859 research outputs found
Assessment of fine scale population genetic diversity and regeneration in Congo basin logged forests
In the Congo Basin most of the light-demanding timber tree species display a deficit of natural regeneration which is a major handicap for sustainable production and certification. Whilst the majority of scientists investigate abiotic and biotic factors explaining that pattern, we hypothesize that tree population density or individual spatial isolation may also affect the tree fitness through inbreeding. In this study, we integrate ecological and genetic approaches to characterize the regeneration potential of a set of priority timber species by (i) estimating pollen dispersal distances at various tree population densities, and (ii) evaluating the impact of increasing spatial isolation on mating characteristics and tree fitness. The ultimate goal is the proposal of minimum population density that prevents inbreeding consequences.
Method
This ongoing study focuses on 10 timber species (Pericopsis elata, Milicia excelsa, Baillonella toxisperma, Entandrophragma cylindricum, E. utile, E. angolense, E. candollei, Afzelia bipindensis, Erythrophleum suaveloens, Terminalia superba). The data collection was carried out in the logging concession granted to Pallisco in Cameroon.
We established two 400-ha plots, where all individuals (DBH > 10 cm) of the target species were inventoried and mapped. A sample of leave or cambium was collected for each of these individuals, as well as for seedlings to characterize patterns of gene flow using genetic tools (nuclear microsatellites). Dispersal agents were identified by direct observations and camera traps. Germination success was characterized in nursery for seeds collected on trees under an increasing isolation gradient.
Results
Main dispersal agents (wind, bat, rodent) and predators (rodent) were identified for all the species. The gene flow and germination data is still being analyzed and the main results will be presented in the poster.
Conclusion
Our data will allow characterizing the reproductive biology of a set of important timber species from the Congo basin. These information will strengthen sustainable forest management and the application of certification by adjusting harvesting norms through the use of scientifically-relevant data. In particular, we will tentatively define a maximum distance to be maintained between two adults to allow a qualitative reproduction
Power Spectra for Cold Dark Matter and its Variants
The bulk of recent cosmological research has focused on the adiabatic cold
dark matter model and its simple extensions. Here we present an accurate
fitting formula that describes the matter transfer functions of all common
variants, including mixed dark matter models. The result is a function of
wavenumber, time, and six cosmological parameters: the massive neutrino
density, number of neutrino species degenerate in mass, baryon density, Hubble
constant, cosmological constant, and spatial curvature. We show how
observational constraints---e.g. the shape of the power spectrum, the abundance
of clusters and damped Lyman-alpha systems, and the properties of the
Lyman-alpha forest--- can be extended to a wide range of cosmologies, including
variations in the neutrino and baryon fractions in both high-density and
low-density universes.Comment: 20 pages, LaTeX, 4 figures. Submitted to ApJ. Electronic versions of
the fitting formula, as well as simple codes to output cosmological
quantities (e.g. sigma_8) as a function of parameters and illustrative
animations of parameter dependence, are available at
http://www.sns.ias.edu/~whu/transfer/transfer.htm
Origin and evolution of halo bias in linear and non-linear regimes
We present results from a study of bias and its evolution for galaxy-size
halos in a large, high-resolution simulation of a LCDM model. We consider the
evolution of bias estimated using two-point correlation function (b_xi), power
spectrum (b_P), and a direct correlation of smoothed halo and matter
overdensity fields (b_d). We present accurate estimates of the evolution of the
matter power spectrum probed deep into the stable clustering regime
(k~[0.1-200]h/Mpc at z=0). The halo power spectrum evolves much slower than the
power spectrum of matter and has a different shape which indicates that the
bias is time- and scale-dependent. At z=0, the halo power spectrum is
anti-biased with respect to the matter power spectrum at wavenumbers
k~[0.15-30]h/Mpc, and provides an excellent match to the power spectrum of the
APM galaxies at all probed k. In particular, it nicely matches the inflection
observed in the APM power spectrum at k~0.15h/Mpc. We complement the power
spectrum analysis with a direct estimate of bias using smoothed halo and matter
overdensity fields and show that the evolution observed in the simulation in
linear and mildly non-linear regimes can be well described by the analytical
model of Mo & White (1996), if the distinction between formation redshift of
halos and observation epoch is introduced into the model. We present arguments
and evidence that at higher overdensities, the evolution of bias is
significantly affected by dynamical friction and tidal stripping operating on
the satellite halos in high-density regions of clusters and groups; we
attribute the strong anti-bias observed in the halo correlation function and
power spectrum to these effects. (Abridged)Comment: submitted to the Astrophys.Journal; 19 pages, 9 figures LaTeX (uses
emulateapj.sty
El Niño–Southern Oscillation (ENSO)-driven hypersedimentation in the Poechos Reservoir, northern Peru
Although extreme El Niño events (EENEs) have always impacted
hydrological anomalies and sediment transport in South America, their
intensification by global warming and their association with changes in
human activities and land cover after humid periods may lead to the
acceleration of sediment transfers in river systems and dam reservoirs. This
situation may threaten soil and water resources in arid and semiarid regions
highly dependent on water originating from large dams. In this study, we
investigated the sediment sequence accumulated in the Poechos Reservoir
(northern Peru) and provided a retrospective reconstruction of the
interactions of El Niño–Southern Oscillation (ENSO), agricultural
practices and vegetation cover changes with sediment dynamics (1978–2019). To
this end, a sediment core was dated and characterized by physical and
chemical analyses (e.g., scanner tomography, X-ray fluorescence, particle
size analysis) to estimate the evolution of sedimentation rates and
changes in sediment sources during the last 5Â decades.
Sediment tracing results indicated the occurrence of changes in sediment
sources associated with positive and negative phases of the Eastern Pacific
index with a greater contribution of the lowland dry-forest area in
comparison to that of the Andean area to sediment during the El Niño
events (mean contribution of 76 %; up to 90 % during the coastal El
Niño events (CENEs) of 2016–2017). This source contribution was mostly
controlled by the stationary rainfall occurring during the EENEs in the lowland dry-forest area characterized by a low vegetation cover. Overall, after an extreme phase of ENSO, like after the
EENE 1982–1983, the normal discharges and persistent sediment supplies from
the middle- and upper-catchment parts led to river aggradation and the
storage of substantial amounts of sediment in alluvial plains. In the
absence of a significant EENE between 1983 and 1997, the large volume of
sediment stored in the alluvial plains was exported by the EENE 1997–1998
resulting in an increase in sedimentation rate of 140 % after 1997 with
a significant aggradation of the deltaic zone of the reservoir. In addition
to the impact of extreme climate events on sediment dynamics, the
development of agriculture along the riverine system after an extreme phase
of ENSO increased the availability of sediments in the main channel of the
rivers, easily transported by the next EENE. This study suggests that
intensification of human activities associated with a higher frequency of
extreme rainfall events amplified the quantity of sediment transported by
the river system, which will significantly decrease the lifespan of the
reservoir, which is essential to meeting the freshwater demands of the farmers and the
populations living in this arid and semiarid region.</p
Galaxies in N-body simulations: overcoming the overmerging problem
We present analysis of the evolution of dark matter halos in dense
environments of groups and clusters in dissipationless cosmological
simulations. The premature destruction of halos in such environments, known as
the overmerging, reduces the predictive power of N-body simulations and makes
difficult any comparison between models and observations. We analyze the
possible processes that cause the overmerging and assess the extent to which
this problem can be cured with current computer resources and codes. Using both
analytic estimates and high resolution numerical simulations, we argue that the
overmerging is mainly due to the lack of numerical resolution. We find that the
force and mass resolution required for a simulated halo to survive in galaxy
groups and clusters is extremely high and was almost never reached before: ~1-3
kpc and 10^8-10^9 Msun, respectively. We use the high-resolution Adaptive
Refinement Tree (ART) N-body code to run cosmological simulations with the
particle mass of \approx 2x10^8/h Msun} and the spatial resolution of \approx
1-2/h kpc, and show that in these simulations the halos do survive in regions
that would appear overmerged with lower force resolution. Nevertheless, the
halo identification in very dense environments remains a challenge even with
the resolution this high. We present two new halo finding algorithms developed
to identify both isolated and satellite halos that are stable (existed at
previous moments) and gravitationally bound. To illustrate the use of the
satellite halos that survive the overmerging, we present a series of halo
statistics, that can be compared with those of observed galaxies. (Abridged)Comment: Accepted for publication in ApJ, substantional revisions after the
first version, LaTeX 23 pages, 18 figs. (uses emulateapj.sty),
Full-resolution version of Fig.9 is available upon reques
Tracing cosmic evolution with clusters of galaxies
The most successful cosmological models to date envision structure formation
as a hierarchical process in which gravity is constantly drawing lumps of
matter together to form increasingly larger structures. Clusters of galaxies
currently sit atop this hierarchy as the largest objects that have had time to
collapse under the influence of their own gravity. Thus, their appearance on
the cosmic scene is also relatively recent. Two features of clusters make them
uniquely useful tracers of cosmic evolution. First, clusters are the biggest
things whose masses we can reliably measure because they are the largest
objects to have undergone gravitational relaxation and entered into virial
equilibrium. Mass measurements of nearby clusters can therefore be used to
determine the amount of structure in the universe on scales of 10^14 to 10^15
solar masses, and comparisons of the present-day cluster mass distribution with
the mass distribution at earlier times can be used to measure the rate of
structure formation, placing important constraints on cosmological models.
Second, clusters are essentially ``closed boxes'' that retain all their gaseous
matter, despite the enormous energy input associated with supernovae and active
galactic nuclei, because the gravitational potential wells of clusters are so
deep. The baryonic component of clusters therefore contains a wealth of
information about the processes associated with galaxy formation, including the
efficiency with which baryons are converted into stars and the effects of the
resulting feedback processes on galaxy formation. This article reviews our
theoretical understanding of both the dark-matter component and the baryonic
component of clusters. (Abridged)Comment: 54 pages, 15 figures, Rev. Mod. Phys. (in press
Sensitivity of source sediment fingerprinting to tracer selection methods
In a context of accelerated soil erosion and sediment supply to water bodies, sediment fingerprinting techniques have received an increasing interest in the last 2Â decades. The selection of tracers is a particularly critical step for the subsequent accurate prediction of sediment source contributions. To select tracers, the most conventional approach is the three-step method, although, more recently, the consensus method has also been proposed as an alternative. The outputs of these two approaches were compared in terms of identification of conservative properties, tracer selection, modelled contributions and performance on a single dataset. As for the three-step method, several range test criteria were compared, along with the impact of the discriminant function analysis (DFA).
The dataset was composed of tracer properties analysed in soil (three potential sources; n = 56) and sediment core samples (n = 32). Soil and sediment samples were sieved to 63 µm and analysed for organic matter, elemental geochemistry and diffuse visible spectrometry. Virtual mixtures (n = 138) with known source proportions were generated to assess model accuracy of each tracer selection method. The Bayesian un-mixing model MixSIAR was then used to predict source contributions on both virtual mixtures and actual sediments.
The different methods tested in the current research can be distributed into three groups according to their sensitivity to the conservative behaviour of properties, which was found to be associated with different predicted source contribution tendencies along the sediment core. The methods selecting the largest number of tracers were associated with a dominant and constant contribution of forests to sediment. In contrast, the methods selecting the lowest number of tracers were associated with a dominant and constant contribution of cropland to sediment. Furthermore, the intermediate selection of tracers led to more balanced contributions of both cropland and forest to sediments.
The prediction of the virtual mixtures allowed us to compute several evaluation metrics, which are generally used to support the evaluation of model accuracy for each tracer selection method. However, strong differences or the absence of correspondence were observed between the range of predicted contributions obtained for virtual mixtures and those values obtained for actual sediments. These divergences highlight the fact that evaluation metrics obtained for virtual mixtures may not be directly transferable to models run for actual samples and must be interpreted with caution to avoid over-interpretation or misinterpretation. These divergences may likely be attributed to the occurrence of a not (fully) conservative behaviour of potential tracer properties during erosion, transport and deposition processes, which could not be fully reproduced when generating the virtual mixtures with currently available methods.
Future research should develop novel metrics to quantify the conservative behaviour of tracer properties during erosion and transport processes. Furthermore, new methods should be designed to generate virtual mixtures closer to reality and to better evaluate model accuracy. These improvements would contribute to the development of more reliable sediment fingerprinting techniques, which are needed to better support the implementation of effective soil and water conservation measures at the catchment scale.</p
The DAFT/FADA survey. I.Photometric redshifts along lines of sight to clusters in the z=[0.4,0.9] interval
As a contribution to the understanding of the dark energy concept, the Dark
energy American French Team (DAFT, in French FADA) has started a large project
to characterize statistically high redshift galaxy clusters, infer cosmological
constraints from Weak Lensing Tomography, and understand biases relevant for
constraining dark energy and cluster physics in future cluster and cosmological
experiments. The purpose of this paper is to establish the basis of reference
for the photo-z determination used in all our subsequent papers, including weak
lensing tomography studies. This project is based on a sample of 91 high
redshift (z>0.4), massive clusters with existing HST imaging, for which we are
presently performing complementary multi-wavelength imaging. This allows us in
particular to estimate spectral types and determine accurate photometric
redshifts for galaxies along the lines of sight to the first ten clusters for
which all the required data are available down to a limit of I_AB=24/24.5 with
the LePhare software. The accuracy in redshift is of the order of 0.05 for the
range 0.2<z<1.5. We verified that the technique applied to obtain photometric
redshifts works well by comparing our results to with previous works. In
clusters, photoz accuracy is degraded for bright absolute magnitudes and for
the latest and earliest type galaxies. The photoz accuracy also only slightly
varies as a function of the spectral type for field galaxies. As a consequence,
we find evidence for an environmental dependence of the photoz accuracy,
interpreted as the standard used Spectral Energy Distributions being not very
well suited to cluster galaxies. Finally, we modeled the LCDCS 0504 mass with
the strong arcs detected along this line of sight.Comment: Accepted in A&
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