21,363 research outputs found
Fire effects on gambel oak in southwestern ponderosa pine-oak forests
Gambel oak (Quercus gambelii) is ecologically and aesthetically valuable in southwestern ponderosa pine (Pinus ponderosa) forests. Fire effects on Gambel oak are important because fire may be used in pine-oak forests to manage oak directly or to accomplish other management objectives. We used published literature to: (1) ascertain historical fire regimes in pine-oak forests, (2) discern prescribed burning effects on Gambel oak survival and diameter growth, and (3) provide suggestions for using fire to manage oak. Frequent fire is part of Gambel oak’s historical environment, as historical fire return intervals often averaged less than 10 years in pine-oak forests. More than 66 percent of oaks greater than 6 inches (15 cm) in diameter were alive at least 5 years after two contemporary prescribed fires, whereas survival was low (percent) for small oaks less than 2 inches (5 cm) in diameter. Top-killed oaks resprout prolifically, suggesting that fire can maintain shrub-sprout forms of oak constituting browse and cover for some wildlife species. Unlike mechanically thinning competing trees, burning has not been found to increase oak diameter growth. We conclude that fire can be used to manage Gambel oak densities and growth forms, and that large oaks can be maintained during low-intensity burning. Several tactics may enhance survival of large oaks during prescribed fire: keeping pine slash away from oak boles, avoiding lighting near oaks or reducing fire intensity near oaks, and raking fuels away from oak boles
Changes in gambel oak densities in southwestern ponderosa pine forests since Euro-American settlement
Densities of small-diameter ponderosa pine (Pinus ponderosa) trees have increased in southwestern ponderosa pine forests during a period of fire exclusion since Euro-American settlement in the late 1800s. However, less well known are potential changes in Gambel oak (Quercus gambelii) densities during this period in these forests. We reviewed published literature to summarize changes in oak density in ponderosa pine forests over the past 140 years and evaluated the possibility that large-diameter oaks have decreased in density. All nine studies examining oak density changes found that densities of small-diameter oaks have escalated. Increases ranged from 4- to more than 63-fold. These increases are comparable on many sites to those of ponderosa pine. Studies in northern Arizona, which analyzed cut stumps and past and present diameter distributions, did not find strong evidence that large-diameter oaks on average have declined in density. However, since oak cutting varied across the landscape, this important question needs additional study. Actively or passively managing Gambel oak requires decisions about desired future conditions and how to attain them. A possible contention for passive management—that the overall abundance of oak has decreased—is not supported by research published to date
Revealing Challenges of Teaching Secrecy
All teaching has something to do with transmission of hidden knowledge, secrecy, and revelation. But the teaching of secrecy itself faces particular challenges. Drawing on the authors’ experiences teaching secrecy-themed seminars to first-year university students, this paper pinpoints four such challenges: how to determine the range of phenomena to cover in a short course, how to prevent excessive interpretation of secrets, how to encourage students to take a fun topic with seriousness, and how to engage students in their own practices of secrecy. In laying out these challenges, we aim to contribute to a secrecy literacy: a needed competency so people can better evaluate efforts to keep secrets and appreciate the need for certain types of secrets while also being able to critique problematic forms. This secrecy literacy can provide a foundation for the skills needed to better manage secrecy in our professional and everyday lives
Anomalous quantum reflection of Bose-Einstein condensates from a silicon surface: the role of dynamical excitations
We investigate the effect of inter-atomic interactions on the
quantum-mechanical reflection of Bose-Einstein condensates from regions of
rapid potential variation. The reflection process depends critically on the
density and incident velocity of the condensate. For low densities and high
velocities, the atom cloud has almost the same form before and after
reflection. Conversely, at high densities and low velocities, the reflection
process generates solitons and vortex rings that fragment the condensate. We
show that this fragmentation can explain the anomalously low reflection
probabilities recently measured for low-velocity condensates incident on a
silicon surface.Comment: 5 figures, 5 pages, references correcte
Birationality of \'etale morphisms via surgery
We use a counting argument and surgery theory to show that if is a
sufficiently general algebraic hypersurface in , then any local
diffeomorphism of simply connected manifolds which is a
-sheeted cover away from has degree or (however all
degrees are possible if fails to be a local diffeomorphism at even
a single point). In particular, any \'etale morphism of
algebraic varieties which covers away from such a hypersurface must be
birational.Comment: 17 pages. Replaced to add further references and make language more
consistent with the literatur
Recovery From Monocular Deprivation Using Binocular Deprivation: Experimental Observations and Theoretical Analysis
Ocular dominance (OD) plasticity is a robust paradigm for examining the functional consequences of synaptic plasticity. Previous experimental and theoretical results have shown that OD plasticity can be accounted for by known synaptic plasticity mechanisms, using the assumption that deprivation by lid suture eliminates spatial structure in the deprived channel. Here we show that in the mouse, recovery from monocular lid suture can be obtained by subsequent binocular lid suture but not by dark rearing. This poses a significant challenge to previous theoretical results. We therefore performed simulations with a natural input environment appropriate for mouse visual cortex. In contrast to previous work we assume that lid suture causes degradation but not elimination of spatial structure, whereas dark rearing produces elimination of spatial structure. We present experimental evidence that supports this assumption, measuring responses through sutured lids in the mouse. The change in assumptions about the input environment is sufficient to account for new experimental observations, while still accounting for previous experimental results
Management of localized energy in discrete nonlinear transmission lines
The manipulation of locked intrinsic localized modes/discrete breathers is
studied experimentally in nonlinear electric transmission line arrays.
Introducing a static lattice impurity in the form of a capacitor, resistor or
inductor has been used both to seed or destroy and attract or repel these
localized excitations. In a nonlinear di-element array counter propagating
short electrical pulses traveling in the acoustic branch are used to generate a
stationary intrinsic localized mode in the optic branch at any particular
lattice site. By changing the pulse polarity the same localized excitation can
be eliminated demonstrating that the dynamical impurity associated with the
propagating electrical pulse in the acoustic branch can trigger optical
localized mode behavior.Comment: submitte
A Co-moving Coordinate System for Relativistic Hydrodynamics
The equations of relativistic hydrodynamics are transformed so that steps
forward in time preserves local simultaneity. In these variables, the
space-time coordinates of neighboring points on the mesh are simultaneous
according to co-moving observers. Aside from the time step varying as a
function of the location on the mesh, the local velocity gradient and the local
density then evolve according to non-relativistic equations of motion. Analytic
solutions are found for two one-dimensional cases with constant speed of sound.
One solution has a Gaussian density profile when mapped into the new
coordinates. That solution is analyzed for the effects of longitudinal
acceleration in relativistic heavy ion collisions at RHIC, especially in
regards to two-particle correlation measurements of the longitudinal size
Observation of genuine three-photon interference
Multiparticle quantum interference is critical for our understanding and
exploitation of quantum information, and for fundamental tests of quantum
mechanics. A remarkable example of multi-partite correlations is exhibited by
the Greenberger-Horne-Zeilinger (GHZ) state. In a GHZ state, three particles
are correlated while no pairwise correlation is found. The manifestation of
these strong correlations in an interferometric setting has been studied
theoretically since 1990 but no three-photon GHZ interferometer has been
realized experimentally. Here we demonstrate three-photon interference that
does not originate from two-photon or single photon interference. We observe
phase-dependent variation of three-photon coincidences with 90.5 \pm 5.0 %
visibility in a generalized Franson interferometer using energy-time entangled
photon triplets. The demonstration of these strong correlations in an
interferometric setting provides new avenues for multiphoton interferometry,
fundamental tests of quantum mechanics and quantum information applications in
higher dimensions.Comment: 7 pages, 7 figure
- …