9,269 research outputs found
Interactions of arbuscular mycorrhizal fungi, critical loads of nitrogen deposition, and shifts from native to invasive species in a southern California shrubland
Anthropogenic nitrogen (N) deposition and invasive species are causing declines in global biodiversity, and both factors impact the diversity and functioning of arbuscular mycorrhizal (AM) fungi. Shifts in arbuscular mycorrhizal fungal (AMF) communities can generate feedback to native plants and affect their success, as was observed in California’s coastal sage scrub, which is a Mediterranean-type shrubland threatened by invasive grasses. As vegetation-type conversion from native shrubland to exotic annual grassland increased along a gradient of increasing N deposition, the richness of native plant species and of spore morphotypes decreased. Rapid declines in all plant and fungal values occurred at the critical load (CL) of 10–11 kg N·ha−1·year−1, indicating that AM fungi respond to the same environmental signals as the plants, and can be used to assess CL. Shrub root colonization also decreased along the N gradient, but colonization of the invasive grass was dominated by a fine AMF endophyte that was unresponsive to elevated N. A greenhouse experiment to assess AMF functioning showed that the native shrub Artemisia californica Less. had a negative growth response to an inoculum from high-N but not low-N soils, whereas the invasive grass Bromus rubens L. had a positive response to both inocula. Differential functioning of AM fungi under N deposition may in part explain vegetation-type conversion and the decline of this native shrubland
A case study in small-body aquifer testing
Determination of aquifer characteristics for small bodies requires careful evaluation of local geology. Many assumptions inherent in the use of the Theis equation and its variations may be violated. Limited areal extent, leakage, local heterogeneities, and the effects of dewatering may skew results. Results from a variety of analyses will illustrate these effects, and aid in proper evaluation of hydrologic properties. This study undertakes such an analysis. Various methods are used to determine transmissivity and storativity in a channel sand aquifer. These include time-drawdown, distance-drawdown, type curve matching, time recovery, and image well analyses. Values of transmissivity range from 3306 to 15,500 gpd/ft, and storativity ranges from 0.0009 to 0.005. Evaluation of the limits of each method in small-body analysis indicates that actual transmissivity ranges from 10,000 to 15,000 gpd/ft, and storativity is on the order of 10^-4
Knowing Where To Hit It: A Conceptual Framework for the Sustainable Development of the Himalayas
This paper originated in a short exploratory piece of work for the United Nations Environment Programme (UNEP), in the nature of a "strategic reconnaissance" of a complex knot of environmental-social problems. However, early forays indicated strong underlying parallels with issues being explored in other case studies in the Institutional Settings and Environmental Policies (INS) project at IIASA. For this reason, and despite its limited external resources, the Himalaya work written up here took its place as one part of a troika INS project, that also involved research on hazardous waste management and on energy policy modeling.
Though the main effort has been on hazardous waste management, the intellectual connections between these apparently diverse research fields have proved most instructive. These connections have to do with the ways in which uncertainties (both technical and social) are defined by institutions, especially regulatory and policy analytic bodies. The thrust of research on all these fronts has been to better characterise different kinds of uncertainty and to explore the origins and policy implications of conflicting problem definitions. The practical aim in this effort has been to encourage better policy design. An understanding of the institutional roots of uncertainty and of multiple problem definitions opens the way for an anticipatory style of policy formulation that is capable of evaluating the implementability, and thus viability, of different policy options and institutional arrangements. This strategic concern is a direct evolution from the previous work of the Risk Group--the predecessor to INS
Electric field sensing with a scanning fiber-coupled quantum dot
We demonstrate the application of a fiber-coupled quantum-dot-in-a-tip as a
probe for scanning electric field microscopy. We map the out-of-plane component
of the electric field induced by a pair of electrodes by measurement of the
quantum-confined Stark effect induced on a quantum dot spectral line. Our
results are in agreement with finite element simulations of the experiment.
Furthermore, we present results from analytic calculations and simulations
which are relevant to any electric field sensor embedded in a dielectric tip.
In particular, we highlight the impact of the tip geometry on both the
resolution and sensitivity.Comment: 10 pages, 4 figure
Thermal fluctuations in moderately damped Josephson junctions: Multiple escape and retrapping, switching- and return-current distributions and hysteresis
A crossover at a temperature T* in the temperature dependence of the width s
of the distribution of switching currents of moderately damped Josephson
junctions has been reported in a number of recent publications, with positive
ds/dT and IV characteristics associated with underdamped behaviour for lower
temperatures T<T*, and negative ds/dT and IV characteristics resembling
overdamped behaviour for higher temperatures T>T*. We have investigated in
detail the behaviour of Josephson junctions around the temperature T* by using
Monte Carlo simulations including retrapping from the running state into the
supercurrent state as given by the model of Ben-Jacob et al. We develop
discussion of the important role of multiple escape and retrapping events in
the moderate-damping regime, in particular considering the behaviour in the
region close to T*. We show that the behaviour is more fully understood by
considering two crossover temperatures, and that the shape of the distribution
and s(T) around T*, as well as at lower T<T*, are largely determined by the
shape of the conventional thermally activated switching distribution. We show
that the characteristic temperatures T* are not unique for a particular
Josephson junction, but have some dependence on the ramp rate of the applied
bias current. We also consider hysteresis in moderately damped Josephson
junctions and discuss the less commonly measured distribution of return
currents for a decreasing current ramp. We find that some hysteresis should be
expected to persist above T* and we highlight the importance, even well below
T*, of accounting properly for thermal fluctuations when determining the
damping parameter Q.Comment: Accepted for publication in PR
Temporal relations among multidimensional perceptions of competence and trichotomous achievement goal adoption in physical education
This article was published in the journal Psychology of Sport and Exercise [© Elsevier Ltd]. The definitive version is available at: http://dx.doi.org/10.1016/j.psychsport.2011.04.007Objectives: The purpose of the present study was two-fold: (1) To empirically establish whether young
people differentiate their perceived competence in physical education (PE) in terms of the self, mastery
of tasks, and others, and (2) To examine longitudinal relations between these three ways of defining
perceived competence and trichotomous achievement goals.
Methods: At the start of the study, students (n = 227 males, n = 205 females; M age = 13.18, SD =.87 years)
completed measures of mastery-approach, performance-approach- and performance-avoidance goals,
along with other-, self- and mastery-referent forms of perceived competence. The same measures were
subsequently recorded three, six and nine months later.
Results: Analyses supported longitudinal factorial invariance for each goal and each type of perceived
competence. Partial support was found for the positive influence of other-referent perceived competence
on approach- and avoidance-performance goal adoption over time.
Conclusion: Young people can construe their competence in PE in various ways. Relative to one’s classmates,
increases in other-referenced perceptions of competence can subsequently lead to increased
adoption of both performance goals
New calculations of the PNC Matrix Element for the 0 doublet in N
A new calculation of the predominantly isoscalar PNC matrix element between
the (E 8.7 MeV) states in N has
been carried out in a (0+1+2+3+4) model space with the
Warburton-Brown interaction. The magnitude of the PNC matrix element of 0.22 to
0.34 eV obtained with the DDH PNC interaction is substantially suppressed
compared with previous calculations in smaller model spaces but shows agreement
with the preliminary Seattle experimental data. The calculated sign is opposite
to that obtained experimentally, and the implications of this are discussed.Comment: REVTEX, 28 page
Quantum dot opto-mechanics in a fully self-assembled nanowire
We show that fully self-assembled optically-active quantum dots (QDs)
embedded in MBE-grown GaAs/AlGaAs core-shell nanowires (NWs) are coupled to the
NW mechanical motion. Oscillations of the NW modulate the QD emission energy in
a broad range exceeding 14 meV. Furthermore, this opto-mechanical interaction
enables the dynamical tuning of two neighboring QDs into resonance, possibly
allowing for emitter-emitter coupling. Both the QDs and the coupling mechanism
-- material strain -- are intrinsic to the NW structure and do not depend on
any functionalization or external field. Such systems open up the prospect of
using QDs to probe and control the mechanical state of a NW, or conversely of
making a quantum non-demolition readout of a QD state through a position
measurement.Comment: 20 pages, 6 figure
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