Tratamiento numérico de materiales compuestos mediante la teoría de homogeneización

El presente trabajo se emmarca dentro de las teorías y métodos que hacen posible la representación por ordenador de los materiales compuesto. Como se sabe, la búsqueda de leyes matemáticas que determinan el comportamiento de esta clase de materiales ha sido objeto de estudio durante varias décadas. Pero, su elavada dificultad conduce a elaborar complejas teorías; o por el contrario, se utlizan grandes simplificaciones cuyo resultado genera modelos poco acertados.Postprint (published version

X-ray flares from propagation instabilities in long Gamma-Ray Burst jets

We present a numerical simulation of a gamma-ray burst jet from a long-lasting engine in the core of a 16 solar mass Wolf-Rayet star. The engine is kept active for 6000 s with a luminosity that decays in time as a power-law with index -5/3. Even though there is no short time-scale variability in the injected engine luminosity, we find that the jet's kinetic luminosity outside the progenitor star is characterized by fluctuations with relatively short time scale. We analyze the temporal characteristics of those fluctuations and we find that they are consistent with the properties of observed flares in X-ray afterglows. The peak to continuum flux ratio of the flares in the simulation is consistent with some, but not all, the observed flares. We propose that propagation instabilities, rather than variability in the engine luminosity, are responsible for the X-ray flares with moderate contrast. Strong flares such as the one detected in GRB 050502B, instead, cannot be reproduced by this model and require strong variability in the engine activity.Comment: 6 pages, MNRAS in pres

Water Availability Is the Main Climate Driver of Neotropical Tree Growth

• Climate models for the coming century predict rainfall reduction in the Amazonian region, including change in water availability for tropical rainforests. Here, we test the extent to which climate variables related to water regime, temperature and irradiance shape the growth trajectories of neotropical trees. • We developed a diameter growth model explicitly designed to work with asynchronous climate and growth data. Growth trajectories of 205 individual trees from 54 neotropical species censused every 2 months over a 4-year period were used to rank 9 climate variables and find the best predictive model. • About 9% of the individual variation in tree growth was imputable to the seasonal variation of climate. Relative extractable water was the main predictor and alone explained more than 60% of the climate effect on tree growth, i.e. 5.4% of the individual variation in tree growth. Furthermore, the global annual tree growth was more dependent on the diameter increment at the onset of the rain season than on the duration of dry season. • The best predictive model included 3 climate variables: relative extractable water, minimum temperature and irradiance. The root mean squared error of prediction (0.035 mm.d–1) was slightly above the mean value of the growth (0.026 mm.d–1). • Amongst climate variables, we highlight the predominant role of water availability in determining seasonal variation in tree growth of neotropical forest trees and the need to include these relationships in forest simulators to test, in silico, the impact of different climate scenarios on the future dynamics of the rainforest

Source water, phenology and growth of two tropical dry forest tree species growing on shallow karst soils

Seasonally dry tropical forests are dominated by deciduous and evergreen tree species with a wide range of leaf phenology. We hypothesized that Piscidia piscipula is able to extend leaf senescence until later in the dry season due to deeper and more reliable water sources than Gymnopodium floribundum, which loses leaves earlier in the dry season. Physiological performance was assessed as timing of leaf production and loss, growth, leaf water potential, depth of water uptake determined by stable isotopes, and leaf stable isotopic composition of carbon (δ¹³C) and oxygen (δ¹⁸O). P. piscipula took water primarily from shallow sources, whereas G. floribundum took water from shallow and deep sources. The greatest variation in water sources occurred during the onset of the dry season, when G. floribundum was shedding old leaves and growing new leaves, but P. piscipula maintained its leaves from the previous wet season. P. piscipula showed greater relative growth rate, greater leaf expansion rates, and more negative predawn and midday water potentials than G. floribundum. P. piscipula also exhibited greater leaf organic δ¹³C and lower δ¹⁸O values, indicating that the decrease in photosynthetic carbon isotope discrimination was associated with greater stomatal conductance and greater photosynthesis. Our results indicate that the contrasting early and late dry season leaf loss phenology of these two species is not simply determined by rooting depth, but rather a more complicated suite of characteristics based on opportunistic use of dynamic water sources, maximizing carbon gain, and maintenance of water potential during the dry season

Termite sensitivity to temperature affects global wood decay rates.

Deadwood is a large global carbon store with its store size partially determined by biotic decay. Microbial wood decay rates are known to respond to changing temperature and precipitation. Termites are also important decomposers in the tropics but are less well studied. An understanding of their climate sensitivities is needed to estimate climate change effects on wood carbon pools. Using data from 133 sites spanning six continents, we found that termite wood discovery and consumption were highly sensitive to temperature (with decay increasing >6.8 times per 10°C increase in temperature)-even more so than microbes. Termite decay effects were greatest in tropical seasonal forests, tropical savannas, and subtropical deserts. With tropicalization (i.e., warming shifts to tropical climates), termite wood decay will likely increase as termites access more of Earth's surface

What is in a name? That which we call cecropia peltata by any other name would be as invasive?

The recent opinion piece by Sheil and Padmanaba (2011) argues that greater attention is required for invasive species management procedures that are relevant to and realistic for developing countries. They use the example of the Neotropical tree Cecropia as an introduction toWest Java to illustrate their point. In our invited response we contend that the assumptions and data on the dynamics of Cecropia in Java presented in their paper, as well as their review of global Cecropia introductions, are of reduced scientific value. Even so, we agree with the paper’s opinion that the naturalised species of Cecropia in West Java represent a considerable invasion risk and that funding must be improved so that the capacity for invasion ecology research and management of invasive alien species in developing countries is more effective. Unlike Sheil and Padmanaba (2011), however, we conclude that there is already enough evidence to be concerned by the threat of Cecropia to natural ecosystems, but that knowledge of the relevant taxa is currently insufficient to recommend the most appropriate control options not only for Java, but also for other Cecropia introductions elsewhere in the world. © 2011 Botanical Society of Scotland and Taylor & Francis

Seed polyphenols in a diverse tropical plant community

1. Polyphenols are one of the most common groups of secondary metabolites in plants and thought to play a key role in enhancing plant fitness by protecting plants against enemies. Although enemy-inflicted mortality at the seed stage can be an important regulator of plant populations and a key determinant of community structure, few studies have assessed community-level patterns of polyphenol content in seeds. 2. We describe the distribution of the main seed polyphenol groups across 196 tree and liana species on Barro Colorado Island (Panama) and community-level patterns in two aspects of their biological activity (protein precipitation and oxidative capacity). Taking advantage of substantial variation in morphological and ecological traits in the studied plant community, we test for correlations and trade-offs between seed polyphenols and non-chemical plant traits hypothesised to make plant species more or less likely to invest in polyphenol production. 3. The majority of species have polyphenols in their seeds. The incidence and concentrations of polyphenols were related to a set of non-chemical plant traits. Polyphenols were most likely to be present (and where present, to be expressed in high concentrations) in species with large seeds, short seed dormancy times, low investment in mechanical seed defences, high wood density, high leaf mass per area, tough leaves, and slow growth rates. 4. Synthesis: Our study reveals a potential trade-off between chemical and mechanical seed defences and shows that plant species that invest in physical defences at later life stages (high wood density and tough leaves) tend not to invest in physical defences of seeds but instead produce secondary metabolites likely to act as seed defences. Overall, our results conform to predictions from the resource availability hypothesis, which states that species in resource-limited environments (such as slow-growing shade-tolerant tree species) will invest more in defences than fast-growing pioneer species