Theses and Dissertations at Montana State University (MSU)
Permanent URI for this collectionhttps://scholarworks.montana.edu/handle/1/733
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Item Long-term environmental history of two low-elevation mixed-conifer forests, Mission Valley, Montana(Montana State University - Bozeman, College of Letters & Science, 2021) LaPierre, Kari Richard; Chairperson, Graduate Committee: David McWethyLow elevation mixed-conifer forests are widespread throughout the Northern Rocky Mountains, yet there are few long-term environmental histories from these structurally and compositionally heterogenous ecosystems. We reconstructed >10,000 years of vegetation change, fire activity, and human presence (e.g., pollen, charcoal, biomarkers) for two closed-basin lakes in mixed-conifer forests in the Mission Valley, western Montana. Environmental reconstructions highlight periods of pronounced changes in climate, vegetation, and fire activity. The late glacial period (>18,000-11,000 cal yr. BP) was characterized by post-glacial warming, generally wet conditions, establishment of mixed-conifer forests and infrequent fires. Following an abrupt, short-lived return to Juniper/Douglas fir parkland associated with the Younger Dryas (~12,900-11,500 cal yr. BP), warming temperatures during the early Holocene (11,000-6,000 cal yr. BP) promoted the expansion of open parkland/grasslands and frequent fire activity until cooler summers and warm, wet winters facilitated the development of modern-day closed mixed-conifer forests. Organic biomarker analyses indicate human presence within the Rainbow Lake watershed for millennia c. 7,000-3,000 cal yr. BP. Regional fire frequency increased during this period at Rainbow Lake, suggesting a possible increased role of human influence.Item 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 McWethyMid-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.Item The influence of forest on the distribution and size of surface hoar in small meadows(Montana State University - Bozeman, College of Letters & Science, 2015) Wieland, Matthew Allen; Chairperson, Graduate Committee: Jordy HendrikxSnow avalanches pose a significant hazard to winter recreationalists travelling in the backcountry and are difficult to predict on individual slopes. Weak layers responsible for these avalanches may form at the surface multiple times during the winter season and are buried by subsequent snowfall. Understanding causes of slope-scale weak layer variations during formation and destruction periods is crucial for gaining an understanding of their distribution after burial. Persistent weak snow layers, such as surface hoar, can pose hazards for months after burial. This study examines surface hoar crystals on the surface, directly after formation, in two small meadow openings in southwest Montana. Data collection occurred during two winter seasons for three surface hoar formation events. Three environmental metrics associated with surface hoar growth processes in meadow openings are explored and their relationships with crystal size examined using spatial regression and regression tree analysis. The spatial structure for each event is described using multiple crystal sizing measures through semi-variograms. Surface hoar crystals tended to grow largest in areas that were both shaded and possessed large unobstructed views of the sky on north and south aspects. The range of spatial autocorrelation for surface hoar crystal sizes varied from 7 m to beyond 25 m and differed depending on event or crystal sizing measure. Results vary between events and suggest the drivers controlling surface hoar growth are unique to each area and not consistent between events. This research highlights the need for multiple slope-scale snow stability assessments for understanding the distribution of a buried weak layer of surface hoar in a meadow opening. Targeted areas for assessment should incorporate a basic understanding of a meadow's shading and canopy openness and how this varies over a winter season.