Scholarship & Research
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Item Biophysical gradients and performance of whitebark pine plantings in the Greater Yellowstone Ecosystem(Montana State University - Bozeman, College of Letters & Science, 2019) Laufenberg, David Anton; Chairperson, Graduate Committee: Andrew J. HansenThe efficacy of planting efforts for species of conservation interest is important for ecosystem managers. Planting efforts represent an opportunity to conserve and manage species during a population crisis. Although federal agencies have been planting whitebark pine (WBP) in the Greater Yellowstone Ecosystem (GYE) for three decades, these efforts have been met with varying success. In this study, we use a combination of field sampling and remote sensing approaches in order to investigate local biophysical gradients as explanatory variables for WBP performance in GYE planting units. Present-day field sampling affords an opportunity to evaluate WBP performance relative to earlier planting and monitoring records. We used remotely-sensed temperature and precipitation alongside field measurements of elevation, aspect, slope, shading, and soils to utilize an adapted Thornthwaite-type water balance model to explain individual growth rates and site density change ratios (essentially survival and natural recruitment). We found that planting sites varied greatly in their biophysical characteristics and WBP performance. Five of twenty-nine sites had higher present-day density than at date of planting, therefore indicating some amount of natural regeneration occurring within those sites since time of planting. These sites were often higher in elevation, not south or southwest facing, and had soils that could hold moisture later in the season and for longer periods following precipitation events. Sites that experienced reductions in the density of WBP were often lower in elevation, with poorer soils, and facing south or southwest. They therefore experience greater potential evapotranspiration, and also greater water deficit when water demands are not being met. Notably, our two response variables, individual growth rate and site density change ratio represent short and long-term performance variables respectively. Although our results suggest that individual growth rates are likely more often limited by energy than water demands, the site density change ratio associated with this late to mature, long-lived species is likely a better benchmark for success. If they make it to maturity, trees planted this season will not begin to produce cones until the end of this century or the beginning of the next. Therefore, they must overcome forecasted periods of greater water stress in the coming decades and centuries. We strongly recommend planting efforts that seek to reduce the effects of increased drought stress by planting at sites with soils of greater water holding capacities (non-rhyolitic), planting on northerly and easterly aspects, and utilizing microsites particularly when planting at sites with potentially higher water stress. We also detected a negative relationship between the density of local competitors and WBP performance, but only at higher densities. Ecosystem managers will continue to plant WBP in the GYE for years to come, and this research helps to inform and identify high quality habitat during a period of changing climate and high GYE WBP mortality rates.Item Spatiotemporal variation in grassland biomass and quality across the upper Yellowstone River basin : variation across phenology and land use gradients and validation of remote sensing vegetation indices(Montana State University - Bozeman, College of Letters & Science, 2016) Garroutte, Erica Lynn; Chairperson, Graduate Committee: Andrew J. HansenSpatial and temporal heterogeneity in forage biomass and quality is known to play an integral role in the movement and population dynamics of migratory ungulates. Once limited by field-based forage assessments, the Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI) have gained considerable attention as proxies for landscape-scale forage biomass and quality at fine temporal scales. In the Greater Yellowstone Ecosystem (GYE), these indices have become especially important for understanding how potential advances in the timing of spring green-up due to climate change and human land use may be influencing the forage patterns available to migratory elk (Cervus elaphus). Given this concern, more information is needed on how the forage biomass and quality available to elk varies across elevation-related phenology and land use gradients and on the reliability of using NDVI and EVI to map forage patterns across the GYE. Using 250m2 MODIS NDVI and EVI and field estimates of grassland biomass and quality, we examined how the rate and magnitude of seasonal variation in forage biomass and quality differed across elevation-related phenology and land use gradients, assessed how the accuracy of NDVI and EVI as proxies for forage biomass and quality differed across the landscape, and then mapped spatiotemporal variation in the abundance of high quality forage for elk across the Upper Yellowstone River Basin (UYRB). We found that: (1) Grasslands with late onset of growth and irrigated agriculture had a faster rate of growth and a greater seasonal magnitude of biomass and quality for elk than all other grasslands; (2) 250m2 NDVI and EVI explained minimal variation in grassland biomass and quality across the UYRB; and (3) the accuracy of NDVI and EVI differed across elevation-related phenology and land use gradients in the UYRB. These results provide novel information on the rate and magnitude of seasonal variation in forage biomass and quality and on the reliability of using NDVI and EVI to map the forage patterns available to migratory elk.Item A multi-scale assessment of animal aggregation patterns to understand increasing pathogen seroprevalence(Ecological Society of America, 2014-10) Brennan, Angela; Cross, Paul C.; Higgs, Megan D.; Edwards, W. Henry; Scurlock, Brandon M.; Creel, ScottUnderstanding how animal density is related to pathogen transmission is important to develop effective disease control strategies, but requires measuring density at a scale relevant to transmission. However, this is not straightforward or well-studied among large mammals with group sizes that range several orders of magnitude or aggregation patterns that vary across space and time. To address this issue, we examined spatial variation in elk (Cervus canadensis) aggregation patterns and brucellosis across 10 regions in the Greater Yellowstone Area where previous studies suggest the disease may be increasing. We hypothesized that rates of increasing brucellosis would be better related to the frequency of large groups than mean group size or population density, but we examined whether other measures of density would also explain rising seroprevalence. To do this, we measured wintering elk density and group size across multiple spatial and temporal scales from monthly aerial surveys. We used Bayesian hierarchical models and 20 years of serologic data to estimate rates of increase in brucellosis within the 10 regions, and to examine the linear relationships between these estimated rates of increase and multiple measures of aggregation. Brucellosis seroprevalence increased over time in eight regions (one region showed an estimated increase from 0.015 in 1991 to 0.26 in 2011), and these rates of increase were positively related to all measures of aggregation. The relationships were weaker when the analysis was restricted to areas where brucellosis was present for at least two years, potentially because aggregation was related to disease-establishment within a population. Our findings suggest that (1) group size did not explain brucellosis increases any better than population density and (2) some elk populations may have high densities with small groups or lower densities with large groups, but brucellosis is likely to increase in either scenario. In this case, any one control method such as reducing population density or group size may not be sufficient to reduce transmission. This study highlights the importance of examining the density-transmission relationship at multiple scales and across populations before broadly applying disease control strategies.