Theses and Dissertations at Montana State University (MSU)
Permanent URI for this collectionhttps://scholarworks.montana.edu/handle/1/733
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Item 20th Century forest-grassland ecotone shift and effects of livestock herbivory(Montana State University - Bozeman, College of Agriculture, 2005) Sankey, Temuulen Tsagaan; Chairperson, Graduate Committee: Cliff Montagne.I studied 20th Century lower forest-grassland ecotone shift in the Centennial Valley in southwestern Montana, USA and the Darhad Valley in northern Mongolia and investigated the effects of livestock herbivory on ecotone dynamics. A total of 525 aspen (Populus tremuloides) and 1,703 Douglas-fir (Pseudotsuga menziesii) trees were cored and 10,168 seedlings were counted at five sites along the ecotone in the Centennial Valley. A total of 2,968 Siberian larch (Larix sibirica) were cored and 4,709 seedlings were counted at five ecotones in the Darhad Valley. Tree-age distribution was constructed to determine 20th Century tree establishment. Tree age and location within the ecotone were correlated to describe the process of ecotone shift into the adjacent grassland. To examine livestock herbivory effects on ecotone shift, the number of new trees was correlated with ten different levels of cattle grazing intensity during the last 60 years in the Centennial Valley and with five different grazing regimes during the last 80 years in the Darhad Valley. Three different types of ecotone shift into the adjacent grassland were documented: forest boundary shift, densification, and fairy ring establishment. No evidence of ecotone shift upslope towards the forest was found. Grazing intensity had a complex relationship with tree encroachment. Aspen and Douglas-fir tree encroachment was low at medium levels of grazing intensity, but aspen establishment was higher at low and high grazing levels and Douglas-fir establishment was higher at low grazing levels. Siberian larch tree encroachment was higher at low and high grazing intensities by sheep and cattle, but it was lower at low and medium grazing levels by goat-sheep and goatsheep- cattle mixes. My results implied that grazing can both facilitate and inhibit tree encroachment. I propose a model of grazing effects on tree encroachment that integrates both inhibition and facilitation effects of grazing disturbance. I also propose a conceptual model of lower forest-grassland ecotone shift and a conceptual model of ecotone shift and livestock herbivory effects. My models suggest that grazing can be used as a tool to maintain the equilibrium between forest and grassland vegetation and to increase or decrease forest expansion.Item Temporal and spatial patterns at alpine treeline in the Sierra Nevada USA : implications for global change(Montana State University - Bozeman, College of Agriculture, 2004) Bunn, Andrew Godard; Chairperson, Graduate Committee: Lisa J. Graumlich.This work focuses on developing understanding of the role of climate variability in shaping montane landscapes. Due to the temperature-related stresses on treeline populations it is thought that change at treeline is an indicator of global warming. Interpretation of treeline changes has been hampered by issues of scale and the paucity of landscape-scale data. The application of remotely sensed imagery and computer-based mapping programs has filled this gap with datasets that have large extents and fine spatial grain. I used geospatial information about treeline in concert with population and paleoecological data to answer questions on ecological patterns and processes. My research focused on the treeline tree communities in the southern Sierra Nevada. There, on the eastern crest of the Sierra Nevada foxtail pine (Pinus balfouriana) forms an abrupt treeline in the vicinity of Sequoia National Park which gives way to a less well defined treeline of whitebark pine (P. albicaulis), limber pine (P. flexilis), and lodgepole pine (P. contorta) north to Yosemite National Park. By analyzing treeline growth patterns and spatial composition at the level of the region (109 m2), stand (103 m2), and organism (10-1 m2) I was able to make inference about the interactions of autecology, biophysical setting, and climate variability in shaping these subalpine forests over the last millennium. At the regional scale I improved paleoclimatic understanding derived from long chronologies of tree rings by relating species-specific differences in the climategrowth relationship over time to the realized niche space of each species in geographic space. At the level of the stand I was able to show how decadal versus centennial modes of growth and stand density vary with biophysical setting related to drought stress. At the level of the organism I was able to show that differences in tree growth and seedling patterns are related to fine scale physical variations in surrounding each tree as well as tree autecology. Over the course of my dissertation, I developed fresh insights into the complex interactions that govern the growth and structure of forests and improved the state-of-science for monitoring treeline as a critical indicator of global change.