Glenn R. Glover

COMPETITION AMONG SECONDARY‐SUCCESSIONAL PINE COMMUNITIES: A FIELD STUDY OF EFFECTS AND RESPONSES

Three common associates on secondary-successional pine sites (Andropogon virginicus, Liquidambar styracifiua, 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 early- successional 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 water- stress integral. Analysis of the water-stress integral suggested that Liquidambar and An- dropogon 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 C4 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.

Vegetation response to intensity of herbaceous weed control in a newly planted loblolly pine plantation

Growth of loblolly pine (Pinus taeda L.) seedlings through three growing seasons after planting increased with intensity of herbaceous weed control using herbicides. Weed control had no effect on pine survival. Two years of complete herbaceous weed control (CHC, control throughout the first two growing seasons after planting) and ‘operational’ herbaceous weed control (OHC, sulfometuron at 0.42 kg ai/ha at the beginning of the first growing season), resulted in lower biomass of weeds plus trees than with no herbaceous weed control (NHC) during the first growing season. Differences in total biomass during the first year were due to differences in biomass of herbaceous weeds. Total biomass on CHC and OHC plots was at least as great as NHC the second year, and greater by the third year, as pines assumed dominance as a result of increased growth from reduction of herbaceous weeds. The operational herbicide treatment had no significant impact on overall herbaceous weed biomass and cover, and little effect on species composition compared to no herbaceous weed control two and three growing seasons after treatment. The CHC treatment significantly reduced herbaceous weed biomass, cover and composition through three growing seasons.

A simulation study of hardwood rootstock populations in young loblolly pine plantations

A computer program to simulate spatial distribution of hard wood rootstock populations is presented. Nineteen 3 to 6 year- old loblolly pine (Pinus taeda L.) plantations in Alabama and Georgia were measured to provide information for the simula tor. Spatial pattern, expressed as Pielous nonrandomness index (PNI), ranged from 0.47 to 2.45. Scatterplots illustrated no rela tionship between pattern, species relative density, site prepara tion, or stand age. Newnhams point pattern generator was modified to reduce execution time. Equations to predict program inputs as a func tion of PNI and desired number of points were developed for uniform, random, and randomly clumped populations. Total rootstock height distribution was fitted using a two-parameter, left- and right-truncated Weibull function. Crown area was de termined conditionally by total rootstock height. The simulator may be used to generate populations for evaluating different sampling methods for hardwood rootstock attributes or in in dividual tree growth and yield models.