Showing papers by "University of Los Andes published in 1995"

Environmental and physiological regulation of transpiration in tropical forest gap species: the influence of boundary layer and hydraulic properties.

Abstract: Environmental and physiological regulation of transpiration were examined in several gap-colonizing shrub and tree species during two consecutive dry seasons in a moist, lowland tropical forest on Barro Colorado Island, Panama. Whole plant transpiration, stomatal and total vapor phase (stomatal + boundary layer) conductance, plant water potential and environmental variables were measured concurrently. This allowed control of transpiration (E) to be partitioned quantitatively between stomatal (gs) and boundary layer (gb) conductance and permitted the impact of invividual environmental and physiological variables on stomatal behavior and E to be assessed. Wind speed in treefall gap sites was often below the 0.25 m s−1 stalling speed of the anemometer used and was rarely above 0.5 m s−1, resulting in uniformly low gb (c. 200–300 mmol m−2 s−1) among all species studied regardless of leaf size. Stomatal conductance was typically equal to or somewhat greater than gb. This strongly decoupled E from control by stomata, so that in Miconia argentea a 10% change in gs when gs was near its mean value was predicted to yield only a 2.5% change in E. Porometric estimates of E, obtained as the product of gs and the leaf-bulk air vapor pressure difference (VPD) without taking gb into account, were up to 300% higher than actual E determined from sap flow measurements. Porometry was thus inadequate as a means of assessing the physiological consequences of stomatal behavior in different gap colonizing species. Stomatal responses to humidity strongly limited the increase in E with increasing evaporative demand. Stomata of all species studied appeared to respond to increasing evaporative demand in the same manner when the leaf surface was selected as the reference point for determination of external vapor pressure and when simultaneous variation of light and leaf-air VPD was taken into account. This result suggests that contrasting stomatal responses to similar leaf-bulk air VPD may be governed as much by the external boundary layer as by intrinsic physiological differences among species. Both E and gs initially increased sharply with increasing leaf area-specific total hydraulic conductance of the soil/root/leaf pathway (Gt), becoming asymptotic at higher values of Gt. For both E and gs a unique relationship appeared to describe the response of all species to variations in Gt. The relatively weak correlation observed between gs and midday leaf water potential suggested that stomatal adjustment to variations in water availability coordinated E with water transport efficiency rather than bulk leaf water status.

Partitioning of water resources among plants of a lowland tropical forest

Abstract: Source water used by plants of several species in a semi-evergreen lowland tropical forest on Barro Colorado Island, Panama, was assessed by comparing the relative abundance of deuterium, D, versus hydrogen, H (stable hydrogen isotope composition, δD) in xylem sap and in soil water at different depths, during the dry season of 1992. Ecological correlates of source water were examined by comparing xylem water δD values with leaf phenology, leaf water status determined with a pressure chamber, and rates of water use determined as mass flow of sap using the stem heat balance method. Soil water δD values decreased sharply to 30 cm, then remained relatively constant with increasing depth. Average δD values were-13‰, for 0–30 cm depth and-36.7‰ for 30–100 cm depth. Soil water δD values were negatively associated with soil water content and soil water potential. Concurrent analyses of xylem water revealed a high degree of partitioning of water resources among species of this tropical forest. Xylem water δD of deciduous trees (average=-25.3±1.4‰) was higher than that of evergreen trees (average=-36.3±3.5‰), indicating that evergreen species had access to the more abundant soil water at greater depth than deciduous species. In evergreen shade-tolerant and high-light requiring shrubs and small trees, δD of xylem water was negatively correlated with transpiration rate and leaf water potential indicating that species using deeper, more abundant water resources had both higher rates of water use and more favorable leaf water status.

Search for High Mass Top Quark Production in pp̄ Collisions at s = 1.8 TeV

Abstract: We present new results on the search for the top quark in p anti-p collisions at S**(1/2) = 1.8 TeV with an integrated luminosity of 13.5 +- 1.6 pb**(-1). We have considered t anti-t production in the Standard Model using electron and muon dilepton decay channels (t anti-t -> e mu + jets, e e + jets, and mu mu + jets) and single-lepton decay channels (t anti-t -> e + jets and mu + jets) with and without tagging of b quark jets. From all channels, we have 9 events with an expected background of 3.8 +- 0.9. If we assume that the excess is due to t anti-t production, and assuming a top mass of 180 GeV/c**2, we obtain a cross section of 8.2 +- 5.1 pb.

Search for squarks and gluinos in pp collisions at s=1.8 TeV

Abstract: We present a search for events consistent with the production and decay of the squarks and gluinos of the minimal supersymmetric standard model (MSSM) in the D0 detector at the Fermilab Tevatron {ital p{bar p}} collider. We examined data for events containing large missing transverse energy and three or more jets. We observed no excess of events above the expected yield from standard model processes. For a choice of MSSM parameter values, we set a lower limit at the 95% confidence level on the mass of the gluino of 144 GeV/{ital c}{sup 2} for all squark masses and a lower limit of 212 GeV/{ital c}{sup 2} for equal squark and gluino masses.

Lithium and body weight gain.

Abstract: Weight gain is an undesirable side-effect of long-term lithium administration which notably interferes with treatment compliance. The mechanisms of this weight gain remain unclear, making its management in patients difficult. In this paper, studies describing the features of this weight gain in patients and in rats treated with chronic lithium administration are reviewed. The effects of lithium on body weight differ between patients and rats in a number of ways, including the observation that excessive weight gain is observed in both male and female patients, but only in female rats. Nevertheless, an animal model of lithium-induced weight gain may be able to provide useful insights into some of the specific mechanisms involved, particularly those related to interactions with gonadal steroid function. We discuss the effects of lithium on the endocrine system, neurotransmitters, metabolism, electrolyte regulation, and feeding behavior, which might underlie lithium's effects on body weight. Finally, suggestions for the management of weight gain in the clinical setting are presented. These include, in the long term, dietary control and physical activity and, in the short term, choosing among several drugs that have been tested either in patients or in animal models of obesity. If weight gain still cannot be controlled and treatment compliance is at risk, the mood stabilizers carbamazepine or valproic acid might be substituted for lithium treatment.

Inclusive and b-quark production cross sections in pp» collisions at s=1.8 TeV

Abstract: We report a measurement of the inclusive muon and {ital b}-quark production cross sections in {ital p{bar p}} collisions at {radical}{ital s}=1.8 TeV using the D0 detector at the Fermilab Tevatron collider. The inclusive muon spectrum extends over the kinematic range {vert_bar}{ital y}{sup {mu}}{vert_bar}{lt}0.8 and 3.5{lt}{ital p}{sub {ital T}}{sup {mu}}{lt}60GeV/{ital c}, and is well described by the expected contributions from various known sources. The {ital b}-quark production cross section for {vert_bar}{ital y}{sup {ital b}}{vert_bar}{lt}1.0 and {ital p}{sup {ital b}}{sub {ital T}}{gt}6GeV/{ital c} is extracted, and agrees with next-to-leading order QCD predictions within the experimental and theoretical uncertainties.

A sliding mode strategy for adaptive learning in Adalines

Abstract: A dynamical sliding mode control approach is proposed for robust adaptive learning in analog Adaptive Linear Elements (Adalines), constituting basic building blocks for perceptron-based feedforward neural networks. The zero level set of the learning error variable is regarded as a sliding surface in the space of learning parameters. A sliding mode trajectory can then be induced, in finite time, on such a desired sliding manifold. Neuron weights adaptation trajectories are shown to be of continuous nature, thus avoiding bang-bang weight adaptation procedures. Sliding mode invariance conditions determine a least squares characterization of the adaptive weights average dynamics whose stability features may be studied using standard time-varying linear systems results. Robustness of the adaptative learning algorithm, with respect to bounded external perturbation signals, and measurement noises, is also demonstrated. The article presents some simulation examples dealing with applications of the proposed algorithm to forward and inverse plant dynamics identification.

Measurement of the $W$ boson mass

Abstract: We present a measurement of the W boson mass in W -> ev decays using 1 fb^-1 of data collected with the D0 detector during Run II of the Fermilab Tevatron collider. With a sample of 499830 W -> ev candidate events, we measure M_W = 80.401 +- 0.043 GeV. This is the most precise measurement from a single experiment.

Leaf-cutting ants and forest groves in a tropical parkland savanna of Venezuela: facilitated succession?

Abstract: Some savanna areas in the Orinoco Llanos region in Venezuela are characterized by the abundance of small forest groves, resulting in a parkland landscape. We hypothesized that forest groves are the result of colonization of the open savanna by forest species, facilitated by the activity of leaf-cutting ants. In this paper we present results from a field study conducted on an ecotonal belt between a deciduous gallery forest and an open savanna in the western Llanos of Venezuela. In seven transects, trees, groves and leaf-cutting ant nests were counted, size parameters evaluated and distance from grove to forest measured. Soil chemical analysis and microclimatic measurements were performed in the forest, grassland and groves. Floristic composition of the woodland component of the savanna (scattered trees) was very dissimilar from that of the forest, but the composition of the groves was essentially a mixture of both woodland and forest species. Woodland trees were present in all groves, whereas forest trees were present only in larger groves. The size structures of populations of the two types of trees differed significandy according to the size of the grove; also, woodland tree population structure differed significantly between open savanna and groves. The frequency of groves with Atta laevigata nests increased with the size of the grove, and groves with forest trees were positively associated with the presence of nests. A. laevigata nests improved the soil of groves, increasing N, Mg, Ca and organic carbon, but other soil properties were not modified. Temperatures at ground level in the dry season were ameliorated in the groves compared with the grassland, and this effect increased with the size of the grove. We concluded that a dynamic process of grove formation is facilitated by the aggregation of a few woodland trees, followed by the successful invasion of forest trees and a leaf-cutting ant nest. The results did not show any clear sequence of arrival of the ants or the forest trees, but it seems clear that these two would act synergistically, improving conditions for each other.

Symmetry nonrestoration at high temperature and the monopole problem.

Abstract: We show that there exists a range of parameters in SU(5) theory for which the grand unified theory symmetry remains broken at high temperature, thus avoiding the phase transition that gives rise to the overproduction of monopoles. The thermal production of monopoles can be naturally suppressed, keeping their number density below the cosmological limits. {copyright} {ital 1995 The American Physical Society.}

Solubilization of polar oils in microemulsion systems

Abstract: A new type of amphiphile that contains both conventional surfactant and lipophilic linker features in a single molecule was designed and tested. In these so-called extended surfactants, a polypropylene oxide chain of variable length is inserted in between the conventional polar and apolar groups.

Top quark search with the D0 1992 - 1993 data sample

Abstract: We present results on the search for the top quark in {ital p{bar p}} collisions at {radical}{ital s} =1.8 TeV with an integrated luminosity of 13.5{plus_minus}1.6 pb{sup {minus}1}. We have considered {ital t{bar t}} production in the standard model using electron and muon dilepton decay channels ({ital t{bar t}}{r_arrow}{ital e}{mu}+jets, {ital ee}+jets, and {mu}{mu}+jets) and single-lepton decay channels ({ital t{bar t}}{r_arrow}{ital e}+jets and {mu}+jets) with and without tagging of {ital b} quark jets. An analysis of these data optimized for top quark masses below 140 GeV/{ital c}{sup 2} gives a lower top quark mass limit of 128 GeV/{ital c}{sup 2}. An analysis optimized for higher top quark masses yields 9 events with an expected background of 3.8{plus_minus}0.9. If we assume that the excess is due to {ital t{bar t}} production, and assuming a top quark mass of 180 GeV/{ital c}{sup 2}, we obtain a cross section of 8.2{plus_minus}5.1 pb.

On the Dielectric Constants of AIBIIIC2VI Chalcopyrite Semiconductor Compounds

Abstract: Values ofthe static, e 0 , and high-frequency, e∞, dielectric constants of I-III-VI 2 chalcopyrite compounds are estimated by using an empirical model proposed by Nag for cubic semiconductors, which relates these constants to the average atomic number of its constituent atoms Z av . It is found, as in the cubic semiconductors, that the inverse of both these constants is linearly related to Z aV . It is also suggested that e 0 and e∞ thus obtained should be more reliable than those calculated from the energy gap Eg, due to the wide range of values of this parameter reported in some I-III-VI 2 chalcopyrite compounds.

Determinants of dirac boundary value problems over odd-dimensional manifolds

Abstract: We present a canonical construction of the determinant of an elliptic selfadjoint boundary value problem for the Dirac operatorD over an odd-dimensional manifold. For 1-dimensional manifolds we prove that this coincides with the ζ-function determinant. This is based on a result that elliptic self-adjoint boundary conditions forD are parameterized by a preferred class of unitary isomorphisms between the spaces of boundary chiral spinor fields. With respect to a decompositionS 1=X 0∪X 1, we explain how the determinant of a Dirac-type operator overS 1 is related to the determinants of the corresponding boundary value problems overX 0 andX 1.

Two-dimensional sections of the yield locus of a Ti6%Al4%V alloy with a strong transverse-type crystallographic α-texture

Abstract: Results obtained from mechanical tests on a Ti6%Al4%V sheet alloy were used to obtain the biaxial normal stress and the in-plane simple shear flow stress sections of its yield locus The critical resolved shear stress (CRSS) of the different activated slip systems is derived from these experimental results of mechanical tests combined with microstructural observations A strong asymmetry of the CRSS and some influence on it of the hydrostatic stress state was found for pyramidal 〈 c + a 〉 systems, ie a deviation from the Schmid law Owing to the high sharpness of the texture in this alloy, a quasi-single-crystal approximation of the plastic anisotropy on the rolling plane (RP) and of the yield locus sections measured was made The good agreement between the simulated and the experimental results encourages the use of the proposed CRSS values for a simulation of polycrystalline texture development and yield locus derivation

Potential effects of global climatic change on the phenology and yield of maize in venezuela

Abstract: Simulated impacts of global and regional climate change, induced by an enhanced greenhouse effect and by Amazonian deforestation, on the phenology and yield of two grain corn cultivars in Venezuela (CENIAP PB-8 and OBREGON) are reported. Three sites were selected:Turen, Barinas andYaritagua, representing two important agricultural regions in the country. The CERES-Maize model, a mechanistic process-based model, in theDecision Support System for Agrotechnology Transfer (DSSAT) was used for the crop simulations. These simulations assume non-limiting nutrients, no pest damage and no damage from excess water; therefore, the results indicate only the difference between baseline and perturbed climatic conditions, when other conditions remain the same. Four greenhouse-induced global climate change scenarios, covering different sensitivity levels, and one deforestation-induced regional climate change scenario were used. The greenhouse scenarios assume increased air temperature, increased rainfall and decreased incoming solar radiation, as derived from atmospheric GCMs for doubled CO2 conditions. The deforestation scenarios assume increased air temperature, increased incoming solar radiation and decreased rainfall, as predicted by coupled atmosphere-biosphere models for extensive deforestation of a portion of the Amazon basin. Two baseline climate years for each site were selected, one year with average precipitation and another with lower than average rainfall. Scenarios associated with the greenhouse effect cause a decrease in yield of both cultivars at all three sites, while the deforestation scenarios produce small changes. Sensitivity tests revealed the reasons for these responses. Increasing temperatures, especially daily maximum temperatures, reduce yield by reducing the duration of the phenological phases of both cultivars, as expected from CERES-Maize. The reduction of the duration of the kernel filling phase has the largest effect on yield. Increases of precipitation associated with greenhouse warming have no effects on yield, because these sites already have adequate precipitation; however, the crop model used here does not simulate potential negative effects of excess water, which could have important consequences in terms of soil erosion and nutrient leaching. Increases in solar radiation increased yields, according to the non-saturating light response of the photosynthesis rate of a C4 plant like corn, compensating for reduced yields from increased temperatures in deforestation scenarios. In the greenhouse scenarios, reduced insolation (due to increased cloud cover) and increased temperatures combine to reduce yields; a combination of temperature increase with a reduction in solar radiation produces fewer and lighter kernels.

Analytic results for N particles with 1/r2 interaction in two dimensions and an external magnetic field

Abstract: The $2N$-dimensional quantum problem of $N$ particles (e.g., electrons) with interaction $\ensuremath{\beta}/{r}^{2}$ in a two-dimensional parabolic potential ${\ensuremath{\omega}}_{0}$ (e.g., quantum dot), and magnetic field $B$, reduces exactly to solving a $(2N\ensuremath{-}4)$-dimensional problem which is independent of $B$ and ${\ensuremath{\omega}}_{0}$. An exact, infinite set of relative mode excitations are obtained for any $N$. The $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3$ problem reduces to that of a fictitious particle in a two-dimensional, nonlinear potential of strength $\ensuremath{\beta}$, subject to a fictitious magnetic field ${B}_{\mathrm{fic}}\ensuremath{\propto}J$, the relative angular momentum.

Constitutional design and economic performance

Abstract: This paper is motivated by the belief that some cultural traits favor economic performance more than others. One trait examined is the ease with which individuals in a community drift away from the spirit of the law for their own benefit; this, it is argued, generates verbose legislation and high-transaction-cost institutions with deleterious effects on economic performance. An empirical comparison between the number of articles in a country's constitution, as a proxy for length and lack of simplicity, and economic performance as measured by GDP per capita finds that no country with a high GDP per capita has a long constitution or, restated, that long constitutions are invariably associated with low levels of GDP per capita.

W and Z boson production in pp̄ collisions at s=1.8 TeV

Abstract: The inclusive cross sections times leptonic branching ratios for $W$ and $Z$ boson production in $p\overline{p}$ collisions at $\sqrt{s}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.8$ TeV were measured using the D0 detector at the Fermilab Tevatron collider: ${\ensuremath{\sigma}}_{W}B(W\ensuremath{\rightarrow}e\ensuremath{ u})\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.36\ifmmode\pm\else\textpm\fi{}0.07\ifmmode\pm\else\textpm\fi{}0.13\mathrm{nb}$, ${\ensuremath{\sigma}}_{W}B\left(W\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{ u}\right)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.09\ifmmode\pm\else\textpm\fi{}0.23\ifmmode\pm\else\textpm\fi{}0.11\mathrm{nb}$, ${\ensuremath{\sigma}}_{Z}B\left(Z\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}\right)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.218\ifmmode\pm\else\textpm\fi{}0.011\ifmmode\pm\else\textpm\fi{}0.012\mathrm{nb}$, and ${\ensuremath{\sigma}}_{Z}B\left(Z\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}\right)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.178\ifmmode\pm\else\textpm\fi{}0.030\ifmmode\pm\else\textpm\fi{}0.009\mathrm{nb}$. The first error is the combined statistical and systematic uncertainty, and the second reflects the uncertainty in the luminosity. For the combined electron and muon analyses we find ${\ensuremath{\sigma}}_{W}B(W\ensuremath{\rightarrow}l\ensuremath{ u})/{\ensuremath{\sigma}}_{Z}B(Z\ensuremath{\rightarrow}{l}^{+}{l}^{\ensuremath{-}})\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}10.90\ifmmode\pm\else\textpm\fi{}0.49$. Assuming standard model couplings, this result is used to determine the width of the $W$ boson, $\ensuremath{\gamma}(W)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.044\ifmmode\pm\else\textpm\fi{}0.093\mathrm{GeV}$.

Review of Colombian Conchostraca (Crustacea) — ecological aspects and life cycles — family Cyclestheriidae

Abstract: This study gives an overview and describes special aspects of the biology, ecology and the life cycle of Cyclestheria hislopi (Baird, 1859). This species is most commonly found in parthenogenically reproducing populations which produce large, directly developing nondiapause eggs. But periodically and under certain environmental conditions, sexually reproducing generations appear and produce diapause eggs. The sexual generations include males and particularly constituted females, which undergo a complete transformation into a special type of ephippium. Cyclestheria is the only known conchostracan species which does not occur in the initial phases of temporary water bodies, but develops in older temporary pools and even in permanent waters. It survives in the presence of effective depredators like fish by hiding within a special self-made mucus capsule.

Selective determination of antimony(III) and antimony(V) in liver tissue by microwave-assisted mineralization and hydride generation atomic absorption spectrometry.

Abstract: Antimony(III) and antimony(V) species have been selectively determined in liver tissues by optimizing the acidic conditions for the evolution of stibine using the reduction with sodium borohydride. The results show that a response for Sb(III) of 0.5 to 20 microg l(-1) was selectively obtained from samples in a 1 mol l(-1) acetic acid medium. The best response for total antimony from 1 to 20 microg l(-1) is obtained after sample treatment with a 0.5 mol l(-1) sulfuric acid and 10% w/v potassium iodide. Microwave digestion has been necessary to release quantitatively antimony species from sample slurries. The amount of Sb(V) was calculated from the difference between the value for total antimony and Sb(III) concentrations. A relative standard deviation from 2.9 to 3.1% and a detection limit of 0.15 and 0.10 microg l(-1) for Sb(III) and total Sb has been obtained. The average accuracy exceeded 95% in all cases comparing the results obtained from recovery studies, electrothermal atomic absorption spectrometry and the analysis of certified reference materials.