water was largely influenced through non-uptake effects, while uptake effects were predominant in deeper portions of the solum. When competitor abundance was expressed on an aboveground biomass basis, rather than a density basis, species differences in effects on soil water were eliminated. Differences among the species in effects on soil water per unit leaf area or leaf biomass appear to be largely explained by differences in stomatal conductance. Predawn leaf-water potential was integrated over the season using a waterstress integral. Analysis of the water-stress integral suggested that Liquidambar and Andropogon both affected water available to Pinus; however, only Liquidambar affected Andropogon, and only Andropogon affected seasonal water available to Liquidambar. Light was most strongly influenced by Liquidambar density; however, as Andropogon density increased, the effects of Liquidambar were reduced.Andropogon response was correlated with light but not with water stress or leaf nitrogen. This reflects high light requirements and high water use efficiency of C 4 plants. Liquidambar response was related to water stress and leaf nitrogen, perhaps reflecting the greater nitrogen requirements of hardwoods. Pinus response was significantly related to all three resources individually, i.e., water stress, light, and leaf nitrogen. Pinus response was better explained by a regression model that included light and water stress than by water stress or light alone. Pinus growth as a function of water stress and light indicated that communities dominated by Liquidambar largely reduced Pinus growth through reduction in light, while communities dominated by Andropogon reduced Pinus growth primarily by increasing water stress. In mixed communities of Liquidambar and Andropogon, pine growth was constrained more equally by light and water stress.

Herbaceous weed control studies installed by the Auburn University Silvicultural Herbicide Cooperative to examine response to methods and duration of herbaceous weed control in eight loblolly pine (Pinustaeda L.) plantations were analyzed to determine stand response through age 9. Studies were designed to compare weed control treatments with an untreated check, weed control methods (band vs. broadcast), and weed control duration (first year vs. first 2 years). Pine growth was increased by weed control on all sites. Growth was increased by an additional year of weed control (duration) on about one-half of the sites, but did not differ between band and broadcast treatments (method). Age 9 volume response above the check averaged 27.3 m3/ha for first-year weed control and 42.9 m3/ha for the first 2 years of weed control. Individual-tree height growth between ages 7 and 9 did not differ by treatment at most sites, but stand volume growth was higher with weed control at six of the eight sites. Uniformity of individual tree size, as represented by the standard deviation of DBH adjusted for dominant height, was more dependent on survival, hardwood encroachment, and level of fusiform rust stem infection, which varied by treatment and site, than on the result of herbaceous weed control per se. Growth projections made with the least intensive weed control treatment at each site indicated that on average, merchantable volume at age 22 with weed control will equal that of an age 25 stand without weed control. Largest gains were on sites where weed control increased survival.

Plant mixtures were established that differed in both proportion and density of loblolly pine (Pinustaeda L.), sweetgum (Liquidambarstyraciflua L.), and broomsedge (Andropogonvirginicus L.). Soil moisture availability to the pine seedlings was quantified every 2 weeks by measuring predawn xylem pressure potentials. Temporal variation in pine water potential was accounted for by a water stress integral approach. Cumulative water stress integral values were calculated over four overlapping periods, from May to June, May to July, May to August, and May to September and compared with the mean seedling stem volume index at each period to determine competitive responses at the whole plant scale. Diurnal measures of stomatal conductance were taken each month to compare competitive responses at the leaf scale. In addition, environmental and plant responses that may control stomatal behavior were quantified. The pine water stress integral was strongly influenced by competing vegetation after the onset of a period of drought in early summer. The correlation between the water stress integral and pine growth increased after a significant drying period, accounting for more than half of the variation in stem volume index at the end of the first growing season. Stomatal conductance was also influenced by competition, with competitive effects more evident during times of drought. Conductance was most often related to bulk leaf water potential, which in turn was related to competitive effects on soil moisture availability. Vapor pressure deficit also influenced stomatal conductance, but this was largely unrelated to competitive effects.

A site-preparation study installed in 1959 in Fayette County, Alabama, U.S.A., provides data to evaluate long-term effects of varying densities of hardwood on loblolly pine (Pinustaeda L.) plantation growth, yield, and stand structure. Six treatments (hand girdle; bulldoze scarification; herbicide applied to axe frill, chain frill, and with an injector; and an untreated check) were installed as a randomized complete block with five replications. Periodic measurements of pine and hardwood size and density show that higher hardwood densities existing early in the pine plantation had a substantial negative effect on loblolly pine survival and basal area yield, with the bulldoze and herbicide treatments having less hardwood and higher survival and stand basal area. Pine total height and diameter at breast height were reduced by increasing hardwood density early in the life of the plantation, but size of surviving trees differed little among treatments at later ages, except on plots where most pine trees were suppressed by hardwoods. Strong relationships between pine basal area per hectare at age 27 and both number of hardwood stems at age 3 and percentage of stand basal area in hardwood at age 6 were noted. These relationships indicate promise for predicting long-term growth and yield of loblolly pine plantations from early measures of hardwood interference.

Three common associates on secondary-successional pine sites (Andropogon virginicus, Liquidambar styraciflua, and Pinus taeda) were established in a field study in which a wide array of plant densities and species proportions were established using an additive series design. To mimic a specific competitive scenario (i.e., a managed earlysuccessional Pinus stand), Andropogon and Liquidambar were established a year prior to the establishment of Pinus. Competitive effect (the attenuation of resources) and competitive response (the growth of each species as a function of resource availability) were determined.Effect on soil water varied among species, depth of soil, and time. In the surface soil, soil water was largely influenced through non-uptake effects, while uptake effects were predominant in deeper portions of the solum. When competitor abundance was expressed on an aboveground biomass basis, rather than a density basis, species differences in effects on soil water were eliminated. Differences among the species in effects on soil water per unit leaf area or leaf biomass appear to be largely explained by differences in stomatal conductance. Predawn leaf-water potential was integrated over the season using a waterstress integral. Analysis of the water-stress integral suggested that Liquidambar and Andropogon both affected water available to Pinus; however, only Liquidambar affected Andropogon, and only Andropogon affected seasonal water available to Liquidambar. Light was most strongly influenced by Liquidambar density; however, as Andropogon density increased, the effects of Liquidambar were reduced.Andropogon response was correlated with light but not with water stress or leaf nitrogen. This reflects high light requirements and high water use efficiency of C 4 plants. Liquidambar response was related to water stress and leaf nitrogen, perhaps reflecting the greater nitrogen requirements of hardwoods. Pinus response was significantly related to all three resources individually, i.e., water stress, light, and leaf nitrogen. Pinus response was better explained by a regression model that included light and water stress than by water stress or light alone. Pinus growth as a function of water stress and light indicated that communities dominated by Liquidambar largely reduced Pinus growth through reduction in light, while communities dominated by Andropogon reduced Pinus growth primarily by increasing water stress. In mixed communities of Liquidambar and Andropogon, pine growth was constrained more equally by light and water stress.