THE ROLE OF VEGETATION DYNAMICS IN CHOCTAWHATCHEE SAND PINE INVASION OF LONGLEAF PINE FORESTS

The expansion of Choctawhatchee sand pine (Pinus clausa) into longleaf pine forests (Pinus palustris) has been widespread on the Florida Panhandle since the early 1900s. This study examines the population-level processes of this sand pine invasion. Expansion of sand pine started by 1920, but rapidly increased in the 1940s. Population establishment and growth in invasive stands has been rapid, with establishment of many sand pine pioneer individuals followed by rapid infilling as these individuals reached reproductive maturity. Both noninvasive and invasive stands had numerous sand pine seedlings and saplings. Sand pine will likely persist in these communities in the near future. Analyses of hemispherical canopy photographs and regeneration patterns illustrate the ability of sand pine to establish under a variety of light conditions. Radial growth analysis of incremental tree cores highlights the importance of episodic wind disturbances in maintaining these sand pine populations. [Key words: sand pine, longleaf pine, Pinus clausa, Florida, pine invasion, Pinus palustris.]     

Gravitational and magnetic fields of some simple solids of revolution

Summary. Exact spherical harmonic expansions are given for calculating the gravitational and magnetic fields associated with certain uniform solids of revolution. The figures are those made by rotating a conic section about one of its principal axes. The coefficients in the expansions can be computed accurately and efficiently and this approach leads to a very satisfactory method for calculating the fields of geological bodies with approximate circular symmetry about a vertical axis. A complete theory of convergence is given for the expansions. Somewhat unexpectedly, the sphere of convergence is determined by the location of a number of equivalent point or line sources that lie within the body or on its edges.     

Computer simulations of the structural and physical properties of the olivine and spinel polymorphs of Mg2SiO4

The aim of the work presented is to develop a computer simulation technique which will predict the structure and physical properties of forsterite and ringwoodite, the major mantle-forming polymorphs of Mg₂SiO₄. The technique is based upon energy minimization, in which all structural parameters are varied until the configuration with the lowest energy is achieved. The lattice energy and physical properties (e.g. elasticity and dielectric constants) are calculated from interatomic potentials, which generally include electrostatic and short-range terms. We investigate several types of traditional potential models, and present a new type of model which includes partial ionic charges and a Morse potential to describe the effect of covalency on the Si-O bond. This new form of potential model is highly successful, and not only reproduces the zero-pressure structural, elastic and dielectric properties of forsterite and ringwoodite, but also accurately describes their pressure dependence.     

Sulphur uptake by salt grasses,mangroves, and seagrasses in anaerobic sediments

Stable sulfur isotope analyses show that rooted estuarine plants growing in anoxic sediments incorporate substantial amounts of ³⁴S-depleted sulfides, or oxidation products thereof. In roots, this incorporation predominates over sulfate uptake from interstitial porewater. Either the plants incorporate normally toxic sulfides, or they are creating and using a specialized nutrient pool of oxidized sulfides at the root-sediment interface.