Functional analysis of secondary tropical dry forests in a region of the Colombia, Caribbean

Secondary tropical forests are increasingly recognized for their role conserving biodiversity in agricultural landscapes and this role is especially important for seasonally dry tropical forests (SDTF), one of the most threatened tropical forested ecosystems. The conservation value of secondary forest is increased by its capacity to maintain ecosystem properties and provide services to humans; which has been hypothesized to have positive links to the species and functional diversity of ecosystems. However very little information is available on the occurrence of this relationship in secondary forests. This dissertation makes an important contribution to the ecological knowledge of secondary SDTF and describes changes in plant species and functional diversity by using a stratified design considering different successional stages along an environmental gradient in a region of the Caribbean coast of Colombia and a multi-trait approach to study functional diversity at three scales: species, communities and landscape. The analysis of the variation in functional traits of SDTF trees at the species level allowed me to support the hypothesis of coordination between leaves and stem traits. Three dimensions of correlated variation were identified: the first related to leaf and stem economy, the second to leaf thickness and organization and the third to the trade-offs between leaf size, stem density and bark thickness. Secondary forests showed high species turnover during succession and increasing diversity from early to late forests. Species composition similarity was higher among late successional forest than early and intermediate stage forests, showing that environmental characteristics are influencing successional trajectories. Frequency distributions of species in the three successional stages showed evidence of functional trait similarity among species and underlined the importance of changes in species abundances determining functional composition during succession. A shift in abundance from individuals with traits that favour survival after disturbance to individuals with denser stems and a more conservative resource use profile was observed from early to late stages of succession. Functional composition was also strongly influenced by environmental variables, especially leaf traits, and a shift of traits from acquisitive to conservative type species was observed with increasing nutrient content and flooding, proxies of water availability. Assessment of ecosystem services was conducted using two independent approaches: semistructured interviews and field data. A high richness of useful woody plants was recorded and the provision of services was related to a high variability in functional traits among species. The provision of the ecosystem services determined by the presence and abundance of useful species showed significant differences between stages. The relationships observed between ecosystem sevices and functional and species diversity indices were not consistent. In contrast species richess showed significant negative relationships at the plot level with most of the ecosystem services assessed, showing a trade-off among the conservation of species richness and the maintenance of service provision. Overall, this research provides novel insights into the dynamic relationships between biodiversity, ecosystem function and ecosystem services in this globally important, but under-researched forest type

Grupos funcionales y variación en rasgos foliares y de tallo de 113 especies leñosas en un bosque seco tropical

In seasonally dry tropical forest (SDTF) plant trait variation has been associated with adaptation to light and water availability and a coordinated tolerance of plants to shortage of both resources has been proposed. We tested this hypothesis in a set of 113 species by analyzing the relationships amongst eleven leaf and stem traits that have been related to shade and drought tolerance. In addition, the usefulness of different types of functional classifications describing community plant trait variation was evaluated. Strong relationships were observed between leaf and stem traits, potentially conferring coordinated tolerance to shade and drought, and three axes of variation were identified by means of a principal component analysis. The first axis described leaf and stem economy, the second was related to leaf thickness and organization and the third was related to the trade-offs between leaf size, stem density and bark thickness. Stem density was correlated strongly with several plant traits, emphasizing its key role in explaining variation in life history strategies of SDTF species. Significant differences were found between functional groups categorized by phylogeny and leaf phenology, whereas for life forms differences were only observed for palms and bamboos. En los bosques secos tropicales (BST) la variación en rasgos vegetales se ha relacionado con adaptaciones a cambios en la disponibilidad de luz y agua y se ha propuesto una tolerancia coordinada a la escasez de ambos recursos. En este estudio probamos dicha hipótesis en un conjunto de 113 especies mediante el análisis de la relación entre once rasgos funcionales de hojas y tallos, que se han asociado con la tolerancia a la sombra y la sequía. Adicionalmente, evaluamos la utilidad de diferentes clasificaciones funcionales para describir la variación de los rasgos en la comunidad de plantas estudiada. Se observaron relaciones fuertes entre rasgos de las hojas y el tallo, otorgando potencialmente tolerancia a la sombra y la sequía, además se identificaron tres ejes de variación a través de un análisis de componentes principales. El primer eje se relacionó con la economía de hojas y tallo, el segundo con el grosor y la organización de las hojas y el tercero con compromisos entre el tamaño de hoja, la densidad del tallo y el grosor de la corteza. La densidad del tallo se relacionó fuertemente con varios rasgos, enfatizando su papel clave explicando la variación en las estrategias de historia de vida de las especies de BST. Se observaron diferencias significativas entre grupos funcionales categorizados por filogenia y fenología, mientras que entre formas de vida solo se observaron diferencias para las palmas y guaduas

Electric Field Effects on Graphene Materials

Understanding the effect of electric fields on the physical and chemical properties of two-dimensional (2D) nanostructures is instrumental in the design of novel electronic and optoelectronic devices. Several of those properties are characterized in terms of the dielectric constant which play an important role on capacitance, conductivity, screening, dielectric losses and refractive index. Here we review our recent theoretical studies using density functional calculations including van der Waals interactions on two types of layered materials of similar two-dimensional molecular geometry but remarkably different electronic structures, that is, graphene and molybdenum disulphide (MoS$_2$). We focus on such two-dimensional crystals because of they complementary physical and chemical properties, and the appealing interest to incorporate them in the next generation of electronic and optoelectronic devices. We predict that the effective dielectric constant ($\varepsilon$) of few-layer graphene and MoS$_2$ is tunable by external electric fields ($E_{\rm ext}$). We show that at low fields ($E_{\rm ext}^{}<0.01$ V/\AA) $\varepsilon$ assumes a nearly constant value $\sim$4 for both materials, but increases at higher fields to values that depend on the layer thickness. The thicker the structure the stronger is the modulation of $\varepsilon$ with the electric field. Increasing of the external field perpendicular to the layer surface above a critical value can drive the systems to an unstable state where the layers are weakly coupled and can be easily separated. The observed dependence of $\varepsilon$ on the external field is due to charge polarization driven by the bias, which show several similar characteristics despite of the layer considered.Comment: Invited book chapter on Exotic Properties of Carbon Nanomatter: Advances in Physics and Chemistry, Springer Series on Carbon Materials. Editors: Mihai V. Putz and Ottorino Ori (11 pages, 4 figures, 30 references

Design, modelling and optimisation of a small-scale Solar Organic Rankine Cycle system for rural power generation

This research study develops the design and model of a Solar Organic Rankine Cycle (SORC) coupled to a bio-digester for small-scale generation in rural areas, in Betulia, Colombia. Moreover, the model is optimised employing a Genetic Algorithm with the software Matlab and the thermodynamic library CoolProp. The objective variables were the mass flow rate of the working fluid, the pressure and temperature of the expander inlet, the solar collectors’ type and the temperature of the water circuit for the bio-digester. The results indicate an overall efficiency between 8.42 and 9.45% with a Levelized Cost of Energy (LCE) between 3.85 and 5.63 £/W. Additionally, the power output is directly related to the mass flow rate of the working fluid. Likewise, increasing the scale of the SORC decreases the LCE. Finally, the results suggest that a superheated fluid reduces the efficiency and the LCE and can deliver more heat to the bio-digester. It is advisable the utilisation of a scroll expander and a counter-flow plate exchanger with a Direct Vapour Generation configuration. The model is a flexible tool capable of integrating more equations and components, with the evaluation of different fitness functions

Vertical Field Effect Transistor based on Graphene-WS2 Heterostructures for flexible and transparent electronics

The celebrated electronic properties of graphene have opened way for materials just one-atom-thick to be used in the post-silicon electronic era. An important milestone was the creation of heterostructures based on graphene and other two-dimensional (2D) crystals, which can be assembled in 3D stacks with atomic layer precision. These layered structures have already led to a range of fascinating physical phenomena, and also have been used in demonstrating a prototype field effect tunnelling transistor - a candidate for post-CMOS technology. The range of possible materials which could be incorporated into such stacks is very large. Indeed, there are many other materials where layers are linked by weak van der Waals forces, which can be exfoliated and combined together to create novel highly-tailored heterostructures. Here we describe a new generation of field effect vertical tunnelling transistors where 2D tungsten disulphide serves as an atomically thin barrier between two layers of either mechanically exfoliated or CVD-grown graphene. Our devices have unprecedented current modulation exceeding one million at room temperature and can also operate on transparent and flexible substrates