Browsing by Author "McGuire, MA"

Now showing 1 - 2 of 2
  • Thumbnail Image
    ItemOpen Access
    A range-wide experiment to investigate nutrient and soil moisture interactions in loblolly pine plantations
    (Forests, 2015-01-01) Will, RE; Fox, T; Akers, M; Domec, JC; González-Benecke, C; Jokela, EJ; Kane, M; Laviner, MA; Lokuta, G; Markewitz, D; McGuire, MA; Meek, C; Noormets, A; Samuelson, L; Seiler, J; Strahm, B; Teskey, R; Vogel, J; Ward, E; West, J; Wilson, D; Martin, TA
    © 2015 by the authors.The future climate of the southeastern USA is predicted to be warmer, drier and more variable in rainfall, which may increase drought frequency and intensity. Loblolly pine (Pinus taeda) is the most important commercial tree species in the world and is planted on ~11 million ha within its native range in the southeastern USA. A regional study was installed to evaluate effects of decreased rainfall and nutrient additions on loblolly pine plantation productivity and physiology. Four locations were established to capture the range-wide variability of soil and climate. Treatments were initiated in 2012 and consisted of a factorial combination of throughfall reduction (approximate 30% reduction) and fertilization (complete suite of nutrients). Tree and stand growth were measured at each site. Results after two growing seasons indicate a positive but variable response of fertilization on stand volume increment at all four sites and a negative effect of throughfall reduction at two sites. Data will be used to produce robust process model parameterizations useful for simulating loblolly pine growth and function under future, novel climate and management scenarios. The resulting improved models will provide support for developing management strategies to increase pine plantation productivity and carbon sequestration under a changing climate.
  • Thumbnail Image
    ItemOpen Access
    Thermoelectric transport properties of CaMg 2Bi 2, EuMg 2Bi 2, and YbMg 2Bi 2
    (Physical Review B - Condensed Matter and Materials Physics, 2012-01-11) May, AF; McGuire, MA; Singh, DJ; Ma, J; Delaire, O; Huq, A; Cai, W; Wang, H
    The thermoelectric transport properties of CaMg 2Bi 2, EuMg 2Bi 2, and YbMg 2Bi 2 were characterized between 2 and 650 K. As synthesized, the polycrystalline samples are found to have lower p-type carrier concentrations than single-crystalline samples of the same empirical formula. These low carrier concentration samples possess the highest mobilities yet reported for materials with the CaAl 2Si 2 structure type, with a mobility of ∼740 cm2/V/s observed in EuMg 2Bi 2 at 50 K. Despite decreases in the Seebeck coefficient (α) and electrical resistivity (ρ) with increasing temperature, the power factor (α2ρ) increases for all temperatures examined. This behavior suggests a strong asymmetry in the conduction of electrons and holes. The highest figure of merit (zT) is observed in YbMg 2Bi 2, with zT approaching 0.4 at 600 K for two samples with carrier densities of approximately 2×1018cm -3 and 8×1018 cm -3 at room temperature. Refinements of neutron powder diffraction data yield similar behavior for the structures of CaMg 2Bi 2 and YbMg 2Bi 2, with smooth lattice expansion and relative expansion in c being ∼35% larger than relative expansion in a at 973 K. First-principles calculations reveal an increasing band gap as Bi is replaced by Sb and then As, and subsequent Boltzmann transport calculations predict an increase in α for a given n associated with an increased effective mass as the gap opens. The magnitude and temperature dependence of α suggests higher zT is likely to be achieved at larger carrier concentrations, roughly an order of magnitude higher than those in the current polycrystalline samples, which is also expected from the detailed calculations. © 2012 American Physical Society.