396 research outputs found
Estimating carbon storage in windbreak trees on U.S. agricultural lands
Assessing carbon (C) capture and storage potential by the agroforestry practice of windbreaks has been limited. This is due, in part, to a lack of suitable data and associated models for estimating tree biomass and C for species growing under more opengrown conditions such as windbreaks in the Central Plains region of the United States (U.S.). We evaluated 15 allometric models using destructively sampled Pinus ponderosa (Lawson & C. Lawson) data from field windbreaks in Nebraska and Montana. Several goodness-of-fit metrics were used to select the optimal model. The Jenkins’ et al. model was then used to estimate biomass for 16 tree species in windbreaks projected over a 50 year time horizon in nine continental U.S. regions. Carbon storage potential in the windbreak scenarios ranged from 1.07 ± 0.21 to 3.84 ± 0.04 Mg C ha-1 year-1 for conifer species and from 0.99 ± 0.16 to 13.6 ± 7.72 Mg C ha-1 year-1 for broadleaved deciduous species during the 50 year period. Estimated mean C storage potentials across species and regions were 2.45 ± 0.42 and 4.39 ± 1.74 Mg C ha-1 year-1 for conifer and broadleaved deciduous species, respectively. Such information enhances our capacity to better predict the C sequestration potential of windbreaks associated with whole farm/ranch operations in the U.S
Estimating carbon storage in windbreak trees on U.S. agricultural lands
Assessing carbon (C) capture and storage potential by the agroforestry practice of windbreaks has been limited. This is due, in part, to a lack of suitable data and associated models for estimating tree biomass and C for species growing under more opengrown conditions such as windbreaks in the Central Plains region of the United States (U.S.). We evaluated 15 allometric models using destructively sampled Pinus ponderosa (Lawson & C. Lawson) data from field windbreaks in Nebraska and Montana. Several goodness-of-fit metrics were used to select the optimal model. The Jenkins’ et al. model was then used to estimate biomass for 16 tree species in windbreaks projected over a 50 year time horizon in nine continental U.S. regions. Carbon storage potential in the windbreak scenarios ranged from 1.07 ± 0.21 to 3.84 ± 0.04 Mg C ha-1 year-1 for conifer species and from 0.99 ± 0.16 to 13.6 ± 7.72 Mg C ha-1 year-1 for broadleaved deciduous species during the 50 year period. Estimated mean C storage potentials across species and regions were 2.45 ± 0.42 and 4.39 ± 1.74 Mg C ha-1 year-1 for conifer and broadleaved deciduous species, respectively. Such information enhances our capacity to better predict the C sequestration potential of windbreaks associated with whole farm/ranch operations in the U.S
Intermanifold similarities in partial photoionization cross sections of helium
Using the eigenchannel R-matrix method we calculate partial photoionization
cross sections from the ground state of the helium atom for incident photon
energies up to the N=9 manifold. The wide energy range covered by our
calculations permits a thorough investigation of general patterns in the cross
sections which were first discussed by Menzel and co-workers [Phys. Rev. A {\bf
54}, 2080 (1996)]. The existence of these patterns can easily be understood in
terms of propensity rules for autoionization. As the photon energy is increased
the regular patterns are locally interrupted by perturber states until they
fade out indicating the progressive break-down of the propensity rules and the
underlying approximate quantum numbers. We demonstrate that the destructive
influence of isolated perturbers can be compensated with an energy-dependent
quantum defect.Comment: 10 pages, 10 figures, replacement with some typos correcte
Mirror Position Determination for the Alignment of Cherenkov Telescopes
Imaging Atmospheric Cherenkov Telescopes (IACTs) need imaging optics with
large apertures to map the faint Cherenkov light emitted in extensive air
showers onto their image sensors. Segmented reflectors fulfill these needs
using mass produced and light weight mirror facets. However, as the overall
image is the sum of the individual mirror facet images, alignment is important.
Here we present a method to determine the mirror facet positions on a segmented
reflector in a very direct way. Our method reconstructs the mirror facet
positions from photographs and a laser distance meter measurement which goes
from the center of the image sensor plane to the center of each mirror facet.
We use our method to both align the mirror facet positions and to feed the
measured positions into our IACT simulation. We demonstrate our implementation
on the 4 m First Geiger-mode Avalanche Cherenkov Telescope (FACT).Comment: 11 figures, small ray tracing performance simulation, and
implementation demonstratio
Resonance structure in the Li^- photodetachment cross section
We report on the first observation of resonance structure in the total cross
section for the photodetachment of Li^-. The structure arises from the
autodetaching decay of doubly excited ^1P states of Li^- that are bound with
respect to the 3p state of the Li atom. Calculations have been performed for
both Li^- and H^- to assist in the identification of these resonances. The
lowest lying resonance is a symmetrically excited intrashell resonance. Higher
lying asymmetrically excited intershell states are observed which converge on
the Li(3p) limit.Comment: 4 pages, 2 figure, 19 references, RevTeX, figures in ep
Electronic and structural properties of vacancies on and below the GaP(110) surface
We have performed total-energy density-functional calculations using
first-principles pseudopotentials to determine the atomic and electronic
structure of neutral surface and subsurface vacancies at the GaP(110) surface.
The cation as well as the anion surface vacancy show a pronounced inward
relaxation of the three nearest neighbor atoms towards the vacancy while the
surface point-group symmetry is maintained. For both types of vacancies we find
a singly occupied level at mid gap. Subsurface vacancies below the second layer
display essentially the same properties as bulk defects. Our results for
vacancies in the second layer show features not observed for either surface or
bulk vacancies: Large relaxations occur and both defects are unstable against
the formation of antisite vacancy complexes. Simulating scanning tunneling
microscope pictures of the different vacancies we find excellent agreement with
experimental data for the surface vacancies and predict the signatures of
subsurface vacancies.Comment: 10 pages, 6 figures, Submitted to Phys. Rev. B, Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
Scanning tunneling microscopy and spectroscopy at low temperatures of the (110) surface of Te doped GaAs single crystals
We have performed voltage dependent imaging and spatially resolved
spectroscopy on the (110) surface of Te doped GaAs single crystals with a low
temperature scanning tunneling microscope (STM). A large fraction of the
observed defects are identified as Te dopant atoms which can be observed down
to the fifth subsurface layer. For negative sample voltages, the dopant atoms
are surrounded by Friedel charge density oscillations. Spatially resolved
spectroscopy above the dopant atoms and above defect free areas of the GaAs
(110) surface reveals the presence of conductance peaks inside the
semiconductor band gap. The appearance of the peaks can be linked to charges
residing on states which are localized within the tunnel junction area. We show
that these localized states can be present on the doped GaAs surface as well as
at the STM tip apex.Comment: 8 pages, 8 figures, accepted for publication in PR
FACT -- the First Cherenkov Telescope using a G-APD Camera for TeV Gamma-ray Astronomy (HEAD 2010)
Geiger-mode Avalanche Photodiodes (G-APD) bear the potential to significantly
improve the sensitivity of Imaging Air Cherenkov Telescopes (IACT). We are
currently building the First G-APD Cherenkov Telescope (FACT) by refurbishing
an old IACT with a mirror area of 9.5 square meters and construct a new, fine
pixelized camera using novel G-APDs. The main goal is to evaluate the
performance of a complete system by observing very high energy gamma-rays from
the Crab Nebula. This is an important field test to check the feasibility of
G-APD-based cameras to replace at some time the PMT-based cameras of planned
future IACTs like AGIS and CTA. In this article, we present the basic design of
such a camera as well as some important details to be taken into account.Comment: Poster shown at HEAD 2010, Big Island, Hawaii, March 1-4, 201
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