332 research outputs found
A Dynamic Single E-Beam Short/Open Testing Technique
Several electron beam techniques for electrical testing of interconnection modules have been presented by different authors in recent years. Most techniques use two or more electron beam energies to establish a charging and a non-loading reading mode. The present paper discusses the feasibility of employing the same beam energy for charging contact pads and reading pad potentials. This avoids the necessity of high voltage switching as used for altering the beam energy. A switching time of 100 us between 2 kV and 4 kV beam voltage which is restricted to this range has been reported earlier. Without switching, higher beam energies may be used with smaller transition times between charging and reading of the test pads
Aharonov-Bohm interference in the presence of metallic mesoscopic cylinders
This work studies the interference of electrons in the presence of a line of
magnetic flux surrounded by a normal-conducting mesoscopic cylinder at low
temperature. It is found that, while there is a supplementary phase
contribution from each electron of the mesoscopic cylinder, the sum of these
individual supplementary phases is equal to zero, so that the presence of a
normal-conducting mesoscopic ring at low temperature does not change the
Aharonov-Bohm interference pattern of the incident electron. It is shown that
it is not possible to ascertain by experimental observation that the shielding
electrons have responded to the field of an incident electron, and at the same
time to preserve the interference pattern of the incident electron. It is also
shown that the measuring of the transient magnetic field in the region between
the two paths of an electron interference experiment with an accuracy at least
equal to the magnetic field of the incident electron generates a phase
uncertainty which destroys the interference pattern.Comment: 15 pages, 5 Postscript figure
Evidence for anisotropic final state interactions in the two-photon ionization of rare gases
Synopsis Anisotropic final state interactions are exhibited most clearly in the angular momentum dependent position of Cooper minima in the photoionization partial cross section and angular distribution asymmetry parameter. We show first indication of this effect in two-photon ionization of rare gases
Tree migration-rates : narrowing the gap between inferred post-glacial rates and projected rates
Faster-than-expected post-glacial migration rates of trees have puzzled ecologists for a long time. In Europe, post-glacial migration is assumed to have started from the three southern European peninsulas (southern refugia), where large areas remained free of permafrost and ice at the peak of the last glaciation. However, increasing palaeobotanical evidence for the presence of isolated tree populations in more northerly microrefugia has started to change this perception. Here we use the Northern Eurasian Plant Macrofossil Database and palaeoecological literature to show that post-glacial migration rates for trees may have been substantially lower (60–260 m yr–1) than those estimated by assuming migration from southern refugia only (115–550 m yr–1), and that early-successional trees migrated faster than mid- and late-successional trees. Post-glacial migration rates are in good agreement with those recently projected for the future with a population dynamical forest succession and dispersal model, mainly for early-successional trees and under optimal conditions. Although migration estimates presented here may be conservative because of our assumption of uniform dispersal, tree migration-rates clearly need reconsideration. We suggest that small outlier populations may be a key factor in understanding past migration rates and in predicting potential future range-shifts. The importance of outlier populations in the past may have an analogy in the future, as many tree species have been planted beyond their natural ranges, with a more beneficial microclimate than their regional surroundings. Therefore, climate-change-induced range-shifts in the future might well be influenced by such microrefugia
LPJ-GM 1.0: simulating migration efficiently in a dynamic vegetation model
Dynamic global vegetation models are a common tool to assess the effect of
climate and land use change on vegetation. Though most applications of
dynamic global vegetation models use plant functional types, some also
simulate species occurrences. While the current development aims to include
more processes, e.g. the nitrogen cycle, the models still typically assume
an ample seed supply allowing all species to establish once the climate
conditions are suitable. Pollen studies have shown that a number of plant
species lag behind in occupying climatological suitable areas (e.g. after a
change in the climate) as they need to arrive at and establish in the newly
suitable areas. Previous attempts to implement migration in dynamic
vegetation models have allowed for the simulation of either only small areas or have
been implemented as a post-process, not allowing for feedbacks within the
vegetation. Here we present two novel methods simulating migrating and
interacting tree species which have the potential to be used for simulations
of large areas. Both distribute seeds between grid cells, leading to
individual establishment. The first method uses an approach based on fast
Fourier transforms, while in the second approach we iteratively shift the
seed production matrix and disperse seeds with a given probability. While
the former method is computationally faster, it does not allow for
modification of the seed dispersal kernel parameters with respect to terrain
features, which the latter method allows.
We evaluate the increase in computational demand of both methods. Since
dispersal acts at a scale no larger than 1 km, all dispersal simulations
need to be performed at maximum at that scale. However, with the currently
available computational power it is not feasible to simulate the local
vegetation dynamics of a large area at that scale. We present an option to
decrease the required computational costs through a reduction in the number of grid cells
for which the local dynamics are simulated only along migration transects.
Evaluation of species patterns and migration speeds shows that
simulating along transects reduces migration speed, and both methods
applied on the transects produce reasonable results. Furthermore, using
the migration transects, both methods are sufficiently computationally
efficient to allow for large-scale DGVM simulations with migration.</p
How robust are future projections of forest landscape dynamics? Insights from a systematic comparison of four forest landscape models
Projections of landscape dynamics are uncertain, partly due to uncertainties in model formulations. However, quantitative comparative analyses of forest landscape models are lacking. We conducted a systematic comparison of all forest landscape models currently applied in temperate European forests (LandClim, TreeMig, LANDIS-II, iLand). We examined the uncertainty of model projections under several future climate, disturbance, and dispersal scenarios, and quantified uncertainties by variance partitioning. While projections under past climate conditions were in good agreement with observations, uncertainty under future climate conditions was high, with between-model biomass differences of up to 200 t ha−1. Disturbances strongly influenced landscape dynamics and contributed substantially to uncertainty in model projections (~25–40% of observed variance). Overall, model differences were the main source of uncertainty, explaining at least 50% of observed variance. We advocate a more rigorous and systematic model evaluation and calibration, and a broader use of ensemble projections to quantify uncertainties in future landscape dynamics
Dataset on the activation of Muller cells through macrophages upon hypoxia in the retina
The dataset presented in this article complements the article entitled
“Myeloid cells contribute indirectly to VEGF expression upon hypoxia via
activation of Müller cells” (C. Nürnberg, N. Kociok, C. Brockmann, T. Lischke,
S. Crespo-Garcia, N. Reichhart, S. Wolf, R. Baumgrass, S.A. Eming, S. Beer-
Hammer, and A.M. Joussen). This complementary dataset provides further insight
into the experimental validation of the VEGFfl/fl LysMCre (here named
VEGFmcko) knockout model used in the main article through genomic and
quantitative Real-Time PCR in various murine tissues as well as additional
flow cytometry data and immunohistochemical stainings. By providing these
data, we aim to enable researcher to reproduce and critically analyze our
data
Testing an optimality-based model of rooting zone water storage capacity in temperate forests
Rooting zone water storage capacity Sr is a crucial parameter for
modeling hydrology, ecosystem gas exchange and vegetation dynamics. Despite
its importance, this parameter is still poorly constrained and subject to
high uncertainty. We tested the analytical, optimality-based model of
effective rooting depth proposed by Guswa (2008, 2010) with regard to its
applicability for parameterizing Sr in temperate forests. The
model assumes that plants dimension their rooting systems to maximize net
carbon gain. Results from this model were compared against values obtained by
calibrating a local water balance model against latent heat flux and soil
moisture observations from 15 eddy covariance sites. Then, the effect of
optimality-based Sr estimates on the performance of local water
balance predictions was assessed during model validation.The agreement between calibrated and optimality-based Sr varied
greatly across climates and forest types. At a majority of cold and temperate
sites, the Sr estimates were similar for both methods, and the
water balance model performed equally well when parameterized with calibrated
and with optimality-based Sr. At spruce-dominated sites,
optimality-based Sr were much larger than calibrated values.
However, this did not affect the performance of the water balance model. On
the other hand, at the Mediterranean sites considered in this study,
optimality-based Sr were consistently much smaller than
calibrated values. The same was the case at pine-dominated sites on sandy
soils. Accordingly, performance of the water balance model was much worse at
these sites when optimality-based Sr were used. This rooting
depth parameterization might be used in dynamic (eco)hydrological models
under cold and temperate conditions, either to estimate Sr
without calibration or as a model component. This could greatly increase the
reliability of transient climate-impact assessment studies. On the other
hand, the results from this study do not warrant the application of this
model to Mediterranean climates or on very coarse soils. While the cause of
these mismatches cannot be determined with certainty, it is possible that
trees under these conditions follow rooting strategies that differ from the
carbon budget optimization assumed by the model.</p
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