Can savannas become forests? A coupled analysis of nutrient stocks and fire thresholds in central Brazil

Aims: The effects of fire ensure that large areas of the seasonal tropics are maintained as savannas. The advance of forests into these areas depends on shifts in species composition and the presence of sufficient nutrients. Predicting such transitions, however, is difficult due to a poor understanding of the nutrient stocks required for different combinations of species to resist and suppress fires. Methods: We compare the amounts of nutrients required by congeneric savanna and forest trees to reach two thresholds of establishment and maintenance: that of fire resistance, after which individual trees are large enough to survive fires, and that of fire suppression, after which the collective tree canopy is dense enough to minimize understory growth, thereby arresting the spread of fire. We further calculate the arboreal and soil nutrient stocks of savannas, to determine if these are sufficient to support the expansion of forests following initial establishment. Results: Forest species require a larger nutrient supply to resist fires than savanna species, which are better able to reach a fire-resistant size under nutrient limitation. However, forest species require a lower nutrient supply to attain closed canopies and suppress fires; therefore, the ingression of forest trees into savannas facilitates the transition to forest. Savannas have sufficient N, K, and Mg, but require additional P and Ca to build high-biomass forests and allow full forest expansion following establishment. Conclusions: Tradeoffs between nutrient requirements and adaptations to fire reinforce savanna and forest as alternate stable states, explaining the long-term persistence of vegetation mosaics in the seasonal tropics. Low-fertility limits the advance of forests into savannas, but the ingression of forest species favors the formation of non-flammable states, increasing fertility and promoting forest expansion. © 2013 Springer Science+Business Media Dordrecht.

Theoretical and experimental study of the excitonic binding energy in GaAs/AlGaAs single and coupled double quantum wells

This paper discusses the theoretical and experimental results obtained for the excitonic binding energy (Eb) in a set of single and coupled double quantum wells (SQWs and CDQWs) of GaAs/AlGaAs with different Al concentrations (Al%) and inter-well barrier thicknesses. To obtain the theoretical Eb the method proposed by Mathieu, Lefebvre and Christol (MLC) was used, which is based on the idea of fractional-dimension space, together with the approach proposed by Zhao et al., which extends the MLC method for application in CDQWs. Through magnetophotoluminescence (MPL) measurements performed at 4 K with magnetic fields ranging from 0 T to 12 T, the diamagnetic shift curves were plotted and adjusted using two expressions: one appropriate to fit the curve in the range of low intensity fields and another for the range of high intensity fields, providing the experimental Eb values. The effects of increasing the Al% and the inter-well barrier thickness on E b are discussed. The Eb reduction when going from the SQW to the CDQW with 5 Å inter-well barrier is clearly observed experimentally for 35% Al concentration and this trend can be noticed even for concentrations as low as 25% and 15%, although the Eb variations in these latter cases are within the error bars. As the Zhao's approach is unable to describe this effect, the wave functions and the probability densities for electrons and holes were calculated, allowing us to explain this effect as being due to a decrease in the spatial superposition of the wave functions caused by the thin inter-well barrier. © 2013 Elsevier B.V.

Gap solitons in a spin-orbit-coupled bose-einstein condensate

We report a diversity of stable gap solitons in a spin-orbit-coupled Bose-Einstein condensate subject to a spatially periodic Zeeman field. It is shown that the solitons can be classified by the main physical symmetries they obey, i.e., symmetries with respect to parity (P), time (T), and internal degree of freedom, i.e., spin (C), inversions. The conventional gap and gap-stripe solitons are obtained in lattices with different parameters. It is shown that solitons of the same type but obeying different symmetries can exist in the same lattice at different spatial locations. PT and CPT symmetric solitons have antiferromagnetic structure and are characterized, respectively, by nonzero and zero total magnetizations. © 2013 American Physical Society.

Estudo analítico/numérico do problema de ablação em corpos rombudos com simetria axial

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Bancada experimental para ensaios em geradores elétricos utilizados em aerogeradores de pequeno porte

Este trabalho apresenta uma bancada para testes em geradores elétricos, utilizados em aerogeradores de pequeno porte. O sistema é composto basicamente por um motor de indução trifásico, que simula um rotor eólico, acoplado mecanicamente ao gerador elétrico submetido ao ensaio; um inversor de frequência, usado para variação da velocidade do motor; transdutores de grandezas elétricas, que efetuam a medição dos parâmetros de interesse (tensões e correntes instantâneas) na saída do gerador; um sensor de velocidade angular, para medição da rotação que é fornecida ao eixo do motor; um microcomputador, para monitoração e armazenamento dos parâmetros medidos, e controle do inversor de frequência, por meio de um programa computacional desenvolvido; e uma placa de aquisição de dados, que serve como interface entre os transdutores/sensores e o microcomputador. A velocidade angular fornecida ao gerador em ensaio pode ser variada conforme valores fornecidos de velocidades de vento, as quais são associadas a valores correspondentes de rotação, a fim de simular o comportamento de um rotor eólico. Dessa forma, é possível traçar a curva de potência (potência versus velocidade de vento) do gerador ensaiado e analisar seu desempenho. Apresenta-se também neste trabalho os resultados de medições realizadas na bancada, referentes aos ensaios de um gerador comercial de pequena potência (1 kW) e de outro desenvolvido em projeto de pesquisa, ambos do tipo de imã permanente com fluxo axial.

Desenvolvimento de um conjunto de modelagem de alta resolução para os métodos eletromagnéticos indutivos

A Modelagem Eletromagnética Analógica tem sido uma poderosa ferramenta no estudo das respostas eletromagnéticas de uma grande variedade das técnicas de levantamentos e de modelos geológicos. Nas aplicações de métodos eletromagnéticos nas investigações de contaminação no subsolo, recentemente em grande demanda, são obtidas anomalias muito baixas (da ordem de 0,01% do campo primário). Com o objetivo de estudar em laboratório os problemas de levantamentos eletromagnéticos para alvos de baixo número de indução, um conjunto de modelagem em escala reduzida de alta sensibilidade, foi projetado e construído. Este conjunto opera na faixa de freqüência de 1 kHz até 1 MHz com excelente linearidade. O transdutor de transmissão gera suficiente densidade de fluxo magnético com corrente de até 3 App (pico a pico) circulando nele. O sistema de recepção, isto é, o conjunto de bobinas receptoras mais o pré-amplificador acoplado apresenta sensibilidade 3 V/A/m na freqüência de 10 kHz com inclinação de 20 dB/dec de freqüência. A alta sensibilidade do sistema de recepção combinada com a boa capacidade de geração do sistema transmissor, permite alcançar a resolução de 0,025% do campo primário. O conjunto dispõe ainda, de um sistema mecânico que permite posicionar o sistema de transdutores (transmissor e receptor) em posições arbitrárias com resolução de 1 mm e precisão de 0,05 mm. A precisão no deslocamento linear do sistema de sonda numa linha de 1715 mm de comprimento é alcançada por meio de um motor de passo controlado por um microcontrolador, que é rigidamente acoplado a um fuso que converte o movimento angular do motor em movimento linear. Com este conjunto, é possível simular uma grande variedade de técnicas de levantamentos eletromagnéticos no formato Slingram, isto é, dipolo-dipolo. Além da boa resolução mecânica e boa resolução de medida eletromagnética do conjunto, todo o processo de controle e aquisição de dados é operacionalizado por meio de um computador mestre.

A simple and rapid screening method for sulfonamides in honey using a flow injection system coupled to a liquid waveguide capillary cell

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Localization of a spin-orbit-coupled Bose-Einstein condensate in a bichromatic optical lattice

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

A new closed-vessel conductively heated digestion system: fostering plant analysis by inductively coupled plasma optical emission spectroscopy

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Identification of species of the Euterpe genus by rare earth elements using inductively coupled plasma mass spectrometry and linear discriminant analysis

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Flame retardant properties of the bark powder of Anadenanthera peregrina var. falcata (Benth.) Altschul (angico) studied by coupled thermogravimetry-Fourier transform infrared spectroscopy

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

THE PARASITOID, Cotesia flavipes (CAMERON) (HYMENOPTERA: BRACONIDAE), INFLUENCES FOOD CONSUMPTION AND UTILIZATION BY LARVAL Diatraea saccharalis (F.) (LEPIDOPTERA: CRAMBIDAE)

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

The dynamic behavior of a cantilever beam coupled to a non-ideal unbalanced motor through numerical and experimental analysis

An excitation force that is not influenced by the system state is said to be an ideal energy source. In real situations, a direct and feedback coupling between the excitation source and the system must always exist at a certain level. This manifestation of the law of conservation of energy is known as the Sommerfeld effect. In the case of obtaining a mathematical model for such a system, additional equations are usually necessary to describe the vibration sources with limited power and its coupling with the mechanical system. In this work, a cantilever beam and a non-ideal DC motor fixed to its free end are analyzed. The motor has an unbalanced mass that provides excitation to the system which is proportional to the current applied to the motor. During the coast up operation of the motor, if the drive power is increased slowly, making the excitation frequency pass through the first natural frequency of the beam, the DC motor speed will remain the same until it suddenly jumps to a much higher value (simultaneously its amplitude jumps to a much lower value) upon exceeding a critical input power. It was found that the Sommerfeld effect depends on some system parameters and the motor operational procedures. These parameters are explored to avoid the resonance capture in the Sommerfeld effect. Numerical simulations and experimental tests are used to help gather insight of this dynamic behavior. (C) 2014 Elsevier Ltd. All rights reserved.

Vacuum polarization by a magnetic flux in a cosmic string background.

In this paper we analyse the vacuum polarization effects due to a magnetic flux on massless fermionic fields in a cosmic string background. Three distinct configurations of magnetic fields are considered. In all of them the magnetic fluxes are confined in a long cylindrical tube of finite radius.

High-throughput sugarcane leaf analysis using a low cost closed-vessel conductively heated digestion system and inductively coupled plasma optical emission spectroscopy

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)