A Computational Parametric Study of the Relationship between the Characteristic Geometry, Flow Structure, and Hemodynamics of Intracranial Bifurcation Aneurysms

This work utilized computational numerical models in parametric studies to develop a relationship between the characteristic geometry of intracranial bifurcation aneurysms and the intra-aneurysmal blood flow structures. In general, intra-aneurysmal flow structures can be categorized by two flow types: Type I (one vortex) and Type II (two vortices). Flow structure can profoundly influence the intra-aneurysmal hemodynamic stresses, which are generally accepted as a major factor responsible for either the stabilization or the eventual rupture of intracranial aneurysms. However, there are currently no known reliable methods that can determine whether an aneurysm is prone to rupture. Current technology limits clinicians to the use of geometric data obtained via angiography as the only means of assessing rupture risk. Therefore, a method of obtaining a description of the intra-aneurysmal hemodynamics from aneurysmal geometry is a potentially valuable tool. Many studies conducted on this subject have the weaknesses of either assuming that one geometric parameter is sufficient to predict and describe intra-aneurysmal hemodynamics, or neglecting the combined effects of multiple parameters in studies involving more than one geometric parameter. Many of these studies are purely statistical and incorporate no mechanistic explanations or hypotheses for rupture. The work of this thesis considers the combine effects of multiple geometric parameters in a systematic manner which relates geometric parameters to flow structure, and flow structure to hemodynamic stress. By using this approach, an extensive systematic guide to intra-aneurysmal flow structure and hemodynamics has been developed which can readily offer clinicians a general description of intra-aneurysmal hemodynamic conditions in a clinical setting. Furthermore, the theory in this work can not only predict the existence of unfavorable hemodynamics, but can also identify the geometric feature(s) in particular that is (are) responsible for unfavorable hemodynamics. To provide evidence which can substantiate these notions, the predictive ability of the theory developed form the parametric study was tested by evaluating its ability to predict the flow structures in 27 clinical aneurysms. An evaluation of the theory’s performance concludes this work

A Numerical Investigation into the Hemodynamics, Oxygen Transport, and Flow Stability of Cerebral Aneurysms

A cerebral or intra-cranial aneurysm (IA) is a pathological saccular bulge occurring in the cerebral arteries of the brain. These structures have a propensity to rupture due to their structurally deficient arising from their pathological nature. A ruptured IA can have disastrous or fatal consequence for a patient. Surgical intervention furthermore carries its own innate risks. Therefore, an understanding of IA initiation, growth and rupture remains imperative in the treatment of the disease. However, these processes remain poorly understood. Hemodynamics, the mechanical forces imparted on the vessel wall from the flowing blood contained within, is thought to be a substantial contributing factor in the progression of the disease. The study of aneurysmal hemodynamics and their impact on the aneurysm wall remains challenging due to the inaccessibility from their location deep within the brain that clinicians are faced with. Therefore, computational fluid dynamics (CFD) studies are frequently utilized in the study of aneurysmal heodynamics. The work herein focuses on advancing the study of aneurysmal hemodynamics in four major areas. The first is an extensive categorization of the blood flow waveforms found within the cerebral circulation from a uniquely large data-set of 272 cardiovascular patent waveforms that quantifies the impact on the hemodynamics from the variation in this large data-set. The second section focuses on quantifying the multi-parameter relationship between aneurysmal geometry and intra-saccular flow-structure via parametric study. The third section explores the impact of the pathological morphology of aneurysms on the blood’s ability to transport oxygen to the wall tissue within the aneurysm. Finally, this work identifies geometric features additional to those previously known which initiate pathological high-frequency fluctuations in the blood flow and examines possible solution strategies in answering the open question as to what impact do these flow features have on the development of IAs.

Mitochondrial genomes reveal slow rates of molecular evolution and the timing of speciation in beavers (Castor), one of the largest rodent species

BACKGROUND: Beavers are one of the largest and ecologically most distinct rodent species. Little is known about their evolution and even their closest phylogenetic relatives have not yet been identified with certainty. Similarly, little is known about the timing of divergence events within the genus Castor. METHODOLOGY/PRINCIPAL FINDINGS: We sequenced complete mitochondrial genomes from both extant beaver species and used these sequences to place beavers in the phylogenetic tree of rodents and date their divergence from other rodents as well as the divergence events within the genus Castor. Our analyses support the phylogenetic position of beavers as a sister lineage to the scaly tailed squirrel Anomalurus within the mouse related clade. Molecular dating places the divergence time of the lineages leading to beavers and Anomalurus as early as around 54 million years ago (mya). The living beaver species, Castor canadensis from North America and Castor fiber from Eurasia, although similar in appearance, appear to have diverged from a common ancestor more than seven mya. This result is consistent with the hypothesis that a migration of Castor from Eurasia to North America as early as 7.5 mya could have initiated their speciation. We date the common ancestor of the extant Eurasian beaver relict populations to around 210,000 years ago, much earlier than previously thought. Finally, the substitution rate of Castor mitochondrial DNA is considerably lower than that of other rodents. We found evidence that this is correlated with the longer life span of beavers compared to other rodents. CONCLUSIONS/SIGNIFICANCE: A phylogenetic analysis of mitochondrial genome sequences suggests a sister-group relationship between Castor and Anomalurus, and allows molecular dating of species divergence in congruence with paleontological data. The implementation of a relaxed molecular clock enabled us to estimate mitochondrial substitution rates and to evaluate the effect of life history traits on it

Tree phylogenetic diversity structures multitrophic communities

1. Plant diversity begets diversity at other trophic levels. While species richness is the most commonly used measure for plant diversity, the number of evolutionary lineages (i.e. phylogenetic diversity) could theoretically have a stronger influence on the community structure of co-occurring organisms. However, this prediction has only rarely been tested in complex real-world ecosystems. 2. Using a comprehensive multitrophic dataset of arthropods and fungi from a species-rich subtropical forest, we tested whether tree species richness or tree phylogenetic diversity relates to the diversity and composition of organisms. 3. We show that tree phylogenetic diversity but not tree species richness determines arthropod and fungi community composition across trophic levels and increases the diversity of predatory arthropods but decreases herbivorous arthropod diver- sity. The effect of tree phylogenetic diversity was not mediated by changed abun- dances of associated organisms, indicating that evolutionarily more diverse plant communities increase niche opportunities (resource diversity) but not necessarily niche amplitudes (resource amount). 4. Our findings suggest that plant evolutionary relatedness structures multitrophic communities in the studied species-rich forests and possibly other ecosystems at large. As global change non-randomly threatens phylogenetically distinct plant species, far-reaching consequences on associated communities are expected

Designing forest biodiversity experiments : general considerations illustrated by a new large experiment in subtropical China

Funded by German Research Foundation. Grant Number: DFG FOR 891/1 and 2 National Natural Science Foundation of China. Grant Numbers: NSFC 30710103907, 30930005, 31170457 , 31210103910 Swiss National Science Foundation (SNSF) Sino-German Centre for Research Promotion in BeijingPeer reviewedPublisher PD

Towards a methodical framework for comprehensively assessing forest multifunctionality

Funded by Deutsche Forschungsgemeinschaft. Grant Number: DFG FOR 891/1-3 National Natural Science Foundation of China. Grant Numbers: 30710103907, 30930005, 31170457, 31210103910 Swiss National Science Foundation (SNSF) Sino-German Centre for Research Promotion in Beijing. Grant Number: GZ 986Peer reviewedPublisher PD

Tree species and genetic diversity increase productivity via functional diversity and trophic feedbacks

Addressing global biodiversity loss requires an expanded focus on multiple dimensions of biodiversity. While most studies have focused on the consequences of plant interspecific diversity, our mechanistic understanding of how genetic diversity within plant species affects plant productivity remains limited. Here, we use a tree species × genetic diversity experiment to disentangle the effects of species diversity and genetic diversity on tree productivity, and how they are related to tree functional diversity and trophic feedbacks. We found that tree species diversity increased tree productivity via increased tree functional diversity, reduced soil fungal diversity, and marginally reduced herbivory. The effects of tree genetic diversity on productivity via functional diversity and soil fungal diversity were negative in monocultures but positive in the mixture of the four tree species tested. Given the complexity of interactions between species and genetic diversity, tree functional diversity and trophic feedbacks on productivity, we suggest that both tree species and genetic diversity should be considered in afforestation

Designing forest biodiversity experiments: general considerations illustrated by a new large experiment in subtropical China


 Biodiversity-ecosystem functioning (BEF) experiments address ecosystem-level consequences of species loss by comparing communities of high species richness with communities from which species have been gradually eliminated. BEF experiments originally started with microcosms in the laboratory and with grassland ecosystems. A new frontier in experimental BEF research is manipulating tree diversity in forest ecosystems, compelling researchers to think big and comprehensively.
 We present and discuss some of the major issues to be considered in the design of BEF experiments with trees and illustrate these with a new forest biodiversity experiment established in subtropical China (Xingangshan, Jiangxi Province) in 2009/2010. Using a pool of 40 tree species, extinction scenarios were simulated with tree richness levels of 1, 2, 4, 8 and 16 species on a total of 566 plots of 25.8 × 25.8 m each.
 The goal of this experiment is to estimate effects of tree and shrub species richness on carbon storage and soil erosion; therefore, the experiment was established on sloped terrain. The following important design choices were made: (i) establishing many small rather than fewer larger plots, (ii) using high planting density and random mixing of species rather than lower planting density and patchwise mixing of species, (iii) establishing a map of the initial 'ecoscape' to characterize site heterogeneity before the onset of biodiversity effects and (iv) manipulating tree species richness not only in random but also in trait-oriented extinction scenarios.
 Data management and analysis are particularly challenging in BEF experiments with their hierarchical designs nesting individuals within-species populations within plots within-species compositions. Statistical analysis best proceeds by partitioning these random terms into fixed-term contrasts, for example, species composition into contrasts for species richness and the presence of particular functional groups, which can then be tested against the remaining random variation among compositions.
 We conclude that forest BEF experiments provide exciting and timely research options. They especially require careful thinking to allow multiple disciplines to measure and analyse data jointly and effectively. Achieving specific research goals and synergy with previous experiments involves trade-offs between different designs and requires manifold design decisions.&#13

Toward a methodical framework for comprehensively assessing forest multifunctionality

Biodiversity-ecosystem functioning (BEF) research has extended its scope from communities that are short-lived or reshape their structure annually to structurally complex forest ecosystems. The establishment of tree diversity experiments poses specific methodological challenges for assessing the multiple functions provided by forest ecosystems. In particular, methodological inconsistencies and nonstandardized protocols impede the analysis of multifunctionality within, and comparability across the increasing number of tree diversity experiments. By providing an overview on key methods currently applied in one of the largest forest biodiversity experiments, we show how methods differing in scale and simplicity can be combined to retrieve consistent data allowing novel insights into forest ecosystem functioning. Furthermore, we discuss and develop recommendations for the integration and transferability of diverse methodical approaches to present and future forest biodiversity experiments. We identified four principles that should guide basic decisions concerning method selection for tree diversity experiments and forest BEF research: (1) method selection should be directed toward maximizing data density to increase the number of measured variables in each plot. (2) Methods should cover all relevant scales of the experiment to consider scale dependencies of biodiversity effects. (3) The same variable should be evaluated with the same method across space and time for adequate larger-scale and longer-time data analysis and to reduce errors due to changing measurement protocols. (4) Standardized, practical and rapid methods for assessing biodiversity and ecosystem functions should be promoted to increase comparability among forest BEF experiments. We demonstrate that currently available methods provide us with a sophisticated toolbox to improve a synergistic understanding of forest multifunctionality. However, these methods require further adjustment to the specific requirements of structurally complex and long-lived forest ecosystems. By applying methods connecting relevant scales, trophic levels, and above? and belowground ecosystem compartments, knowledge gain from large tree diversity experiments can be optimized