433 research outputs found
Concerted changes in tropical forest structure and dynamics: evidence from 50 South American long-term plots
Several widespread changes in the ecology of old-growth tropical forests have recently been documented for the late twentieth century, in particular an increase in stem turnover (pan-tropical), and an increase in above-ground biomass (neotropical). Whether these changes are synchronous and whether changes in growth are also occurring is not known. We analysed stand-level changes within 50 long-term. monitoring plots from across South America spanning 1971-2002. We show that: (i) basal area (BA: sum of the cross-sectional areas of all trees in a plot) increased significantly over time (by 0.10 +/- 0.04 m(2) ha(-1) yr(-1), mean +/- 95% CI); as did both (ii) stand-level BA growth rates (sum of the increments of BA of surviving trees and BA of new trees that recruited into a plot); and (iii) stand-level BA mortality rates (sum of the cross-sectional areas of all trees that died in a plot). Similar patterns were observed on a per-stem basis: (i) stem density (number of stems per hectare; 1 hectare is 10(4) m(2)) increased significantly over time (0.94 +/- 0.63 stems ha(-1) yr(-1)); as did both (ii) stem recruitment rates; and (iii) stem mortality rates. In relative terms, the pools of BA and stem density increased by 0.38 +/- 0.15% and 0.18 +/- 0.12% yr(-1), respectively. The fluxes into and out of these pools-stand-level BA growth, stand-level BA mortality, stem recruitment and stem mortality rates-increased, in relative terms, by an order of magnitude more. The gain terms (BA growth, stem recruitment) consistently exceeded the loss terms (BA loss, stem mortality) throughout the period, suggesting that whatever process is driving these changes was already acting before the plot network was established. Large long-term increases in stand-level BA growth and simultaneous increases in stand BA and stem density imply a continent-wide increase in resource availability which is increasing net primary productivity and altering forest dynamics. Continent-wide changes in incoming solar radiation, and increases in atmospheric concentrations of CO2 and air temperatures may have increased resource supply over recent decades, thus causing accelerated growth and increased dynamism across the world's largest tract of tropical forest
Electric field control of spins in bilayer graphene: Local moment formation and local moment interactions
We study local moment formation for adatoms on bilayer graphene (BLG) within
a mean-field theory of the Anderson impurity model. The wavefunctions of the
BLG electrons induce strong particle-hole asymmetry and band dependence of the
hybridization, which is shown to result in unusual features in the impurity
model phase diagram. We also study the effect of varying the chemical
potential, as well as varying an electric field perpendicular to the bilayer;
the latter modifies the density of states of electrons in BLG and, more
significantly, shifts the impurity energy. We show that this leads to regimes
in the impurity phase diagram where local moments can be turned on or off by
applying modest external electric fields. Finally, we show that the RKKY
interaction between local moments can be varied by tuning the chemical
potential (as has also been suggested in monolayer graphene) or, more
interestingly, by tuning the electric field so that it induces changes in the
band structure of BLG.Comment: Revised discussion and figures, 17 page
Pattern and process in Amazon tree turnover, 1976-2001
Previous work has shown that tree turnover, tree biomass and large liana densities have increased in mature tropical forest plots in the late twentieth century. These results point to a concerted shift in forest ecological processes that may already be having significant impacts on terrestrial carbon stocks, fluxes and biodiversity. However, the findings have proved controversial, partly because a rather limited number of permanent plots have been monitored for rather short periods. The aim of this paper is to characterize regional-scale patterns of 'tree turnover' (the rate with which trees die and recruit into a population) by using improved datasets now available for Amazonia that span the past 25 years. Specifically, we assess whether concerted changes in turnover are occurring, and if so whether they are general throughout the Amazon or restricted to one region or environmental zone. In addition, we ask whether they are driven by changes in recruitment, mortality or both. We find that: (i) trees 10 cm or more in diameter recruit and die twice as fast on the richer soils of southern and western Amazonia than on the poorer soils of eastern and central Amazonia; (ii) turnover rates have increased throughout Amazonia over the past two decades; (iii) mortality and recruitment rates have both increased significantly in every region and environmental zone, with the exception of mortality in eastern Amazonia; (iv) recruitment rates have consistently exceeded mortality rates; (v) absolute increases in recruitment and mortality rates are greatest in western Amazonian sites; and (vi) mortality appears to be lagging recruitment at regional scales. These spatial patterns and temporal trends are not caused by obvious artefacts in the data or the analyses. The trends cannot be directly driven by a mortality driver (such as increased drought or fragmentation-related death) because the biomass in these forests has simultaneously increased. Our findings therefore indicate that long-acting and widespread environmental changes are stimulating the growth and productivity of Amazon forests
Reactivation of Fault Systems by Compartmentalized Hydrothermal Fluids in the Southern Andes Revealed by Magnetotelluric and Seismic Data
In active volcanic arcs such as the Andean volcanic mountain belt, magmaticallyâsourced fluids are channelled through the brittle crust by faults and fracture networks. In the Andes, volcanoes, geothermal springs and major mineral deposits have a spatial and genetic relationship with NNEâtrending, marginâparallel faults and marginâoblique, NWâtrending Andean Transverse Faults (ATF). The Tinguiririca and PlanchĂłnâPeteroa volcanoes in the Andean Southern Volcanic Zone (SVZ) demonstrate this relationship, as their spatially associated thermal springs show strike alignment to the NNEâoriented El Fierro Thrust Fault System. We constrain the fault system architecture and its interaction with volcanically sourced hydrothermal fluids using a combined magnetotelluric (MT) and seismic survey that was deployed for 20 months. High conductivity zones are located along the axis of the active volcanic chain, delineating fluids and/or melt. A distinct WNWâtrending cluster of seismicity correlates with resistivity contrasts, considered to be a reactivated ATF. Seismicity occurs below 4 km, suggesting activity is limited to basement rocks, and the cessation of seismicity at 9 km delineates the local brittleâductile transition. As seismicity is not seen west of the El Fierro fault, we hypothesize that this structure plays a key role in compartmentalizing magmaticallyâderived hydrothermal fluids to the east, where the fault zone acts as a barrier to crossâfault fluid migration and channels faultâparallel fluid flow to the surface from depth. Increases in fluid pressure above hydrostatic may facilitate reactivation. This siteâspecific case study provides the first threeâdimensional seismic and magnetotelluric observations of the mechanics behind the reactivation of an ATF
Clinical characteristics of patients hospitalized with COVID-19 in Spain: Results from the SEMI-COVID-19 Registry
Background: Spain has been one of the countries most affected by the COVID-19 pandemic. Objective: To create a registry of patients with COVID-19 hospitalized in Spain, in order to improve our knowledge of the clinical, diagnostic, therapeutic, and prognostic aspects of this disease. Methods: A multicentre retrospective cohort study, including consecutive patients hospitalized with confirmed COVID-19 throughout Spain. Epidemiological and clinical data, additional tests at admission and at seven days, treatments administered, and progress at 30 days of hospitalization were collected from electronic medical records. Results: Up to June 30th 2020, 15,111 patients from 150 hospitals were included. Their median age was 69.4 years (range: 18-102 years) and 57.2% were male. Prevalences of hypertension, dyslipidemia, and diabetes mellitus were 50.9%, 39.7%, and 19.4%, respectively. The most frequent symptoms were fever (84.2%) and cough (73.5%). High values of ferritin (73.5%), lactate dehydrogenase (73.9%), and D-dimer (63.8%), as well as lymphopenia (52.8%), were frequent. The most used antiviral drugs were hydroxychloroquine (85.6%) and lopinavir/ritonavir (61.4%); 33.1% developed respiratory distress. Overall mortality rate was 21.0%, with a marked increase with age (50-59 years: 4.7%, 60-69 years: 10.5%, 70-79 years: 26.9%, â„ 80 years: 46.0%). Conclusions: The SEMI-COVID-19 Network provides data on the clinical characteristics of patients with COVID-19 hospitalized in Spain. Patients with COVID-19 hospitalized in Spain are mostly severe cases, as one in three patients developed respiratory distress and one in five patients died. These findings confirm a close relationship between advanced age and mortality.Antecedentes: España ha sido uno de los paĂses mĂĄs afectados por la pandemia de COVID-19.Objetivo: Crear un registro de pacientes hospitalizados en España por COVID-19 para mejorar nuestro conocimiento sobre los aspectos clĂnicos, diagnĂłsticos, terapĂ©uticos y pronĂłsticos de esta enfermedad. MĂ©todos: Estudio de cohorte retrospectiva, multicĂ©ntrico, que incluye pacientes consecutivos hospitalizados con COVID-19 confirmada en toda España. Se obtuvieron los datos epidemiolĂłgicos y clĂnicos, las pruebas complementarias al ingreso y a los 7 dĂas de la admisiĂłn, los tratamientos administrados y la evoluciĂłn a los 30 dĂas de hospitalizaciĂłn de las historias clĂnicas electrĂłnicas. Resultados: Hasta el 30 de junio de 2020 se incluyeron 15.111 pacientes de 150 hospitales. Su mediana de edad fue 69,4 años (rango: 18-102 años) y el 57,2% eran hombres. Las prevalencias de hipertensiĂłn, dislipemia y diabetes mellitus fueron 50,9%, 39,7% y 19,4%, respectivamente.Los sĂntomas mĂĄs frecuentes fueron fiebre (84,2%) y tos (73,5%). Fueron frecuentes los valores elevados de ferritina (73,5%), lactato deshidrogenasa (73,9%) y dĂmero D (63,8%), asĂ como lalinfopenia (52,8%). Los fĂĄrmacos antivirales mĂĄs utilizados fueron la hidroxicloroquina (85,6%)y el lopinavir/ritonavir (61,4%). El 33,1% desarrollĂł distrĂ©s respiratorio. La tasa de mortalidad global fue del 21,0%, con un marcado incremento con la edad (50-59 años: 4,7%; 60-69 años:10,5%; 70-79 años: 26,9%; â„ 80 años: 46%).Conclusiones: El Registro SEMI-COVID-19 proporciona informaciĂłn sobre las caracterĂsticas clĂnicas de los pacientes con COVID-19 hospitalizados en España. Los pacientes con COVID-19 hospitalizados en España son en su mayorĂa casos graves, ya que uno de cada 3 pacientes desarrollĂł distrĂ©s respiratorio y uno de cada 5 pacientes falleciĂł. Nuestros datos confirman una estrecha relaciĂłn entre la edad avanzada y la mortalidadLa Sociedad Española de Medicina Interna (SEMI) es la patrocinadora de este estudio
Methods to estimate aboveground wood productivity from long-term forest inventory plots
Forest inventory plots are widely used to estimate biomass carbon storage and its change over time. While there has been much debate and exploration of the analytical methods for calculating biomass, the methods used to determine rates of wood production have not been evaluated to the same degree. This affects assessment of ecosystem fluxes and may have wider implications if inventory data are used to parameterise biospheric models, or scaled to large areas in assessments of carbon sequestration. Here we use a dataset of 35 long-term Amazonian forest inventory plots to test different methods of calculating wood production rates. These address potential biases associated with three issues that routinely impact the interpretation of tree measurement data: (1) changes in the point of measurement (POM) of stem diameter as trees grow over time; (2) unequal length of time between censuses; and (3) the treatment of trees that pass the minimum diameter threshold (ârecruitsâ). We derive corrections that control for changing POM height, that account for the unobserved growth of trees that die within census intervals, and that explore different assumptions regarding the growth of recruits during the previous census interval. For our dataset we find that annual aboveground coarse wood production (AGWP; in Mg haâ1 yearâ1 of dry matter) is underestimated on average by 9.2% if corrections are not made to control for changes in POM height. Failure to control for the length of sampling intervals results in a mean underestimation of 2.7% in annual AGWP in our plots for a mean interval length of 3.6 years. Different methods for treating recruits result in mean differences of up to 8.1% in AGWP. In general, the greater the length of time a plot is sampled for and the greater the time elapsed between censuses, the greater the tendency to underestimate wood production. We recommend that POM changes, census interval length, and the contribution of recruits should all be accounted for when estimating productivity rates, and suggest methods for doing this.European UnionUK Natural Environment Research CouncilGordon and Betty Moore FoundationCASE sponsorship from UNEP-WCMCRoyal Society University Research FellowshipERC Advanced Grant âTropical Forests in the Changing Earth SystemâRoyal Society Wolfson Research Merit Awar
Ultrahigh-energy neutrino follow-up of Gravitational Wave events GW150914 and GW151226 with the Pierre Auger Observatory
On September 14, 2015 the Advanced LIGO detectors observed their first
gravitational-wave (GW) transient GW150914. This was followed by a second GW
event observed on December 26, 2015. Both events were inferred to have arisen
from the merger of black holes in binary systems. Such a system may emit
neutrinos if there are magnetic fields and disk debris remaining from the
formation of the two black holes. With the surface detector array of the Pierre
Auger Observatory we can search for neutrinos with energy above 100 PeV from
point-like sources across the sky with equatorial declination from about -65
deg. to +60 deg., and in particular from a fraction of the 90% confidence-level
(CL) inferred positions in the sky of GW150914 and GW151226. A targeted search
for highly-inclined extensive air showers, produced either by interactions of
downward-going neutrinos of all flavors in the atmosphere or by the decays of
tau leptons originating from tau-neutrino interactions in the Earth's crust
(Earth-skimming neutrinos), yielded no candidates in the Auger data collected
within s around or 1 day after the coordinated universal time (UTC)
of GW150914 and GW151226, as well as in the same search periods relative to the
UTC time of the GW candidate event LVT151012. From the non-observation we
constrain the amount of energy radiated in ultrahigh-energy neutrinos from such
remarkable events.Comment: Published version. Added journal reference and DOI. Added Report
Numbe
Azimuthal asymmetry in the risetime of the surface detector signals of the Pierre Auger Observatory
The azimuthal asymmetry in the risetime of signals in Auger surface detector
stations is a source of information on shower development. The azimuthal
asymmetry is due to a combination of the longitudinal evolution of the shower
and geometrical effects related to the angles of incidence of the particles
into the detectors. The magnitude of the effect depends upon the zenith angle
and state of development of the shower and thus provides a novel observable,
, sensitive to the mass composition of cosmic rays
above eV. By comparing measurements with predictions from
shower simulations, we find for both of our adopted models of hadronic physics
(QGSJETII-04 and EPOS-LHC) an indication that the mean cosmic-ray mass
increases slowly with energy, as has been inferred from other studies. However,
the mass estimates are dependent on the shower model and on the range of
distance from the shower core selected. Thus the method has uncovered further
deficiencies in our understanding of shower modelling that must be resolved
before the mass composition can be inferred from .Comment: Replaced with published version. Added journal reference and DO
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