Michigan Technological University

About: Michigan Technological University is a education organization based out in Houghton, Michigan, United States. It is known for research contribution in the topics: Population & Volcano. The organization has 8023 authors who have published 17422 publications receiving 481780 citations. The organization is also known as: MTU & Michigan Tech.

Carbon allocation and partitioning in aspen clones varying in sensitivity to tropospheric ozone.

Abstract: Clones of aspen (Populus tremuloides Michx.) were identified that differ in biomass production in response to O(3) exposure. (14)Carbon tracer studies were used to determine if the differences in biomass response were linked to shifts in carbon allocation and carbon partitioning patterns. Rooted cuttings from three aspen Clones (216, O(3) tolerant; 271, intermediate; and 259, O(3) sensitive) were exposed to either charcoal-filtered air (CF) or an episodic, two-times-ambient O(3) profile (2x) in open-top chambers. Either recently mature or mature leaves were exposed to a 30-min (14)C pulse and returned to the treatment chambers for a 48-h chase period before harvest. Allocation of (14)C to different plant parts, partitioning of (14)C into various chemical fractions, and the concentration of various chemical fractions in plant tissue were determined. The percent of (14)C retained in recently mature source leaves was not affected by O(3) treatment, but that retained in mature source leaves was greater in O(3)-treated plants than in CF-treated plants. Carbon allocation from source leaves was affected by leaf position, season, clone and O(3) exposure. Recently mature source leaves of CF-treated plants translocated about equal percentages of (14)C acropetally to growing shoots and basipetally to stem and roots early in the season. When shoot growth ceased (August 16), most (14)C from all source leaves was translocated basipetally to stem and roots. At no time did mature source leaves allocate more than 6% of (14)C translocated within the plant to the shoot above. Ozone effects were most apparent late in the season. Ozone decreased the percent (14)C translocated from mature source leaves to roots and increased the percent (14)C translocated to the lower stem. In contrast, allocation from recently mature leaves to roots increased. Partitioning of (14)C among chemical fractions was affected by O(3) more in source leaves than in sink tissue. In source leaves, more (14)C was incorporated into the sugar, organic acid and lipids + pigments fractions, and less (14)C was incorporated into starch and protein fractions in O(3)-treated plants than in CF-treated plants. In addition, there were O(3) treatment interactions between leaf position and clones for (14)C incorporation into different chemical fractions. When photosynthetic data were used to convert percent (14)C transported to the total amount of carbon transported on a mass basis, it was found that carbon transport was controlled more by photosynthesis in the source leaves than proportional changes in allocation to the sinks. Ozone decreased the total amount of carbon translocated to all sink tissue in the O(3)-sensitive Clone 259 because of decreases in photosynthesis in both recently mature and mature source leaves. In contrast, O(3) had no effect on carbon transport from recently mature leaves to lower shoots of either Clone 216 or 271, had no significant effect on transport to roots of Clone 216, and increased transport to roots of Clone 271. The O(3)-induced increase in transport to roots of Clone 271 was the result of a compensatory increase in upper leaf photosynthesis and a relatively greater shift in the percent of carbon allocated to roots. In contrast to those of Clone 271, recently mature leaves of Clone 216 maintained similar photosynthetic rates and allocation patterns in both the CF and O(3) treatments. We conclude that Clone 271 was more tolerant to O(3) exposure than Clone 216 or 259. Tolerance to chronic O(3) exposure was directly related to maintenance of high photosynthetic rates in recently mature leaves and retention of lower leaves.

Brittle fracture in iridium

Abstract: Brittle fracture in fcc metals is uncommon. It is not common knowledge that single crystals of iridium, a high melting point fcc metal, fail by brittle cleavage at room temperature. Furthermore, polycrystalline iridium fails predominantly by brittle inter granular fracture at temperatures below 1000°C. With the aid of several models of brittle fracture we have demonstrated that cleavage in iridium is intrinsic, resulting from apparently very strong and directed atomic binding forces. Intergranular fracture in iridium has been generally assumed to arise from the segregation of harmful impurities to the grain boundaries. We were able to demonstrate using Auger electron spectroscopy that impurity segregation to grain boundaries in iridium was not necessary for grain boundary fracture to occur, thereby demonstrating that intergranular brittle fracture in polycrystalline iridium is also intrinsic and not impurity related.

Spurt phenomena of the Johnson-Segalman fluid and related models

Abstract: We examine the behavior of viscoelastic fluid models which exhibit local extrema of the steady shear stress. For the Johnson-Segalman and Giesekus models, a variety of steady singular solutions with jumps in shear rate are constructed and their stability to one dimensional disturbances analyzed. It is found that flow-rate versus imposed stress curves in slit-die flow fit experimental observation of the “spurt” phenomenon with some precision. The flow curves involve linearly stable singular solutions, but some assumptions on the dynamics of the spurt process are required. These assumptions are tested by a semi-implicit finite element solution technique which allows solutions to be efficiently integrated over the very long time-scale involved. The Johnson-Segalman model with added Newtonian viscosity is used in the calculations. It is found that the assumptions required to model spurf are satisfied by the dynamic model. The dynamic model also displays a characteristic “latency time” before the spurt ensues and a characteristic “shape memory” hysteresis in load/unload cycles. These as well as other features of the computed solutions should be observable experimentally. We conclude that constitutive equations with shear stress extrema are not necessarily flawed, that their predicted behavior may appear to be arrested “wall slip”, and that such behavior may actually have been observed already.

Spatial distributions of particle number concentrations in the global troposphere: Simulations, observations, and implications for nucleation mechanisms

Abstract: [1] Particle number concentration in the troposphere is an important parameter controlling the climate and health impacts of atmospheric aerosols. We show that nucleation rates and total particle number concentrations in the troposphere, predicted by different nucleation schemes, differ significantly. Our extensive comparisons of simulated results with land-, ship-, and aircraft-based measurements indicate that, among six widely used nucleation schemes involving sulfuric acid, only the ion-mediated nucleation (IMN) scheme can reasonably account for both absolute values (within a factor of ∼2) and spatial distributions of particle number concentrations in the whole troposphere. Binary homogeneous nucleation (BHN) schemes significantly underpredict particle number concentration in the lower troposphere (below ∼500 mbar), especially in the boundary layer over major continents (by a factor of up to ∼10). BHN is also insignificant in the upper troposphere based on a recent kinetically self-consistent nucleation model constrained by multiple independent laboratory data. Previous conclusions about the importance of BHN in the upper troposphere should be revisited. Empirical activation and kinetic nucleation formulas significantly overpredict the particle number concentrations over tropical and subtropical oceans (by a factor of up to ∼10 in the boundary layer), and the overpredictions extend from ocean surface to around ∼400 mbar. This study represents the first comprehensive comparison of global particle number simulations with relevant measurements that have a 3-D global spatial coverage. Our results suggest that ion-mediated H2SO4-H2O nucleation appears to dominate over neutral H2SO4-H2O nucleation, not only in the lower troposphere but also in the middle and upper troposphere.

A review of methods for mapping and prediction of inventory attributes for operational forest management

Abstract: Forest inventory attributes are an important source of information for a variety of strategic and tactical forest management purposes. However, it is not possible or feasible for field inventories to be conducted contiguously across large areas, especially at a resolution fine enough to be useful for operational management. Therefore, a large number of quantitative modeling and prediction methods have been and are being developed and applied to predict and map forest attributes, with the goal of providing an accurate, spatially continuous, and detailed information base for practitioners of forestry and ecosystem management. This article reviews the most commonly used prediction techniques in the context of a comprehensive modeling framework that includes a discussion of methods, data sources, variable selection, and model validation. The methods discussed include regression, nearest neighbor, artificial neural networks, decision trees, and ensembles such as random forest. No single technique is revealed as universally superior for predicting forest inventory attributes; the ideal approach depends on goals, available training and ancillary data, and the modeler’s interest in tradeoffs between realism and statistical considerations. Useful ancillary data included in the models tend to include climate and topographic variables as well as vegetation indices derived from optical remote sensing systems such as Landsat. However, the use of airborne LiDAR in modeling of forest inventory attributes is increasing rapidly and shows promise for operational forest management applications. Different considerations are encapsulated within a generalized model development framework that provides a structure against which tradeoffs can be evaluated.