Thesis (Ph.D.)--University of Wollongong, 2005. / Typescript. Includes bibliographical references: leaf 215-246.

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1993. / Vita. Abstract. Includes bibliographical references (leaves 182-218). Also available via the Internet.

Thesis (doctoral)--Universität, Freiburg im Breisgau, 2005. / University publication. Includes bibliographical references (p. 137-151).

Thesis (Ph. D.)--University of Wisconsin, 1967. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

The mesic forests distributed within the Rocky Mountains of northern Idaho are unique because many species contained within the forest are separated from their main distribution along the Pacific Northwest coast. It remains unclear whether most species within the inland disjunction survived the glacial periods of the Pleistocene, or whether they were more recently dispersed from coastal populations. To see if the dominant tree taxa of the mesic forests today could have persisted in a refugium south of the large ice sheets, four sediment cores were used to reconstruct the vegetation and climate history of the region. A nearly continuous record of pollen and sediment composition (biogenic silica and inorganic and organic matter) over the last ca. 120,000 years provides evidence of a dynamic ecosystem. Over a long timescale, the slow shifts in vegetation are attributed to the changes in climate. During the last interglacial period, the region was warmer and drier with a Pinus dominated mixed conifer forest. Approximately 71,000 years ago, a Pseudotsuga/Larix forest became established in the area as a response to the increased available moisture. As climate cooled and glaciers expanded the Pinus and Picea forest was the dominant vegetation type until ca. 40,000 years ago. The environment during the Last Glacial Maximum (LGM) was so harsh that no vegetation record was recorded. After the LGM, climate warmed, enabling a Pinus and Picea forest to establish and persist until the Holocene. The mesic taxa that dominate the modern forests did not arrive in northern Idaho until the mid- to late-Holocene. The recent arrival of the dominant tree species, Thuja plicata and Tsuga mertensiana, suggests that they likely did not persist in a refugium during the last glaciation. Instead, these species recently dispersed from coastal populations, but expansion into their interior distributions was likely limited by both climate and species competition in already established forests. During the late-Quaternary, the deposition of thick tephra layers (>20 cm) from the eruptions of Glacier Peak (ca. 13,400 years ago) and Mt. Mazama (ca. 7,600 years ago) also facilitated an abrupt and persistent change in vegetation in northern Idaho.

No description available.

<p> The forest-tundra-short wave feedback is the dominant short wave (SW) vegetation feedback at mid-to-high northern latitudes and is an important feedback in Earth&rsquo;s climate system, especially due to its potential role in modulating glacial cycles. Little research has been done on how the strength of this feedback might vary with the background climate state. It is hypothesized that the feedback has generally strengthened over the last four million years. The feedback mechanism is hypothesized to be weaker under warm Northern Hemispheric conditions when tundra is primarily confined to the high Arctic than under cooler conditions in which the forest-tundra boundary lies generally south across the interiors of the large continental land masses. To test the hypothesis of the weakened/strengthened feedback, an Earth System Model of Intermediate Complexity is used that consists of a newly-developed simple dynamic terrestrial vegetation model coupled to a general circulation atmospheric model and a slab ocean. The response to the same orbital forcing ("cold orbit", favorable to Northern Hemispheric glacial inception) is analyzed for two eras: the PRISM mid-Pliocene Warm Period and the pre-industrial Holocene. </p><p> The change in top-of-atmosphere short wave net radiation (TOASW_net) that is attributable to including interactivity of vegetation in the systemic response to orbital forcing is decomposed into the product of three terms: the short wave vegetation feedback, an effective orbital forcing term, and the amplification of this effective orbital forcing by the climate system when vegetation cover is held fixed. Further analysis is carried out to determine why these terms differ between each era (mid-Pliocene and pre-industrial). </p><p> The results show that the change in TOASW_net that is attributable to including interactivity of vegetation in the systemic response to orbital forcing is about four times as strong in the pre-industrial as in the mid-Pliocene. The mid-to-high latitude SW vegetation feedback is about twice as strong for the pre-industrial as for the mid-Pliocene. This SW vegetation feedback is stronger in the pre-industrial mostly because its climate system is more sensitive in boreal spring to climate-induced changes in vegetation for various reasons, many of which boil down to geography. Surface albedo change is the principle mechanism by which the forest-tundra-short wave feedback operates, but it is discovered that there is also a component of this feedback that operates through the interactions between atmospheric reflectivity and vegetation. The results suggest that the forest-tundra-short wave feedback in glacial inception strengthens as the baseline climate cools from early Pliocene levels, with important implications for the start of major Northern Hemispheric glaciation and the increasing amplitude of glacial-interglacial oscillations over the last few million years.</p><p>

Despite lake sensitivity to climate change, few Florida paleolimnological studies have focused on changes in hydrology. Evidence from Florida vegetation histories raise questions about long-term hydrologic history of Florida lakes, and a 25-year limnological dataset revealed recent climate-driven effects on Lake Annie. The objectives of this research are (1) to use modern diatom assemblages to develop methods for reconstruction of climatic and anthropogenic change (2) to reconstruct both long-term and recent histories of Lake Annie using diatom microfossils. Paleoenvironmental reconstruction models were developed from diatom assemblages of various habitat types from modern lakes. Plankton and sediment assemblages were similar, but epiphytes were distinct, suggesting differences in sediment delivery from different parts of the lakes. Relationships between a variety of physical and chemical data and the diatoms from each habitat type were explored. Total phosphorus (TP), pH, and color were found to be the most relevant variables for reconstruction, with sediment and epiphyte assemblages having the strongest relationships to those variables, six calibration models were constructed from the combination of these habitat types and environmental variables. Reconstructions utilizing the weighted averaging models in this study may be used to directly reveal TP, color, and pH changes from a sediment record, which might be suggestive of hydrologic change as well. These variables were reconstructed from the diatom record from both a long-term (11,000 year) and short-term (100 year) record and showed an interaction between climate-driven and local land-use impacts on Lake Annie. The long-term record begins with Lake Annie as a wetland, then the lake filled to a high stand around 4000 years ago. A period of relative stability after that point was interrupted near the turn of the last century by subtle changes in diatom communities that indicate acidification. Abrupt changes in the diatom communities around 1970 AD suggest recovery from acidification, but concurrent hydrologic change intensified anthropogenic effects on the lake. Diatom evidence for alkalization and phosphorus loading correspond to changes seen in the limnological record.

Explosive volcanism can release large amounts of particles and gases into the atmosphere. Sulfuric acid droplets in the lower stratosphere are the primary substance interacting with the radiative fluxes over many months and possibly years. Because of their sub-micron size, they are more efficient at scattering incoming shortwave radiation from the sun back into space than absorbing and trapping longwave radiation from the earth. This results in a negative impact on the earth energy balance causing a general cooling below the aerosol layer. The magnitude of the cooling depends mostly on the amount of radiatively active aerosol particles as well as the duration of the perturbation. The cooling signal is largest in the upper troposphere through feedbacks with a slowed hydrologic cycle. At the surface, heat release, mostly from the oceans, can buffer some of the cooling. A combined approach using both observations/proxy data and a state-of-the-art coupled General Circulation Model (GCM) to analyze the volcanic effects on climate can help in our understanding of the possible range of responses. Here, the most recent large eruption of Mt. Pinatubo (June 1991) was used to verify the implementation of the aerosol parameterization in the radiation code of the GCM. From there, an analysis of the volcanic contribution since 1870 A.D. was performed. A simple way of describing the spatial aerosol distribution is presented. In general, only a handful of eruptions were found large enough to significantly perturb the radiative balance of the earth. These few events caused a global climate signal, which is clearly detectable against the background noise of internal variability of the climate system. Next to the influence of isolated events, model simulations confirm earlier suggestions that temporally closely spaced large events can cause a further cooling in climate before the system can recover. Thus, explosive volcanism must be regarded as an important player in decadal to multi-decadal natural climate variations. In case of the 20th-Century, volcanic cooling in the last decades could have offset any possible warming due to increased solar irradiation. The potential volcanic role in other important time periods in the past must be studied, including the cooling in decades generally solely attributed to the sun. Proxy records, in particular tree rings, point to a potentially large role of explosive volcanism in the past. Additionally to the radiative effects, increased atmospheric flow at high latitudes, particularly in winter, is the result of dynamical responses to changes in meridional temperature gradients in the lower stratosphere through heating in the aerosol layer. This effect helps to orchestrate the spatial distribution of the climate signal for several years after the eruption. Currently, no clear influence on other internal modes of variability, such as El Niño, could be unanimously confirmed in both observations/proxy reconstructions and the model simulations. But more work is needed, as better proxy climate data for earlier large events get available.

This project expands on the 4200-year history of Laguna Los Mangos by adding diatom analysis to existing records of pollen, charcoal, elemental composition, and stable light isotope analysis. Diatoms were counted in peroxide-treated samples, and data were analyzed using cluster analysis, PCA, CCA, and Pearson correlation. Diatom variability was correlated with nitrogen and organic material. Before 3430 cal yr BP, diatoms reflect landscape instability with higher lake levels and macrophyte cover. This period was followed by a period of agriculture-induced nutrient pollution, reduced pH, and increased precipitation until about 2450 cal yr BP. Peaks in A. granulata may indicate periods of increased mixing driven by precipitation. Diatom composition reflects reduced agricultural activity and lake desiccation during the TCD and increased precipitation and agricultural collapse during the LIA. Overall, this record reflects a history of slightly alkaline, eutrophic conditions, increasing salinity, and human disturbance from maize agriculture and deforestation. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2021. / FAU Electronic Theses and Dissertations Collection