Theses and Dissertations at Montana State University (MSU)
Permanent URI for this collectionhttps://scholarworks.montana.edu/handle/1/733
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Item Biogeochemical and plant functional group response to long-term snow manipulation in a subalpine grassland(Montana State University - Bozeman, College of Agriculture, 2013) Holsinger, Jordan Paul; Chairperson, Graduate Committee: Jack BrookshireSnow represents an important control over plant communities in seasonally snow-covered ecosystems. It constrains the growing season and affects the availability of important resources including water, nitrogen (N) and phosphorus (P). Snow depth, distribution and duration have been affected by global climate change making it increasingly important to understand the effects of changing snow regimes on terrestrial ecosystems. Here we leverage a 43-year snow manipulation experiment to examine the effects of long-term changes in snow depth on plant community structure, resource availability and interactions therein in a common grassland type of the northern Rocky Mountains in western North America. Long-term experimental doubling and quadrupling of snowpack was associated with a significant shift in plant functional group distribution to a more forb rich community. Snow addition has resulted in a two to three-fold increase in forb to grass biomass ratios over time. Forbs consistently had greater N and P contents and lower nutrient use efficiencies compared to grasses. Forbs also displayed higher rates of net photosynthesis relative to grasses and sustained positive carbon (C) fixation rates late into the growing season after grasses had ceased. Though there is evidence that water exerts considerable control over ecosystem processes, increased snow depth did not have affect soil water availability through the growing season. However, snow depth was associated with significant differences in plant-available phosphate across the entire growing season with approximate 15% and 31% increases in pools of available P relative to ambient snowpack depth for doubled and quadrupled snowpacks respectively. Estimates of direct P inputs via dust and the ratio of available P to total P in the soil suggest that internal cycling was largely responsible for the observed differences in pools of available P. However, growing season net mineralization rates do not differ across treatments. This may suggest that winter processes make significant contributions to nutrient cycles. It is possible that the increased availability of P favors the shift to a forb-rich community under deeper snow because of their increased productivity under dry conditions and that the increased litter quality of forbs likewise promotes increased litter decomposition and mineralization, especially of P.Item Biological and physical controls of CO 2 flux through snow in a forested ecosystem(Montana State University - Bozeman, College of Agriculture, 2013) Rains, Fredrick Aaron; Chairperson, Graduate Committee: Paul C. Stoy; Cliff Montagne (co-chair)Soil CO 2 efflux is the dominant component of carbon loss in many temperate forests. Wintertime respiration accounts for a significant contribution of the annual carbon loss to the atmosphere from terrestrial ecosystems, but the magnitude of this flux and physical transport mechanisms through snow are unclear. This research examines wintertime CO 2 flux in a lodgepole pine forest in the Upper Stringer Creek catchment at the Tenderfoot Creek Experimental Forest, Montana, USA. I hypothesized that: CO 2 production and efflux during the winter contributes a significant amount (10-20%)of CO 2 efflux to the atmosphere in the Tenderfoot Creek Experimental Forest; 2) Snow properties, i.e. depth and density, and thereby porosity and tortuosity vary during the winter via snow metamorphosis, thus changing the impediment to flux through the snow medium and CO 2 production increases when the snowpack becomes isothermal during melt due to increased soil moisture and soil temperature. A micrometeorological stations was installed to measure soil water content, soil temperature, incoming and outgoing radiation, albedo, snow depth, snow/soil interface CO 2 concentration, atmospheric CO 2 concentration, three-dimensional wind speed, and above snow/sub-canopy CO 2 flux on a half-hourly basis. In addition, throughout the winters of 2010/2011 and 2011/2012 snow pit analyses was performed in triplicate approximately once monthly and snow depth, density, and temperature were measured in 10-centimeter increments. Three methodological approaches were used to analyze CO 2 flux through the snow pack: Chamber on snow, two-point Fick's law based diffusivity modeling, and snow-surface/subcanopy eddy covariance. The results of the comparison show a significant difference in measured and estimated flux between methodologies during early and late winter, while demonstrating the Fick's based model is can accurately estimate up 75% of measured flux during mid-winter. Observations are consistent with advection, in addition to diffusion, as a mechanism of CO 2 transport through snow such that observation strategies that do not account for advection may underestimate wintertime efflux. Furthermore, all three methodologies indicate that wintertime respiration is a major contributor to the annual carbon budget when mean flux rates are compared to growing season flux rates.