1,721,160 research outputs found
Technical Note: Approximate Bayesian parameterization of a process-based tropical forest model
No full textInverse parameter estimation of process-based models is a long-standing
problem in many scientific disciplines. A key question for inverse parameter
estimation is how to define the metric that quantifies how well model
predictions fit to the data. This metric can be expressed by general cost or
objective functions, but statistical inversion methods require a particular
metric, the probability of observing the data given the model parameters,
known as the likelihood.
For technical and computational reasons, likelihoods for process-based
stochastic models are usually based on general assumptions about variability
in the observed data, and not on the stochasticity generated by the model.
Only in recent years have new methods become available that allow the
generation of likelihoods directly from stochastic simulations. Previous
applications of these approximate Bayesian methods have concentrated on
relatively simple models. Here, we report on the application of a
simulation-based likelihood approximation for FORMIND, a parameter-rich
individual-based model of tropical forest dynamics.
We show that approximate Bayesian inference, based on a parametric likelihood
approximation placed in a conventional Markov chain Monte Carlo (MCMC)
sampler, performs well in retrieving known parameter values from virtual
inventory data generated by the forest model. We analyze the results of the
parameter estimation, examine its sensitivity to the choice and aggregation
of model outputs and observed data (summary statistics), and demonstrate the
application of this method by fitting the FORMIND model to field data from an
Ecuadorian tropical forest. Finally, we discuss how this approach differs
from approximate Bayesian computation (ABC), another method commonly used to
generate simulation-based likelihood approximations.
Our results demonstrate that simulation-based inference, which offers
considerable conceptual advantages over more traditional methods for inverse
parameter estimation, can be successfully applied to process-based models of
high complexity. The methodology is particularly suitable for heterogeneous
and complex data structures and can easily be adjusted to other model types,
including most stochastic population and individual-based models. Our study
therefore provides a blueprint for a fairly general approach to parameter
estimation of stochastic process-based models
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The carbon fluxes in different successional stages: modelling the dynamics of tropical montane forests in South Ecuador
Full text linkAbstract Background Tropical forests play an important role in the global carbon (C) cycle. However, tropical montane forests have been studied less than tropical lowland forests, and their role in carbon storage is not well understood. Montane forests are highly endangered due to logging, land-use and climate change. Our objective was to analyse how the carbon balance changes during forest succession. Methods In this study, we used a method to estimate local carbon balances that combined forest inventory data with process-based forest models. We utilised such a forest model to study the carbon balance of a tropical montane forest in South Ecuador, comparing two topographical slope positions (ravines and lower slopes vs upper slopes and ridges). Results The simulation results showed that the forest acts as a carbon sink with a maximum net ecosystem exchange (NEE) of 9.3 Mg C∙(ha∙yr)−1 during its early successional stage (0–100 years). In the late successional stage, the simulated NEE fluctuated around zero and had a variation of 0.77 Mg C∙(ha∙yr) –1. The simulated variability of the NEE was within the range of the field data. We discovered several forest attributes (e.g., basal area or the relative amount of pioneer trees) that can serve as predictors for NEE for young forest stands (0–100 years) but not for those in the late successional stage (500–1,000 years). In case of young forest stands these correlations are high, especially between stand basal area and NEE. Conclusion In this study, we used an Ecuadorian study site as an example of how to successfully link a forest model with forest inventory data, for estimating stem-diameter distributions, biomass and aboveground net primary productivity. To conclude, this study shows that process-based forest models can be used to investigate the carbon balance of tropical montane forests. With this model it is possible to find hidden relationships between forest attributes and forest carbon fluxes. These relationships promote a better understanding of the role of tropical montane forests in the context of global carbon cycle, which in future will become more relevant to a society under global change
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Biotic controls on shallow translational landslides
No full textIn undisturbed tropical montane rainforests massive organic layers accommodate the majority of roots and only a small fraction of roots penetrate the mineral soil. We investigated the contribution of vegetation to slope stability in such environments by modifying a standard model for slope stability to include an organic layer with distinct mechanical properties. The importance of individual model parameters was evaluated using detailed measurements of soil and vegetation properties to reproduce the observed depth of 11 shallow landslides in the Andes of southern Ecuador. By distinguishing mineral soil, organic layer and above-ground biomass, it is shown that in this environment vegetation provides a destabilizing effect mainly due to its contribution to the mass of the organic layer (up to 973 t ha-1 under wet conditions). Sensitivity analysis shows that the destabilizing effect of the mass of soil and vegetation can only be effective on slopes steeper than 37.9°. This situation applies to 36% of the study area. Thus, on the steep slopes of this megadiverse ecosystem, the mass of the growing forest promotes landsliding, which in turn promotes a new cycle of succession. This feedback mechanism is worth consideration in further investigations of the impact of landslides on plant diversity in similar environments. © 2012 John Wiley & Sons, Ltd
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Photoprotective effect of Pothomorphe umbellata on UVB radiation-induced biomarkers involved in carcinogenesis of hairless mouse epidermis
No full textIn this study we assessed the protective effect of topical application of Pothomorphe umbellata extract on ultraviolet B (UVB)-induced skin lesion parameters in hairless mouse epidermis. A single dose of UVB irradiation (0.23 kJ/m(2)) resulted in a significant decrease in thymine dimer-positive cells and apoptotic sunburn cells, with an increase in p53 and proliferating cell nuclear antigen-positive cells in the epidermis. After 5 weeks (total dose 13.17 kJ/m(2)) and 15 weeks (total dose 55.51 kJ/m(2)) of irradiation, P. umbellata treatment inhibited the hyperplasic response and induced an increase in p53-positive cells. These findings suggest that P. umbellata extract affords protection against UVB-induced skin lesions.Fundacao de Amparo a Pesquisa do Estado de Sao Paulo - FAPESPVanessa Vitoriano da Silva[02/13106-7]Cristina Dislich Ropke[03/10917-7]Cristina Dislich Ropke[04/03507-0
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