120 research outputs found
Forest dynamics following spruce budworm outbreaks in the northern and southern mixedwoods of central Quebec
The effects of 20th century spruce budworm (Choristoneura fumiferana (Clem.)) outbreaks on forest dynamics was examined in the southern and northern parts of the mixedwood forest zone in central Quebec, Canada. In each region, three study areas were placed in unmanaged stands that had not burned for more than 200 years. Disturbance impacts and forest succession were evaluated using aerial photographs and dendrochronology. Spruce budworm outbreaks occurred around 1910, 1950, and 1980 in both regions. The 1910 outbreak seemed to have limited impact in both regions, and the 1950 outbreak caused heavy mortality in conifer stands (mostly of balsam fir, Abies balsamea (L.) Mill.) in the southern region. The 1980 outbreak caused major mortality in the northern region, but had little impact in the southern region. Successive spruce budworm outbreaks led to a massive invasion by hardwood species in the last century in the southern region but not in the northern region. The reason for such contrasting dynamics between regions is unknown, but we hypothesize that differences in disturbance intensities, influenced by climate, played a major role. Results from this study emphasize that generalizations about the effect of spruce budworm outbreaks on forest dynamics cannot be derived from observations made during a single outbreak or at a single location
Magnetic Ordering, Orbital Ordering and Resonant X-ray Scattering in Perovskite Titanates
The effective Hamiltonian for perovskite titanates is derived by taking into
account the three-fold degeneracy of orbitals and the strong
electron-electron interactions. The magnetic and orbital ordered phases are
studied in the mean-field approximation applied to the effective Hamiltonian. A
large degeneracy of the orbital states in the ferromagnetic phase is found in
contrast to the case of the doubly degenerate orbitals. Lifting of this
orbital degeneracy due to lattice distortions and spin-orbit coupling is
examined. A general form for the scattering cross section of the resonant x-ray
scattering is derived and is applied to the recent experimental results in
YTiO. The spin wave dispersion relation in the orbital ordered YTiO is
also studied.Comment: 10 pages, 6 figure
Environment and Rural Affairs Monitoring & Modelling Programme - ERAMMP Year 1 Report 24: Welsh national natural capital accounts - Ecosystem service accounts for woodland, farmland and freshwater habitats
This report presents 7 service accounts, containing estimates of the quantity and value of services being supplied by Welsh natural capital in woodland, farmland, and
freshwater broad habitats. These services include food, water, air filtration and recreation.
Farmland is the largest of these three Welsh habitats, at 1,015,693 hectares, followed by woodland at 268,588 hectares (125,117 broadleaf and 143,471 coniferous), and
freshwater at 42,309 hectares.
The value of the stock of Welsh natural capital in woodland, farmland, and freshwater was estimated to be approximately £30.5 billion in 2014. This is a partial value and the
true value is expected to be significantly higher than this figure as only 7 of the benefits received from natural capital in Wales are currently measured. Some notable
examples of ecosystem services which aren’t currently measured are flood protection, hydropower, and tourism. Of the services measured, 76% of this value was
attributable to intangible services not traditionally captured in GDP (recreation, pollution removal and carbon sequestration).
This construction of accounts for Welsh habitats is necessary for potential future commissioning of work to exploit the many condition indicators relevant to provisioning and regulating services which come from the ongoing national monitoring programme
Welsh Government has funded through the GMEP and ERAMMP programmes. These condition metrics are particularly important for assessing the underpinning resilience of
ecosystems to sudden shocks. Examples include change in soil condition - increases in soil pH over time; variable trends in soil carbon - with increases in woodland and
losses in Mountain, Moor and Heath habitats; and condition indicators relevant to cultural services (recreation) which show improvement in the condition of footpaths and public rights of way over time. Other condition metrics include indicators of diversity and connectivity.
The accounts reported here reflect a Welsh-data focus for three habitats. A broader set of ecosystem service accounts would benefit from inclusion of other habitats such
as mountain moors and heaths, and urban and coastal accounts.
The potential to link to environmental monitoring, such as the new indicators developed under the Environmental and Rural Affairs Monitoring and Modelling
Programme (ERAMMP) and National Survey questions should also be explored further. This will particularly benefit measures of habitat condition, which are not typically represented in the reporting of many natural capital accounts.
With resource rent residual value methods in valuing many ecosystem service flows, geographical breakdowns of Natural Capital accounts, such as Welsh ecosystem service accounts, currently rely upon apportioning UK national accounts. Further development work should take place to investigate other possible valuation methodologies outlined in the System of Environmental Economic Accounting (SEEA)
Experimental Ecosystem Accountin
Crystal and magnetic structure of LaTiO3 : evidence for non-degenerate -orbitals
The crystal and magnetic structure of LaTiO3 ~ has been studied by x-ray and
neutron diffraction techniques using nearly stoichiometric samples. We find a
strong structural anomaly near the antiferromagnetic ordering, T=146 K. In
addition, the octahedra in LaTiO3 exhibit an intrinsic distortion which implies
a splitting of the t2g-levels. Our results indicate that LaTiO3 should be
considered as a Jahn-Teller system where the structural distortion and the
resulting level splitting are enhanced by the magnetic ordering.Comment: 4 pages 5 figure
Hidden Symmetries and their Consequences in Cubic Perovskites
The five-band Hubbard model for a band with one electron per site is a
model which has very interesting properties when the relevant ions are located
at sites with high (e. g. cubic) symmetry. In that case, if the crystal field
splitting is large one may consider excitations confined to the lowest
threefold degenerate orbital states. When the electron hopping matrix
element () is much smaller than the on-site Coulomb interaction energy
(), the Hubbard model can be mapped onto the well-known effective
Hamiltonian (at order ) derived by Kugel and Khomskii (KK). Recently
we have shown that the KK Hamiltonian does not support long range spin order at
any nonzero temperature due to several novel hidden symmetries that it
possesses. Here we extend our theory to show that these symmetries also apply
to the underlying three-band Hubbard model. Using these symmetries we develop a
rigorous Mermin-Wagner construction, which shows that the three-band Hubbard
model does not support spontaneous long-range spin order at any nonzero
temperature and at any order in -- despite the three-dimensional lattice
structure. Introduction of spin-orbit coupling does allow spin ordering, but
even then the excitation spectrum is gapless due to a subtle continuous
symmetry. Finally we showed that these hidden symmetries dramatically simplify
the numerical exact diagonalization studies of finite clusters.Comment: 26 pages, 3 figures, 520 KB, submitted Phys. Rev.
Landau Expansion for the Kugel-Khomskii Hamiltonian
The Kugel-Khomskii (KK) Hamiltonian for the titanates describes spin and
orbital superexchange interactions between ions in an ideal perovskite
structure in which the three orbitals are degenerate in energy and
electron hopping is constrained by cubic site symmetry. In this paper we
implement a variational approach to mean-field theory in which each site, ,
has its own single-site density matrix \rhov(i), where , the
number of allowed single-particle states, is 6 (3 orbital times 2 spin states).
The variational free energy from this 35 parameter density matrix is shown to
exhibit the unusual symmetries noted previously which lead to a
wavevector-dependent susceptibility for spins in orbitals which is
dispersionless in the -direction. Thus, for the cubic KK model
itself, mean-field theory does not provide wavevector `selection', in agreement
with rigorous symmetry arguments. We consider the effect of including various
perturbations. When spin-orbit interactions are introduced, the susceptibility
has dispersion in all directions in -space, but the resulting
antiferromagnetic mean-field state is degenerate with respect to global
rotation of the staggered spin, implying that the spin-wave spectrum is
gapless. This possibly surprising conclusion is also consistent with rigorous
symmetry arguments. When next-nearest-neighbor hopping is included, staggered
moments of all orbitals appear, but the sum of these moments is zero, yielding
an exotic state with long-range order without long-range spin order. The effect
of a Hund's rule coupling of sufficient strength is to produce a state with
orbital order.Comment: 20 pages, 5 figures, submitted to Phys. Rev. B (2003
Orbital Wave and its Observation in Orbital Ordered Titanates and Vanadates
We present a theory of the collective orbital excitation termed orbital wave
in perovskite titanates and vanadates with the triply degenerate
orbitals. The dispersion relations of the orbital waves for the orbital ordered
LaVO, YVO and YTiO are examined in the effective spin-orbital
coupled Hamiltonians associated with the Jahn-Teller type couplings. We propose
possible scattering processes for the Raman and inelastic neutron scatterings
from the orbital wave and calculate the scattering spectra for titanates and
vanadates. It is found that both the excitation spectra and the observation
methods of the orbital wave are distinct qualitatively from those for the
orbital ordered systems.Comment: 9 pages, 7 figure
Haptography: Capturing and Recreating the Rich Feel of Real Surfaces
Haptic interfaces, which allow a user to touch virtual and remote environments through a hand-held tool, have opened up exciting new possibilities for applications such as computer-aided design and robot-assisted surgery. Unfortunately, the haptic renderings produced by these systems seldom feel like authentic re-creations of the richly varied surfaces one encounters in the real world. We have thus envisioned the new approach of haptography, or haptic photography, in which an individual quickly records a physical interaction with a real surface and then recreates that experience for a user at a different time and/or place. This paper presents an overview of the goals and methods of haptography, emphasizing the importance of accurately capturing and recreating the high frequency accelerations that occur during tool-mediated interactions. In the capturing domain, we introduce a new texture modeling and synthesis method based on linear prediction applied to acceleration signals recorded from real tool interactions. For recreating, we show a new haptography handle prototype that enables the user of a Phantom Omni to feel fine surface features and textures
Mountain maple and balsam fir early response to partial and clear-cut harvesting under aspen stands of northern Quebec
This study is a component of the Sylviculture et am
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