Prescribed burning is a common management technique to reduce non‐native grass cover and seed availability in temperate forests and grasslands; however, its effectiveness in reducing non‐native forbs ...is unclear. Litter of invasive forbs like Oncosiphon pilulifer are not consumed by fire like invasive grass litter is, resulting in residual singed stands and high heterogeneity in the postburn landscape. We investigated the potential for this incomplete burning to alter postfire establishment of native and non‐native plant species by conducting a field experiment in a prescribed burn in Lake Perris State Park, CA. We investigated the role of microclimate and seed availability on establishment for 2 years following a prescribed burn in both singed stands and completely burned patches by adding or removing litter and adding native seed in a factorial design. Litter presence reduced soil temperatures and light availability, while singed stands had lower soil moisture and temperature. Litter present treatments had 5.6 ± 5.9% (mean ± SE) greater Oncosiphon cover yet doubled Oncosiphon viable seeds in the seedbank. Singed stands had 22.6 ± 4.9% greater Oncosiphon cover and more than doubled Oncosiphon viable seeds. Native seed addition did not influence native or Oncosiphon cover. These results suggest that residual singed stands within the prescribed burn landscape can create a favorable microclimate and allow Oncosiphon to retain seed, increasing reinvasion. Our experiment suggests that litter increased establishment of non‐native species as these species may better utilize postburn establishment opportunities impacting overall community recovery. Management of invasive forbs with prescribed burns may require secondary treatments to reduce reinvasion.
Habitat restoration is an important tool for managing degraded ecosystems, yet the success of restoration projects depends in part on adequately identifying preferred sites for restoration. Species ...distribution modeling using a machine learning approach provides novel tools for mapping areas of interest for restoration projects. Here we use stacked-species distribution models (s-SDMs) to identify candidate locations for installment of manmade reefs, a useful management tool for restoring structural habitat complexity and the associated biota in marine ecosystems. We created species distribution models for 21 species of commercial, recreational, ecological, or conservation importance within the Southern California Bight based on observations from long-term reef surveys combined with high resolution (200 x 200m) geospatial environmental data layers. We then combined the individual species models to create a stacked-species habitat suitability map, identifying over 800 km2 of potential area for reef restoration within the Bight. When considering only the 21 focal species, s-SDM scores were positively associated with observed bootstrap species richness not only on natural reefs (linear model: slope = 0.27, 95% CI = 0.17 - 0.36, w = 1), but also this result was supported by two independent test datasets. The predicted richness from this linear model was associated with observed species richness when considering only the focal species on manmade reefs (linear model: slope = 0.52, 95% CI = 0.13 - 0.92, w = 1) and also when considering 204 other non-focal species on both natural and manmade reefs in southern California (slope = 3.65, 95% CI = 2.93 - 4.37, w = 1). Finally, our results demonstrate that the existing manmade reefs included in our study on average are located in regions with habitat suitability that is not only less suitable than natural reefs (t-value = -5.4; p < 0.05) only, but also only slightly significantly better than random (p < 0.05), demonstrating a need for more biologically informed placement of manmade reefs. The stacked-species distribution model provides insight for marine restoration projects in southern California specifically, but more generally this method can also be widely applied to other types of habitat restoration including both marine and terrestrial.
Ecosystems are undergoing unprecedented rates of change with severe consequences for biodiversity loss, yet natural resilience and restoration of ecosystems provides hope to mitigate these losses. ...Despite the potential for natural recovery, there are thresholds and feedback mechanisms that inhibit recovery that are often driven by invasive species. Consequently, understanding how invasive species interact with their surrounding communities and how they respond to restoration efforts is crucial information for effective management and successful restoration. My dissertation seeks to understand how the impact of invasive plants prevents effective restoration through a combination of field experiments with paired greenhouse components to capture robust processes in the field and disentangle the underlying mechanisms in a controlled greenhouse setting. My first two chapters investigate how a local and regionally obnoxious novel invasive forb, Oncosiphon pilulifer, responds to the common management technique of prescribed fire, and how Oncosiphon interacts with soil biota to inhibit native plant growth. My final chapter focuses on how the multiple factors of seed limitation, invasive litter accumulation, and soil symbiont depletion constrain restoration success in a Northern Californian annual grassland. Taken together, my dissertation projects aim to both address fundamental questions in community ecology and produce actionable science for land management practitioners.
A number of studies have shown that the introduction of lubricious solid films, especially metal oxides, can improve elevated temperature tribological performance. Two such studies have identified a ...class of oxides that offer exceptional tribological performance at elevated temperatures. Certain combinations of ion-implanted titanium and nickel yielded coefficients of friction (COF) of 0.06–0.09 at 800°C. Examination of the surface layers by Auger spectroscopy indicated that the low friction coefficients were obtained on surfaces with a titanium oxide film that had a substantial nickel content. The second study demonstrated a dramatic reduction in the wear of silicon nitride by adding 15
vol.% titanium carbide. Reductions in the COF by up to 50% were also obtained. Auger spectroscopy of the wear tracks identified a mixed titanium and silicon oxide as the lubricious species in the titanium carbide-containing silicon nitride.
We have developed wet chemical methods of depositing the titanium oxide materials believed to be responsible for the improved tribological performance observed in previous studies. Deposition of the oxides as nanostructured materials should promote lubricious behavior at sub-ambient temperatures. The microstructure was examined by X-ray diffraction (XRD) of powders, energy dispersive spectroscopy (EDS) in a scanning electron microscope (SEM), and thermal analysis (TGA/DSC). COF was measured by a reciprocating ball-on-plate at 25 and 500°C. Wear tracks were observed with surface profilometry, optical microscopy and SEM. Titania/nickel oxide coatings reduced the COF by at least 50% at both temperatures and demonstrated the best wear resistance. This paper will discuss these results with respect to microstructure and processing parameters.
Although carbon fiber reinforced carbon matrix (C/C) composites are used in the aerospace and utility industries, carbon materials oxidize readily at temperatures above 425 °C. A variety of coatings ...have been used to protect C/C composites from oxidation. The thermal expansion mismatch creates microcracks that could result in catastrophic failure. To alleviate these difficulties, we have used the approach of modifying the carbon precursor molecule with organometallic functions, which results in uniform and nanosize mixing of antioxidants in the carbon matrix. Mesophase pitch, which is a carbon precursor, was grafted with silicon and aluminum preceramic functions by using organometallic chemistry to produce Compounds
1, 2, 3 and
4. These organometallic pitches produce Chars
A, B, C and
D upon carbonization. Char
C exhibits oxidation resistance superior to that of commercial carbon fiber.
Nanostructure of polymer-derived silicon nitride Schwab, Stuart.T.; Page, Richard A.
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
12/1995, Letnik:
204, Številka:
1
Journal Article, Conference Proceeding
Recenzirano
Although silicon nitride (Si
3N
4) ceramics have considerable potential for use in high temperature applications such as engines, processing-related difficulties and associated high cost have ...hindered their acceptance. While chemical processing methods, such as those based on preceramic polymers, have the potential to overcome many of the difficulties inherent to traditional techniques, many features of polymer-derived Si
3N
4 are poorly understood. Transmission electron microscopy (TEM) has been used to characterize the nanostructure of polymer-derived Si
3N
4. The polymer-derived material is found to consist of large grains composed of very fine crystallites, as well as fibrils 40–50 nm in diameter. If the factors responsible for the formation of these structures were understood, the understanding could be harnessed to produce Si
3N
4 components with enhanced fracture resistance and other engineering properties.
Titanium aluminides are among the more promising intermetallics for use in aerospace and automotive applications; however, their acceptance has been hampered by their lack of ductility. Significant ...improvement in ductility may be obtained from nanostructured intermetallics. Reaction of TiN(SiMe
3)
2
3 with excess alane produces a precursor (Compound 1) to titanium aluminide. We propose compound 1 to be a loose cluster (or family of clusters) in which titanium and aluminum atoms are both bonded directly and bridged by imidosilanes. This chemically synthesized precursor, when heated to 1000 °C, produces nanosized particles of TiAl
3. Nanosized TiAl
3 has been characterized by chemical analysis, solid-state NMR, X-ray diffraction, energy dispersive spectroscopy, and high-resolution electron microscopy.
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