Ecological Controls of Rhizosphere Processes and Soil Organic Matter Dynamics at a Sub-arctic Treeline

Rapid climate change in the Arctic and Sub-Arctic is causing vegetation change across large areas of tundra. Shrubs and trees are undergoing range expansions as part of an over-all trend of ‘greening’ of the tundra. This is of importance because northern peatlands contain around half of total soil carbon (C) and there is a potential for productive vegetation to interact with this C in a number of ways: (1) Ectomycorrhizal fungi (ECM) in symbiosis with trees and shrubs could potentially stimulate decomposition through extracellular enzyme production whilst extracting nitrogen (N) for their hosts; (2) deep snow, trapped by tall vegetation insulates the soil, resulting in higher winter-time microbial activity and has potential to influence growing season microbial activity; (3) the biochemistry of litter and decomposition environment associated with more productive vegetation could result in accelerated mass loss of litter and stimulate decomposition of older soil C. This thesis investigates how productive sub-arctic plant species in Northern Sweden interact with soil C by using ‘space-for-time’ transitions from forests (Betula pubescens), through intermediate shrub vegetation (Betula, Salix), to tundra heath (Empetrum nigrum). This was to test how ECM fungi, winter snow accumulation, defoliation events and litter input influence C cycling. C stocks, respiration rates and ECM growth rates were measured across these ecotones. It was found that birch forests and shrub stands had significantly lower soil C storage and higher respiration rates than adjacent heaths. This is contrary to the predictions of earth system models. Higher ECM growth rates at plots with low C storage and high cycling rates implied that they had an important role in the stimulation of C decomposition. To test whether snow cover in forests over winter had an important effect on C cycling, soils were transplanted between forest and heath (different snow cover), and respiration rates were measured over summer. It was found that deep snow cover over winter increases microbial activity in summer due to a warmer, more stable winter environment; this is hypothesised to be due to the environmental selection of a more active assemblage of decomposing microbes. A defoliation event of part of the birch forest by caterpillars allowed for a natural ‘experiment’. Trees with different degrees of defoliation were compared in their influence over soil C cycling processes. Defoliated plots shifted to slower-cycling states through a shift in the ECM community. This further implied that ECM fungi have an important role to play in rapid cycling of C in forests. A decomposition experiment using the litter of significant plant species in forest, shrub and heath communities was carried out by transplanting them between these key environments. This work showed that rapid decomposition of litter in the forest is driven by an interaction between carbohydrate-rich litter input and an effective decomposer community. This work addresses the relationship between vegetation productivity and C storage in the soil. This theme runs through every experiment as they test specific interactions between different plant groups and the soil. The results from this thesis suggest that increasing productivity and shrub expansion in the Arctic will stimulate decomposition of soil C via a number of pathways. Plant-soil interactions are clearly of importance in determining the fate of C in ecosystems and will play a key part in the balance of C in the future

The elimination of surface cross-hatch from relaxed, limited-area Si1 – xGex buffer layers

The influence of lateral dimensions on the relaxation and surface topography of linearly graded Si1 – xGex buffer layers has been investigated. A dramatic change in the relaxation mechanism has been observed for depositions on Si mesa pillars of lateral dimensions 10 µm and below. Misfit dislocations are able to extend unhindered and terminate at the edges of the growth zone, yielding a surface free of cross-hatch. For lateral dimensions in excess of 10 µm orthogonal misfit interactions occur and relaxation is dominated by the modified Frank–Read (MFR) mechanism. The stress fields associated with the MFR dislocation pile-ups result in a pronounced cross-hatch topography

Global Localization in Unstructured Environments using Semantic Object Maps Built from Various Viewpoints

We present a novel framework for global localization and guided relocalization of a vehicle in an unstructured environment. Compared to existing methods, our pipeline does not rely on cues from urban fixtures (e.g., lane markings, buildings), nor does it make assumptions that require the vehicle to be navigating on a road network. Instead, we achieve localization in both urban and non-urban environments by robustly associating and registering the vehicle's local semantic object map with a compact semantic reference map, potentially built from other viewpoints, time periods, and/or modalities. Robustness to noise, outliers, and missing objects is achieved through our graph-based data association algorithm. Further, the guided relocalization capability of our pipeline mitigates drift inherent in odometry-based localization after the initial global localization. We evaluate our pipeline on two publicly-available, real-world datasets to demonstrate its effectiveness at global localization in both non-urban and urban environments. The Katwijk Beach Planetary Rover dataset is used to show our pipeline's ability to perform accurate global localization in unstructured environments. Demonstrations on the KITTI dataset achieve an average pose error of 3.8m across all 35 localization events on Sequence 00 when localizing in a reference map created from aerial images. Compared to existing works, our pipeline is more general because it can perform global localization in unstructured environments using maps built from different viewpoints.Comment: 8 pages, 6 figures, presented at IROS 202

Spatial patterns in soil organic matter dynamics are shaped by mycorrhizosphere interactions in a treeline forest

Aims In the Swedish sub-Arctic, mountain birch (Betula pubescens ssp. czerepanovii) forests mediate rapid soil C cycling relative to adjacent tundra heaths, but little is known about the role of individual trees within forests. Here we investigate the spatial extent over which trees influence soil processes. Methods We measured respiration, soil C stocks, root and mycorrhizal productivity and fungi:bacteria ratios at fine spatial scales along 3 m transects extending radially from mountain birch trees in a sub-Arctic ecotone forest. Root and mycorrhizal productivity was quantified using in-growth techniques and fungi:bacteria ratios were determined by qPCR. Results Neither respiration, nor root and mycorrhizal production, varied along transects. Fungi:bacteria ratios, soil organic C stocks and standing litter declined with increasing distance from trees. Conclusions As 3 m is half the average size of forest gaps, these findings suggest that forest soil environments are efficiently explored by roots and associated mycorrhizal networks of B. pubescens. Individual trees exert influence substantially away from their base, creating more uniform distributions of root, mycorrhizal and bacterial activity than expected. However, overall rates of soil C accumulation do vary with distance from trees, with potential implications for spatio-temporal soil organic matter dynamics and net ecosystem C sequestration

Thermal and Pressure Characterization of a Wind Tunnel Force Balance Using the Single Vector System

Wind tunnel research at NASA Langley Research Center s 31-inch Mach 10 hypersonic facility utilized a 5-component force balance, which provided a pressurized flow-thru capability to the test article. The goal of the research was to determine the interaction effects between the free-stream flow and the exit flow from the reaction control system on the Mars Science Laboratory aeroshell during planetary entry. In the wind tunnel, the balance was exposed to aerodynamic forces and moments, steady-state and transient thermal gradients, and various internal balance cavity pressures. Historically, these effects on force measurement accuracy have not been fully characterized due to limitations in the calibration apparatus. A statistically designed experiment was developed to adequately characterize the behavior of the balance over the expected wind tunnel operating ranges (forces/moments, temperatures, and pressures). The experimental design was based on a Taylor-series expansion in the seven factors for the mathematical models. Model inversion was required to calculate the aerodynamic forces and moments as a function of the strain-gage readings. Details regarding transducer on-board compensation techniques, experimental design development, mathematical modeling, and wind tunnel data reduction are included in this paper

Climate Change and San Francisco Bay-Delta Tidal Wetlands

Climate change will affect tidal wetlands with higher rates of sea-level rise and higher concentrations of salt in brackish and freshwater tidal systems, in addition to causing increases in atmospheric CO2 concentration, warmer temperatures, and shifts in precipitation. In the San Francisco Bay–Delta, the areas most likely to be affected—brackish and freshwater tidal wetlands—are also the sites with the majority of endemic plant species and the greater biodiversity and productivity. Effects on the San Francisco Bay– Delta estuary are complex and difficult to predict, but a few things are clear. Biodiversity of the tidal wetland system in the San Francisco Bay–Delta region will decline, with subsequent effects on ecosystem functioning and services. Altered plant production, physiological tolerances, and shifts in rates of mortality will modify wetland plant communities in ways not yet predictable. Lower ecosystem productivity from salinity increases will affect both primary and detrital-based food webs. Such changes will cascade via the food webs into invertebrate, bird, and pelagic systems. Tidal wetlands are especially sensitive to processes that climate change will alter. Several of these altered processes are exacerbated by water diversions from the Delta

The mass and density of the dwarf planet (225088) 2007 OR10

The satellite of (225088) 2007 OR10 was discovered on archival Hubble Space Telescope images and along with new observations with the WFC3 camera in late 2017 we have been able to determine the orbit. The orbit's notable eccentricity, e$\approx$0.3, may be a consequence of an intrinsically eccentric orbit and slow tidal evolution, but may also be caused by the Kozai mechanism. Dynamical considerations also suggest that the moon is small, D$_{eff}$ $<$ 100 km. Based on the newly determined system mass of 1.75x10$^{21}$ kg, 2007 OR10 is the fifth most massive dwarf planet after Eris, Pluto, Haumea and Makemake. The newly determined orbit has also been considered as an additional option in our radiometric analysis, provided that the moon orbits in the equatorial plane of the primary. Assuming a spherical shape for the primary this approach provides a size of 1230$\pm$50 km, with a slight dependence on the satellite orbit orientation and primary rotation rate chosen, and a bulk density of 1.75$\pm$0.07 g cm$^{-3}$ for the primary. A previous size estimate that assumed an equator-on configuration (1535$^{+75}_{-225}$ km) would provide a density of 0.92$^{+0.46}_{-0.14}$ g cm$^{-3}$, unexpectedly low for a 1000 km-sized dwarf planet.Comment: Accepted for publication in Icaru