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Termite mounds have been poorly studied in tropical rainforest in contrast to those of savannahs where they create fertility hotspots and influence the spatial pattern of vegetation. An inventory of termite mounds and of 15 tree species with a diameter at breast height ≥ 10 cm was carried out in two 800-ha blocks, in two rainforest sites located in northern Congo. We used inhomogeneous and intertype K functions to study the spatial pattern of termite mounds and of tree species around mounds, respectively. We found that mounds in Loundoungou were over-dispersed within a radius of less than 70 m, while those in Mokabi were randomly spaced. Tree species within a 20-m radius around a mound were aggregated towards the mound, e.g. Entandrophragma cylindricum, randomly distributed, or even repulsed by the mound. The specific responses also differed in the two sites. These results suggest that (i) the mounds in Loundoungou were created by savannah termite species 3,000-2,000 years BP during the Late Holocene Rainforest Crisis and (ii) the mounds in Mokabi are characteristic of forest mounds. The impact of termite mounds on the spatial pattern of tree species is thus site-dependent, and these differences might be due to species seed dispersal strategies and to soil calcium concentrations.
Reducing the spread of multidrug-resistant bacteria in hospitals remains a challenge. Current methods are screening of patients, isolation, and adherence to hygiene measures among healthcare workers (HCWs). More specific measures could rely on a better characterization of the contacts at risk of dissemination.
To quantify how close-proximity interactions (CPIs) affected Staphylococcus aureus dissemination.
Nested case-control study.
French long-term care facility in 2009.
Patients (n=329) and HCWs (n=261).
We recorded CPIs using electronic devices together with S. aureus nasal carriage during 4 months in all participants. Cases consisted of patients showing incident S. aureus colonization and were paired to 8 control patients who did not exhibit incident colonization at the same date. Conditional logistic regression was used to quantify associations between incidence and exposure to demographic, network, and carriage covariables.
The local structure of contacts informed on methicillin-resistant S. aureus (MRSA) carriage acquisition: CPIs with more HCWs were associated with incident MRSA colonization in patients (odds ratio [OR], 1.10 [95% CI, 1.04–1.17] for 1 more HCW), as well as longer CPI durations (1.03 [1.01–1.06] for a 1-hour increase). Joint analysis of carriage and contacts showed increased carriage acquisition in case of CPI with another colonized individual (OR, 1.55 [1.14–2.11] for 1 more HCW). Global network measurements did not capture associations between contacts and carriage.
Electronically recorded CPIs inform on the risk of MRSA carriage, warranting more study of in-hospital contact networks to design targeted intervention strategies.
Infect. Control Hosp. Epidemiol. 2015;36(8):922–929
Despite the important functional role of deep roots in withdrawing water during drought, direct measurements of root distribution are very rare in tropical rain forests. The aim of this study was to investigate the root distribution of Entandrophragma cylindricum, a common tree species in the Central African semi-deciduous rain forest, in Ferralsols and Arenosols. We dug two pits to a depth of 6 m in Ferralsols and two pits to a depth of 3 m in Arenosols, close to E. cylindricum trees. The vertical soil profiles were divided into 10 × 10-cm grid cells and the roots counted were distributed in three diameter classes. We fitted a root distribution model to our dataset. We found that vertical root distribution was shallower in Arenosols than in Ferralsols. Root penetration was not stopped even by a Ferralsol with high gravel content in its subsoil. Overall, our measurements showed that 95% of all roots were distributed to depths of between 258 and 564 cm from the soil surface, which is much deeper than the 95 cm depth previously reported in the literature for tropical rain forests. As sampling depth could explain this discrepancy, we recommend a sampling depth of at least 3–5 m to accurately estimate root distribution. The drier the dry season, the deeper the sampling depth should be. Our results are consistent with global models of root distribution in forest ecosystems, which are driven by climate variables. We thus suggest that deep rooting could be common in rain forests with a marked dry season.
We investigated the microstructural evolution of Sn96.4Ag2.8Cu0.8 solder through in situ heating transmission electron microscopy observations. As-soldered bump consisted of seven layers, containing the nanoeutectic lamella structure of AuSn and Au5Sn phases, and the polygonal grains of AuSn2 and AuSn4, on Au-plated Cu bond pads. Here, we found that there are two nanoeutectic lamellar layers with lamella spacing of 40 and 250 nm. By in situ heating above 140°C, the nanoeutectic lamella of AuSn and Au5Sn was decomposed with structural degradation by sphering and coarsening processes of the lamellar interface. At the third layer neighboring to the lamella layer, on the other hand, Au5Sn particles with a zig-zag shape in AuSn matrix became spherical and were finally dissipated in order to minimize the interface energy between two phases. In the other layers except both lamella layers, polycrystal grains of AuSn2 and AuSn4 grew by normal grain growth during in situ heating. The high interface energy of nanoeutectic lamella and polygonal nanograins, which are formed by rapid solidification, acted as a principal driving force on the microstructural change during the in situ heating.
The effect of microstructural inhomogeneities with different length scale on the plasticity of (Ti45Zr16Be20Cu10Ni9)100–xTax (x = 0, 5, and 10) bulk glassy alloys has been studied. The formation of specific heterogeneous microstructures with a different type of structural inhomogeneity, i.e., short-/medium-range ordered clusters or micrometer-scale ductile dendrites combined with a glassy matrix, evolved by appropriately tuning the alloy chemistry, improves the room temperature plasticity up to ∼12.5% and ∼15%, respectively. The pronouncedly enhanced plasticity is mainly attributed to the retardation of shear localization and multiplication of shear bands by controlling the plastic and failure instabilities otherwise responsible for premature failure.
Unlike the dislocation-based plasticity in crystalline metals, which can be readily explained by their crystal structure and the presence of defects, the nature of the plasticity in amorphous alloys is not completely understood. Experiments have shown that the plasticity in amorphous alloys is strongly dependent on their atomic packing density. This study, based on the combination of experimental and computational techniques, examines the origin of the plasticity in amorphous alloys considering characteristics of the inherent atomic-scale structure as defined by short-range ordered (SRO) clusters. The role of various SRO atomic clusters in creating free volume during shear deformation is discussed. We report that the plasticity exhibited by amorphous alloys is very sensitive to the characteristics of the atomic packing state, which can be described by various SRO atomic structures and quantified by the effective activation energy for crystallization.
W-rich particle-reinforced Ti-based bulk metallic glass (BMG) matrix composites with a compressive strength approaching 3 GPa and a fracture strain of approximately 12% were developed. In contrast to most existing BMG matrix composites, in which the improved ductility was obtained only at the expense of the strength, the composites developed in this study exhibited a significant enhancement in their strength, as well as an improvement in the plasticity. This improvement in the plasticity was attributed to the blocking and circumscription of the shear band propagation, leading to the formation of a large number of shear bands. Using a classical elasticity theory of inclusions, the improvement of the strength was interpreted as resulting from the generation of tensile residual stresses in the matrix due to the difference in the coefficient of thermal expansion between the W-rich particles and the BMG matrix.
The sliding friction and wear performance of Al–Ni–Co–Si quasicrystalline coatings deposited by the high-velocity oxy-fuel technique were investigated under dry sliding conditions. This study indicated that changes in the imposed sliding test conditions modified the friction and wear behavior of quasicrystalline coatings. Qualitative analysis of the contact interface and wear debris were performed with the aim of understanding the role of the third body on the friction and wear processes. The dependence of the coefficient of friction on the sliding velocity and counterpart material was explained by the stick-slip behavior. It was also shown that test conditions favorable for the formation of thick intermediate layers and the densification of the coating subsurface led to low wear rates. Large cylindrical particles, formed by agglomeration of small wear debris, were suggested as a beneficial factor for the reduction of the coefficient of friction.
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