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Reconstructing large herbivore abundance and environmental interactions in postglacial North America
(Montana State University - Bozeman, College of Letters & Science, 2023) Wendt IV, John Arthur Frederic; Chairperson, Graduate Committee: David B. McWethy; This is a manuscript style paper that includes co-authored chapters.
Large herbivores drive critical ecological processes, yet their long-term dynamics and effects are poorly understood due to the limitations of existing paleoherbivore proxies. To address these shortcomings, long-term records of paleoherbivores were constructed by (i) applying new analytical techniques to existing bison fossil datasets; and (ii) examining fecal steroid data that characterize temporal changes in ungulate abundance and community composition. These paleoherbivore reconstructions were analyzed in relation to their environmental contexts to better understand herbivore-ecosystem interactions through time in three separate studies: First, spatiotemporal changes in postglacial bison distribution and abundance in North America were examined by summarizing fossil bison observations. Bison observations were compared with simulated climate variables in a distribution modeling framework to project probable bison distributions in 1000-year intervals from the Last Glacial Maximum to present in light of changing climatic drivers over time. Since the Bolling-Allerod Interstadial (14.7-12.9 ka) the geographic distribution of bison is primarily explained by seasonal temperature patterns. Second, Holocene records of bison abundance were compared to paleofire reconstructions spanning the midcontinental moisture gradient to determine the relative dominance of herbivores and fire as biomass consumers. Bison dominated biomass consumption in dry settings whereas fire dominated consumption in wetter environments. Historical distributions of herbivory and burning resemble those of Sub-Saharan Africa, suggesting a degree of generality in the feedbacks and interactions that regulate long-term consumer dynamics. Third, the utility of fecal steroids in lake sediments for reconstructing past herbivore abundance and identity was tested by (i) characterizing the fecal steroid signatures of key North American ungulates, (ii) comparing these signatures with multiproxy data preserved in lake sediments from the Yellowstone Northern Range, and (iii) comparing influxes of fecal steroids over time to historical records of ungulate biomass and use. Bison and/or elk were abundant at Buffalo Ford Lake over the past c. 2300 years. Ungulate densities in the watershed were highest in the early 20 th century and likely contributed to decreases in forage taxa and possibly increased lake production. These results demonstrate long-term ecological impacts of herbivores and highlight opportunities for continued development of paleoherbivore proxies.
A forest entombed in ice: a unique record of mid-Holocene climate and ecosystem change in the northern Rocky Mountains, USA
(Montana State University - Bozeman, College of Letters & Science, 2022) Stahle, Daniel Kent; Chairperson, Graduate Committee: David McWethy; This is a manuscript style paper that includes co-authored chapters.
Across the high alpine of the northern Rocky Mountains small vestiges of perennial ice have endured for thousands of years. These ice patches reside hundreds of meters above modern treeline, with some persisting through mid-Holocene warmth and others establishing at the onset of a cooler period that began around 5,000-5,500 years BP. Recent warming-driven melting at the margins of one ice patch high on the Beartooth Plateau of northern Wyoming exposed over 30 intact mature whitebark pine (Pinus albicaulis) tree boles, all > 25 cm in diameter. We extracted cross-sectional samples from the stems of 27 preserved logs, and radiocarbon dated annual growth rings from 11 of these trees, anchoring the chronology to a date range spanning 5,947 to 5,436 years BP + or - 51.3 years. From this fossil wood chronology, we developed estimates of warm-season, annual, and biennial average temperatures for upper-elevation treeline during the mid-Holocene. To identify the predominant climate-growth relationships of the subfossil trees, we sampled live whitebark pine trees growing at an adjacent treeline site approximately 120 m lower in elevation. Temperature was found to be the major driver of variability in tree growth at the modern treeline location, with trees producing narrower (wider) rings during periods of cooler (warmer) growing season temperatures. Using linear and non-linear transfer functions based upon the stable statistical relationship between modern tree growth and temperature, we reconstructed past temperature estimates from the ice patch subfossil ring-width chronology. Our results provide estimates of mid-Holocene warm-season (and biennial) average temperatures ranging from 5.7-6.5 °C (-0.44-0.26 °C) respectively. A multi-century regional cooling trend beginning around 5,650 years BP resulted in average temperatures declining below a warm-season (biennial) critical threshold of ~5.8 °C (-0.34 °C), likely leading to the eventual death of the whitebark pine stand and subsequent formation of the ice-patch. This high-quality paleo-ecological dataset reveals a major shift in the alpine and forest ecotone on the Beartooth Plateau following the mid-Holocene warm period and offers further insight on the thermal limits of whitebark pine trees in the Greater Yellowstone Ecosystem.
Reconstructing the age, provenance, and thermal history of the basal Great Valley forearc basin, northern California
(Montana State University - Bozeman, College of Letters & Science, 2022) Romero, Mariah Christina; Chairperson, Graduate Committee: Devon A. Orme; This is a manuscript style paper that includes co-authored chapters.
Forearc basins are important sediment archives of Earth's geologic history, preserving a record of the erosional history of magmatic arcs, subduction zone dynamics, and climatic changes over millions of years. However, questions remain about the early developmental stages of a forearc basin, including the relationship between a forearc basin and its underlying basement, and thermal histories of exhumed, ancient forearc basins that preserve extensive sedimentary successions. This dissertation examines the basement underlying the Great Valley Forearc basin, the upper Coast Range Ophiolite and ophiolitic breccia, and Great Valley Group strata using sedimentology, sandstone modal petrography, geochronology, thermochronology, and cathodoluminescence, photoluminescence, and Raman spectroscopy. Chapter 1 evaluates Upper Jurassic-Lower Cretaceous strata and underlying Coast Range Ophiolite in the northwestern Sacramento basin to constrain the timing of initial deposition within the Great Valley Forearc, identify potential provenances, and provide a tectonic model for the early development of the forearc. Detrital zircon and petrographic data from a localized breccia interval directly underlying basal forearc strata indicate provenance from the Coast Range Ophiolite and North American margin, with formation ongoing by ~151 Ma. Detrital zircon data from Upper Jurassic-Lower Cretaceous strata yield maximum depositional ages between ~165-141 Ma and are interpreted to reflect diachronous deposition in segmented depocenters during the early development of the forearc that was proximal to the Sierra Nevada-Klamath magmatic. Chapter 2 utilizes apatite and zircon (U-Th)/He thermochronology to constrain the thermal history of the Sacramento basin, which includes documenting minimum burial temperatures for the western outcrop belt to have exceeded 85°C with cooling in the Cenozoic, whereas the subsurface strata of the central-eastern parts of the basin reached ~180-200°C with cooling in the middle-Late Cretaceous and Cenozoic. Chapter 3 examines gabbro and granitic basement rock underlying the Great Valley Forearc, along with several zircon U-Pb age standards, to demonstrate that radiation damage in zircon can non-destructively be estimated using cathodoluminescence spectroscopy, a complementary technique to photoluminescence and Raman spectroscopy. This dissertation highlights the significance of studying forearc basins along with their accompanying basement component, as the linked relationship is crucial for understanding how forearc basins evolve.
Late Holocene vegetation and fire history in subalpine forests of northwestern Montana
(Montana State University - Bozeman, College of Letters & Science, 2020) Sly, Shelby Fulton; Chairperson, Graduate Committee: David McWethy
Mid-to high- elevation forests of the Northern Rocky Mountains are dynamic systems that change in time in response to climate, disturbance and human activities. Climate models suggest these ecosystems will experience warmer temperatures, decreased spring snowpack, drier summers, and longer fire seasons, highlighting a need to better understand how these systems respond to changing climatic conditions. Sediment cores were extracted from two lakes in the Mission and Reservation Divide Mountains of Montana and analyzed for pollen and macroscopic charcoal to reconstruct fire and vegetation histories. The records from Mud Lake and Three Lakes Peak span the last 5400 and 4600 years respectively, and highlight the long-term persistence and relative stability of closed, mixed conifer/subalpine forests. During the mid-Holocene (approx. ca. 5000 yr BP), Pinus pollen percentages increased, suggesting closed forests which then transitioned to mesic forest of Pinus, Abies, and Picea over the last ca. 4000 years. Modern forests established between 3500-3000 cal yr BP at both sites. Both sites experience elevated fire activity during the Medieval Climate Anomaly c. 1000-900 cal yr BP which is consistent with other sites in the region. While relatively infrequent (2-4 fires per millennia), wildfires played a role in maintaining early successional vegetation (Poaeceae, Alnus spp.) and taxa that benefit from post-fire reduction in competition such as five-needle pines. Paleoenvironmental records from these two sites indicate subalpine forests of northwestern Montana persisted with relative stability throughout the mid to late-Holocene. With projections for increased warming, longer fire seasons and the possibility for increased occurance of short-interval fires in higher elevation ecosystems of the Northern Rocky Mountains, subalpine forests may become vulnerable to rapid transitions to different forest types or even non-forest systems.
Disentangling anthropogenic and natural drivers of change in vegetation and fire history along the forest-grassland ecotones of the central United States and Patagonia
(Montana State University - Bozeman, College of Letters & Science, 2020) Nanavati, William Parashar; Chairperson, Graduate Committee: Cathy Whitlock; Eric C. Grimm was a co-author of the article, 'Humans, fire, and ecology in the southern Missouri Ozarks' in the journal 'The holocene' which is contained within this dissertation.; Cathy Whitlock, Valeria Outes and Gustavo Villarosa were co-authors of the article, 'A holocene history of Araucaria araucana in northernmost Patagonia' submitted to the journal 'Journal of biogeography' which is contained within this dissertation.; Cathy Whitlock, Virginia Iglesias and Maria Eugenia de Porras were co-authors of the article, 'Postglacial vegetation, fire, and climate history along the eastern Andes, Argentina and Chile (lat. 41-55°S)' in the journal 'Quaternary science reviews' which is contained within this dissertation.
Disentangling anthropogenic and natural drivers of vegetation and fire history at different spatiotemporal scales is a fundamental challenge in Earth Systems science. To better understand the role of past human ignition in altering long-term ecosystem dynamics, we rely on the anthropogenic fire regime conceptual model proposed by Guyette et al. (2002) in the central U.S. Ozarks. The synthesis of new and existing pollen and charcoal records, and their integration with archaeological, ethnographic, and independent paleoclimate records is used to test the anthropogenic fire regime conceptual model at a longer time scale in the central U.S. Ozarks. Following its validation, this conceptual model is applied to the forest-steppe ecotone east of the Patagonian Andes (38-55°S) for the first time. Although it is well established that Patagonian vegetation and fire history for most of the postglacial period was governed by the strength and position of the Southern Westerly Wind (SWW) storm tracks, the influence of land use since the arrival of American Indians to the region ~12,000 years ago remains unclear. From the late glacial to early Holocene, region-wide increases in fire were associated with aridity while the SWW were weakened and south of their present position. Between ~7000-4000 cal yr BP, increased arboreal taxa and decreased fire throughout Patagonia suggest wet conditions as the SWW moved northward to their present position. After ~4000 cal yr BP, a combination of increased land use and greater climate variability, led to spatially heterogeneous but generally rising fire activity along the forest-steppe ecotone. When trends in the vegetation and fire history of individual sites are compared to each other and to the archaeological record, however, it becomes apparent that American Indians may have served as an important source of ignition, locally increasing landscape heterogeneity since their arrival. During the last 100 years, increased Euro-American settlement and land clearance in Patagonia led to native forest loss, more disturbance, and the spread of introduced taxa along the eastern flanks of the Andes. These ecological changes in the recent century far outweigh thousands of years of American Indian influence on fire and vegetation history.
Assessing alternative drivers of occupancy, abundance, and elevational range retraction at the range core of a climate-sensitive mammal
(Montana State University - Bozeman, College of Letters & Science, 2020) Billman, Peter Donn; Chairperson, Graduate Committee: David McWethy
Ecological niche theory and niche conservativism suggest that rising temperatures globally will continue pressuring species to track cooler environments. Examining changes in occupancy and abundance together across bioclimatic gradients can inform forecasts of expected range shifts. Although occupancy and abundance reflect similar aspects of species-environment relationships, they are governed by different underlying processes. Abundance is thought to be more reflective of shorter-term conditions affecting vital rates, whereas site occupancy often reflects habitat suitability. By directly comparing results of both response types, as well as elevational range retraction, we offer a robust method for assessing complex species-climate relationships. In this study, we test how populations of the American pika (Ochotona princeps), a small montane lagomorph, respond to varying climatic conditions. To do so, we tested and compared the drivers of site occupancy, abundance, and upslope retraction, across 760 talus patches, nested within 64 watersheds across the Northern Rocky Mountains, USA. Using mixed-effects modeling, paired with an information-theoretic approach, we tested model suites that reflected hypothesized species-climate relationships to identify the top models of each of our response classes. Approximately one third (33.9%) of patches were found extirpated. The most important environmental predictors differed among occupancy, abundance, and amount of upslope retraction. For site occupancy, the top model included metrics of summer acute heat stress, actual evapotranspiration, and habitat availability. For abundance, acute heat stress and the preceding winter's mean temperature (i.e. chronic cold stress) was the top-ranked model, suggesting rapid responses of populations to recent climatic conditions. Furthermore, we found that a model including both chronic heat and chronic cold stress best predicted the total amount of vertical retraction across watersheds, whereas acute heat stress and summer precipitation best explained the residuals. Our results emphasize the complexity associated with evaluating species responses to environmental change and that results from occupancy analyses should be used with caution when extrapolating to predicting abundances across varied landscapes. Our method for assessing the drivers of elevational retraction across a suite of watersheds has widespread applications for evaluating species response to changing climatic conditions elsewhere.
Late Holocene climate, fire, and vegetation history on the northern range, Yellowstone National Park
(Montana State University - Bozeman, College of Letters & Science, 2019) Firmage, David Samuel; Chairperson, Graduate Committee: Cathy Whitlock
Yellowstone National Park is an iconic natural landscape that encompasses unique geologic features as well as a diverse and ecologically important flora and fauna. The ecological resilience of the Northern Range of the park, home to the park's ungulate herds, faces an uncertain future with a projected warming climate over the next century. Understanding the variability of vegetation organization in response to past changes in climate can help park managers plan for future climate scenarios. Lake sediment cores from two lakes were collected, analyzed for pollen type, charcoal accumulation and lithological components, and compared with other studies to highlight commonalities in fire-histories and vegetation trends across the Northern Range over the past 4000 cal yr BP. Foster Lake the records suggest fire-episodes and changes to lake productivity and between ~3500-2900 cal yr BP, large intense fire-episodes between ~2500-2150 cal yr BP, ~1000 cal yr BP and frequent large fire episodes from ~700 cal yr BP to the present day. Floating Island Lake records between ~4000-3000 cal yr BP suggest the site experienced infrequent, large fire episodes concurrent with periods of protracted drought and decreases in water level. Between ~3000-1500 low intensity fire episodes were common, punctuated by infrequent large fire episodes at ~2900, ~2250, ~2050 and ~1880 cal yr BP. During the last ~1ka two fire episodes were recorded at Floating Island Lake, at ~1030 cal yr BP coincident with the Medieval Climate Anomaly, and 270~ cal yr BP during the Little Ice Age. A comparison of fire histories from studies spanning the Northern Range shows that during periods of protracted drought large fire episodes are common across the landscape, and that during periods of moderate climate fire size and severity is likely modulated by local site controls such as topography and vegetation structure. This study shows that fire episodes in the Northern Range have occurred as a spatial and temporal mosaic, and are likely to continue to do so. Additionally, this study increases our understanding of how vegetation structure and fire regimes in the Northern Range have varied as a result of a range of climate conditions in the past. Such baseline information helps us anticipate some of the ecological responses that may occur in the decades ahead with global warming.
Paleofire patterns in Tasmania: postglacial and Holocene record of fire and vegetation from Cradle Mountain-Lake St. Clair National Park, Tasmania, Australia
(Montana State University - Bozeman, College of Letters & Science, 2017) Stahle, Laura Nicole; Chairperson, Graduate Committee: Cathy Whitlock; Cathy Whitlock was a co-author of the book chapter, 'Getting information from the past: paleoecological studies of terrestrial ecosystems' in the book 'Key methods in geography' which is contained within this thesis.; Cathy Whitlock and Simon G. Haberle were co-authors of the article, 'A 17,000-year-long record of vegetation and fire from Cradle Mountain National Park, Tasmania' submitted to the journal 'Frontiers in ecology and evolution' which is contained within this thesis.; Cathy Whitlock and Simon G. Haberle were co-authors of the article, 'The postglacial and holocene record of fire and vegetation from Cradle Mountain National Park, Tasmania, Australia' which is contained within this thesis.
On centennial to millennial timescales fire regimes are driven by climate changes, vegetation composition, and human activities. In this study, I reconstructed the vegetation and fire history of Cradle Mountain-Lake St Clair National Park in northwestern Tasmania, and linked vegetation changes to variations in the fire regime, large-scale climate patterns, and anthropogenic activity. Postglacial vegetation and fire dynamics were inferred from five high-resolution pollen and charcoal records from lakes in the montane and subalpine zones of the National Park. Watershed-scale reconstructions of fire and regional trends in vegetation composition were compared to independent records of past climate and the regional archeological record to assess long-term climate-vegetation-fire-human linkages. Pollen and charcoal data indicate that during late-glacial period, the vegetation was largely open and fires were rare. During the Pleistocene-Holocene transition, rainforest taxa and subalpine woody shrubs began to increase in abundance. In the early Holocene, a fire activity maximum occurred at the three lower elevation sites while biomass burning remained low at the higher sites. The elevational differences and basin characteristics likely resulted in climate-controlled differences in vegetation and fuel flammability. The high biomass burning in the early Holocene occurred during the warmest interval of the Holocene as recorded by regional paleoclimate proxy records. The mid-Holocene period featured a multi-millennial phase of cool, temperate rainforest dominance at all sites. The relatively wet conditions of the mid-Holocene likely allowed the rainforest to reach its maximum extent. The late Holocene marks a regional shift toward open sclerophyll woodland associated with increased climate variability and decreased precipitation. A large fire episode occurred at all five sites during this period and hastened the shift in compositional balance from rainforest to a mosaic of sclerophyll woodlands and shrublands, rainforests, buttongrass moorlands, and alpine vegetation. Overall, the vegetation became substantially more open in the late Holocene. The human-fire linkages in Cradle Mountain are tenuous. There is no clear evidence that fire regimes or vegetation were extensively modified by humans prior to European settlement. Climate was the primary driver of fire activity over millennial timescales as explained by the close relationship between charcoal and climate proxy data.
Postglacial vegetation and fire history of the southern Mission Valley, Montana
(Montana State University - Bozeman, College of Letters & Science, 2017) Alt, Mio Hazel; Chairperson, Graduate Committee: Cathy Whitlock
Ecosystems shaped by mixed - severity fire regimes cover a large area of the Northern Rocky Mountains, yet relatively little is known about the historical variability and drivers of these ecosystems. The low - and mid - elevations of the Mission Range, Montana, are dominated by mixed conifer forests, and the area has been occupied by humans for at least 10,000 years, making it an ideal location for investigating how climate and humans may have affected vegetation and fire regimes during the late - glacial period through the Holocene. Pollen and charcoal records from lake sediment cores from a small closed - basin lake (Twin Lake) were used to reconstruct the vegetation and fire history of the southern Mission Valley, Montana, and compared to other sites in the region. During the late - glacial period, data show an abundance of Pinus (P. albicaulis or monticola) Artemisia, and Poaceae pollen prior to 13,000 cal yr BP, suggesting the site was dominated by an open landscape with shrubs and grass, cold relatively dry conditions, and minimal fire activity. Increased percentages of Pinus (P. Ponderosa or contorta), Picea, and Abies pollen at 13,000 cal yr BP mark the onset of a closed conifer forest, relatively cool and wet conditions and an increase in fire activity accompanying an increase in biomass. Large increases in Pseudotsuga/Larix and Artemisia pollen between 10,000 - 6000 cal yr BP suggest warmer and drier climatic conditions developed during this interval, consistent with other records from the northwestern U.S. Charcoal influx show this interval of warm and dry conditions led to low severity fires followed by high severity fires as forests of P. contorta or P. ponderosa became more dense between 7000 and 5000 cal yr BP. The mixed - conifer forests that dominate the site today began to develop ca. 6000 cal yr BP when fire frequency and severity became highly variable. Surprisingly, fire activity from ca. 5000 cal yr BP to present remained relatively high despite a cooling and wetting trend in the region. This departure of fire activity from climatic controls suggests other local factors influenced fire activity, and may suggest a greater role of human influence during the late Holocene.
Snowpack driven changes in decadal soil evolution: insights from a 48-year snow manipulation experiment
(Montana State University - Bozeman, College of Letters & Science, 2017) Feldhaus, Aaron Michael; Chairperson, Graduate Committee: Jean Dixon
Soil mantled landscapes are a critical interface that support biological life, weather geologic materials, and develop in response to changes in climate. Climate has long been considered a dynamic control on the evolution of Earth's landscapes. However, we have limited understanding regarding how soils respond to short-term perturbations of key climate variables like precipitation and moisture availability. Furthermore, the timescales over which diverse weathering processes feedback and measurably change soil character are still relatively uncertain, as well as how they respond to swift changes in climate. Here, we explore the role of precipitation in decadal soil evolution by utilizing a 48-year snowpack experiment located in the Greater Yellowstone Ecosystem (GYE) of SW Montana. In this unique field site, we compare soil development across experimental plots with enhanced snowpack, where snow has been doubled (2x) and quadrupled (4x) above ambient conditions for almost five decades. We find that decadal snowpack addition provides multiple pathways for enhancing soil weathering, both physically and chemically. Soils under enhanced snowpack generally contain higher amounts of fine-grained material (clay and silt) and are more acidic (lower soil pH) in nature. Significant (>85%) surface depletions of the fallout radionuclide 210 Pb and reduced surface horizon carbon and nitrogen content, along with reduced above and below ground vegetation biomass provide evidence of increased wind erosion of soils that experience enhanced winter snowpack. Modeling of diffusion-like mixing from 210 Pb profiles also indicates there is increased bioturbation intensity (soil mixing) under enhanced snowpack. We find that snowpack addition, through associated changes in plant communities and vegetation biomass, along with its effects on physical and chemical weathering processes, produces rapid and measurable changes in the weathered state of soils. Our results indicate that short-term, decadal perturbations in snowpack significantly alter weathering mechanisms in this landscape, which measurably overprint thousands of years of soil development. These findings provide novel insight into the fundamental role of climate on short-term soil evolution and have significant implications for how mountainous or snowpack-dominated systems may respond to perturbations in climate.

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