15,653 research outputs found
Mechanisms of Surviving Burial: Dune Grass Interspecific Differences Drive Resource Allocation After Sand Deposition
Sand dunes are important geomorphic formations of coastal ecosystems that are critical in protecting human populations that live in coastal areas. Dune formation is driven by ecomorphodynamic interactions between vegetation and sediment deposition. While there has been extensive research on responses of dune grasses to sand burial, there is a knowledge gap in understanding mechanisms of acclimation between similar, coexistent, dune-building grasses such as Ammophila breviligulata (C3), Spartina patens (C4), and Uniola paniculata (C4). Our goal was to determine how physiological mechanisms of acclimation to sand burial vary between species. We hypothesize that (1) in the presence of burial, resource allocation will be predicated on photosynthetic pathway and that we will be able to characterize the C3 species as a root allocator and the C4 species as leaf allocators. We also hypothesize that (2) despite similarities between these species in habitat, growth form, and life history, leaf, root, and whole plant traits will vary between species when burial is not present. Furthermore, when burial is present, the existing variability in physiological strategy will drive species-specific mechanisms of survival. In a greenhouse experiment, we exposed three dune grass species to different burial treatments: 0 cm (control) and a one-time 25-cm burial to mimic sediment deposition during a storm. At the conclusion of our study, we collected a suite of physiological and morphological functional traits. Results showed that Ammophila decreased allocation to aboveground biomass to maintain root biomass, preserving photosynthesis by allocating nitrogen (N) into light-exposed leaves. Conversely, Uniola and Spartina decreased allocation to belowground production to increase elongation and maintain aboveground biomass. Interestingly, we found that species were functionally distinct when burial was absent; however, all species became more similar when treated with burial. In the presence of burial, species utilized functional traits of rapid growth strategy, although mechanisms of change were interspecifically variable
Optimal Principal Component Analysis in Distributed and Streaming Models
We study the Principal Component Analysis (PCA) problem in the distributed
and streaming models of computation. Given a matrix a
rank parameter , and an accuracy parameter , we
want to output an orthonormal matrix for which where is the best rank- approximation to .
This paper provides improved algorithms for distributed PCA and streaming
PCA.Comment: STOC2016 full versio
Correlation length of hydrophobic polyelectrolyte solutions
The combination of two techniques (Small Angle X-ray Scattering and Atomic
Force Microscopy) has allowed us to measure in reciprocal and real space the
correlation length of salt-free aqueous solutions of highly charged
hydrophobic polyelectrolyte as a function of the polymer concentration ,
charge fraction and chain length . Contrary to the classical behaviour
of hydrophilic polyelectrolytes in the strong coupling limit, is strongly
dependent on . In particular a continuous transition has been observed from
to when decreased from 100% to
35%. We interpret this unusual behaviour as the consequence of the two features
characterising the hydrophobic polyelectrolytes: the pearl necklace
conformation of the chains and the anomalously strong reduction of the
effective charge fraction.Comment: 7 pages, 5 figures, submitted to Europhysics Letter
Recovery of the orbital parameters and pulse evolution of V0332+53 during a huge outburst
The high mass X-ray binary (HMXB) V0332+53 became active at the end of 2004
and the outburst was observed at hard X-rays by RXTE and INTEGRAL. Based on
these hard X-ray observations, the orbital parameters are measured through
fitting the Doppler-shifted spin periods. The derived orbital period and
eccentricity are consistent with those of Stella et al. (1985) obtained from
EXOSAT observations, whereas the projected semimajor axis and the periastron
longitude are found to have changed from 484 to 86 lt-s and
from 31310 to 28314, respectively. This would
indicate an angular speed of 1.50.8 yr for
rotation of the orbit over the past 21 years. The periastron passage time of
MJD 533671 is just around the time when the intensity reached maximum and
an orbital period earlier is the time when the outburst started. This
correlation resembles the behavior of a Type I outburst. During outburst the
source spun up with a rate of 8.01 s
day. The evolution of pulse profile is highly intensity dependent. The
separation of double pulses remained almost constant ( 0.47) when the
source was bright, and dropped to 0.37 within 3 days as the source
became weaker. The pulse evolution of V0332+53 may correlate to the change in
dominance of the emission between fan-beam and pencil-beam mechanisms.Comment: 13 pages, 3 figures, accepted for publication in ApJ
Generation of finite wave trains in excitable media
Spatiotemporal control of excitable media is of paramount importance in the
development of new applications, ranging from biology to physics. To this end
we identify and describe a qualitative property of excitable media that enables
us to generate a sequence of traveling pulses of any desired length, using a
one-time initial stimulus. The wave trains are produced by a transient
pacemaker generated by a one-time suitably tailored spatially localized finite
amplitude stimulus, and belong to a family of fast pulse trains. A second
family, of slow pulse trains, is also present. The latter are created through a
clumping instability of a traveling wave state (in an excitable regime) and are
inaccessible to single localized stimuli of the type we use. The results
indicate that the presence of a large multiplicity of stable, accessible,
multi-pulse states is a general property of simple models of excitable media.Comment: 6 pages, 6 figure
Elucidation of physico-chemical characteristics and mycoflora of bovine milk available in selected area of Karachi, Pakistan.
ISO 14000 has opened the field for enhancing the awareness towards healthy environment and gives the ideology of regular assessment of threshold values not only of nutrients but contaminants as well. Under this statement microbiological studies in terms of mycoflora followed by physical characteristics and level of some essential elements Mg, Fe, Cu and Zn, and deleterious metal Cd were analyzed in fresh milk samples (FMS) and tetra pack milk or UHT processed milk samples (PMS) available in Gulshan-e-Iqbal Town, Karachi for the residents. Samples were collected in the morning time during winter season. Physical parameters like pH, conductivity, density, viscosity, surface tension and refractive index were observed very soon after sample collection. Percentage of ash and total dissolved solids (TDS) were also measured. For fungal flora studies PDA (Potato Dextrose Agar) and Sabourouds Agar were used for the fungal growth. The concentrations of metals considered were estimated after wet digestion of samples using atomic absorption spectroscopy. The ranges of average concentration of Mg, Fe, Cu and Zn were found to be 77.120 – 141.915mg/l, 0.580 – 7.320mg/l, 0.004 – 0.070mg/l and 2.574 – 4.872mg/l. The level of Cd was estimated in the samples between 0.0050 – 0.053mg/l, but was not observed in most of the samples. Among the fungi, the highest diversity was that of Aspergillus spp. that produces aflatoxins. ©JASEMKey words: physical characteristics, essential elements, toxic elements, fungal specie
The Additional Line Component within the Iron K\alpha Profile in MCG-6-30-15: Evidence for Blob Ejection?
The EPIC data of MCG -6-30-15 observed by XMM-Newton were analyzed for the
complexities of the iron K-alpha line. Here we report that the additional line
component (ALC) at 6.9 keV undoubtedly appears within the broad iron Kalpha;
line profile at the high state, whereas it disappears at the low state. These
state-dependent behaviors exclude several possible origins and suggest an
origin of the ALC in matter being ejected from the vicinity of the black hole.
At the low state, the newborn blob ejected from the accretion disk is so
Thomson-thick that hard X-rays are blocked from ionizing the old blobs, leading
to the disappearance of the ALC. When the blob becomes Thomson-thin as a result
of expansion, the hard X-ray will penetrate it and ionize the old ones,
emitting the ALC at the high state. The blob ejection is the key to switching
the ALC on or off.Comment: 6 pages, 4 Figure
Hybridising heuristics within an estimation distribution algorithm for examination timetabling
This paper presents a hybrid hyper-heuristic approach based on estimation distribution algorithms. The main motivation is to raise the level of generality for search methodologies. The objective of the hyper-heuristic is to produce solutions of acceptable quality for a number of optimisation problems. In this work, we demonstrate the generality through experimental results for different variants of exam timetabling problems. The hyper-heuristic represents an automated constructive method that searches for heuristic choices from a given set of low-level heuristics based only on non-domain-specific knowledge. The high-level search methodology is based on a simple estimation distribution algorithm. It is capable of guiding the search to select appropriate heuristics in different problem solving situations. The probability distribution of low-level heuristics at different stages of solution construction can be used to measure their effectiveness and possibly help to facilitate more intelligent hyper-heuristic search methods
The song of the dunes as a self-synchronized instrument
Since Marco Polo (1) it has been known that some sand dunes have the peculiar
ability of emitting a loud sound with a well defined frequency, sometimes for
several minutes. The origin of this sustained sound has remained mysterious,
partly because of its rarity in nature (2). It has been recognized that the
sound is not due to the air flow around the dunes but to the motion of an
avalanche (3), and not to an acoustic excitation of the grains but to their
relative motion (4-7). By comparing several singing dunes and two controlled
experiments, one in the laboratory and one in the field, we here demonstrate
that the frequency of the sound is the frequency of the relative motion of the
sand grains. The sound is produced because some moving grains synchronize their
motions. The existence of a velocity threshold in both experiments further
shows that this synchronization comes from an acoustic resonance within the
flowing layer: if the layer is large enough it creates a resonance cavity in
which grains self-synchronize.Comment: minor changes, essentially more references
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