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Martin J.G.,Oregon State University | Phillips C.L.,Lawrence Livermore National Laboratory | Schmidt A.,Oregon State University | Irvine J.,Yellowstone Ecological Research Center | Law B.E.,Oregon State University
Tree Physiology | Year: 2012

High-frequency soil CO 2 flux data are valuable for providing new insights into the processes of soil CO 2 production. A record of hourly soil CO 2 fluxes from a semi-arid ponderosa pine stand was spatially and temporally deconstructed in attempts to determine if variation could be explained by logical drivers using (i) CO 2 production depths, (ii) relationships and lags between fluxes and soil temperatures, or (iii) the role of canopy assimilation in soil CO 2 flux variation. Relationships between temperature and soil fluxes were difficult to establish at the hourly scale because diel cycles of soil fluxes varied seasonally, with the peak of flux rates occurring later in the day as soil water content decreased. Using a simple heat transport/gas diffusion model to estimate the time and depth of CO 2 flux production, we determined that the variation in diel soil CO 2 flux patterns could not be explained by changes in diffusion rates or production from deeper soil profiles. We tested for the effect of gross ecosystem productivity (GEP) by minimizing soil flux covariance with temperature and moisture using only data from discrete bins of environmental conditions (±1°C soil temperature at multiple depths, precipitation-free periods and stable soil moisture). Gross ecosystem productivity was identified as a possible driver of variability at the hourly scale during the growing season, with multiple lags between ∼5, 15 and 23 days. Additionally, the chamber-specific lags between GEP and soil CO 2 fluxes appeared to relate to combined path length for carbon flow (top of tree to chamber center). In this sparse and heterogeneous forested system, the potential link between CO 2 assimilation and soil CO 2 flux may be quite variable both temporally and spatially. For model applications, it is important to note that soil CO 2 fluxes are influenced by many biophysical factors, which may confound or obscure relationships with logical environmental drivers and act at multiple temporal and spatial scales; therefore, caution is needed when attributing soil CO 2 fluxes to covariates like temperature, moisture and GEP. © 2012 The Author. Source

White P.J.,National Park Service | Davis T.L.,National Park Service | Sheldon J.W.,Yellowstone Ecological Research Center | White J.R.,Gardiner Public School
Journal of Mammalogy | Year: 2012

Understanding mechanisms that influence the grouping tendencies of large herbivores is necessary to predict the influence of environmental and human factors on threatened populations. Locations of 53 adult female pronghorn (Antilocapra americana) in Yellowstone National Park during June 1999April 2005 indicated that mean and typical group sizes and the variation in group size decreased during fawning when females secluded themselves, but became larger and more dynamic during fawn rearing and the rut and winter. Mixed-effects models indicated a strong effect of time of year on mean group sizes, with some evidence that predators negatively affected group sizes during winter. Within-animal variability (0.64) was substantially higher than between-animal variability (0.02). Pronghorn density, snow water equivalent, and predation apparently influenced variations in group size. Multiple regressions indicated effects of pronghorn density and snow water equivalent on typical group size, the size of the group in which the average animal finds itself. Overall, there was fluidity in group cohesion, with female associations changing within and among days. The behavioral plasticity of pronghorn with respect to grouping and social cohesion might confer resilience to changes in environmental conditions, but often makes it difficult to predict the consequences of conservation actions to control disease, protect or restore key habitat, regulate harvests, and limit adverse effects of development and recreation. © 2012 American Society of Mammalogists. Source

Geremia C.,National Park Service | Geremia C.,Colorado State University | White P.J.,National Park Service | Wallen R.L.,National Park Service | And 5 more authors.
PLoS ONE | Year: 2011

Long distance migrations by ungulate species often surpass the boundaries of preservation areas where conflicts with various publics lead to management actions that can threaten populations. We chose the partially migratory bison (Bison bison) population in Yellowstone National Park as an example of integrating science into management policies to better conserve migratory ungulates. Approximately 60% of these bison have been exposed to bovine brucellosis and thousands of migrants exiting the park boundary have been culled during the past two decades to reduce the risk of disease transmission to cattle. Data were assimilated using models representing competing hypotheses of bison migration during 1990-2009 in a hierarchal Bayesian framework. Migration differed at the scale of herds, but a single unifying logistic model was useful for predicting migrations by both herds. Migration beyond the northern park boundary was affected by herd size, accumulated snow water equivalent, and aboveground dried biomass. Migration beyond the western park boundary was less influenced by these predictors and process model performance suggested an important control on recent migrations was excluded. Simulations of migrations over the next decade suggest that allowing increased numbers of bison beyond park boundaries during severe climate conditions may be the only means of avoiding episodic, large-scale reductions to the Yellowstone bison population in the foreseeable future. This research is an example of how long distance migration dynamics can be incorporated into improved management policies. Source

Grafius D.,University of Iowa | Malanson G.,University of Iowa | Weiss D.,Yellowstone Ecological Research Center
Physical Geography | Year: 2012

Alpine treelines can be explained by lower temperatures with increasing elevation at global scales, but regional and local treelines vary because of additional constraints. We examine data from treelines across the western USA, divided into subregions to elucidate the patterns of these other constraints. We determined the best predictor of elevation for 930 treeline sites in 26 mountain ranges by finding a climate variable with least variance. We used the variance at each site from this predictor to compute an elevation anomaly that became the dependent variable in correlation and regression analyses. We derived independent variables from digital elevation models and regional climate interpolations. A "best subsets" regression, which compares all possible combinations, revealed that at the continental scale, latitude and longitude stand out among many significant but weak correlations between the elevation anomaly and the other variables; these retain importance within regions. Our results show variables related to water and energy play a secondary role. We found that the division of the observations into regions and interpretation of spatial patterns within regions was necessary to interpret the relations between the potential predictor variables and the elevation anomaly. Source

Malanson G.P.,University of Iowa | Resler L.M.,Virginia Polytechnic Institute and State University | Bader M.Y.,University of Oldenburg | Holtmeier F.-K.,University of Munster | And 4 more authors.
Arctic, Antarctic, and Alpine Research | Year: 2011

For over 100 years, mountain treelines have been the subject of varied research endeavors and remain a strong area of investigation. The purpose of this paper is to examine aspects of the epistemology of mountain treeline research-that is, to investigate how knowledge on treelines has been acquired and the changes in knowledge acquisition over time, through a review of fundamental questions and approaches. The questions treeline researchers have raised and continue to raise have undoubtedly directed the current state of knowledge. A continuing, fundamental emphasis has centered on seeking the general cause of mountain treelines, thus seeking an answer to the question, "What causes treeline?" with a primary emphasis on searching for ecophysiological mechanisms of low-temperature limitation for tree growth and regeneration. However, treeline research today also includes a rich literature that seeks local, landscape-scale causes of treelines and reasons why treelines vary so widely in three-dimensional patterns from one location to the next, and this approach and some of its consequences are elaborated here. In recent years, both lines of research have been motivated greatly by global climate change. Given the current state of knowledge, we propose that future research directions focused on a spatial approach should specifically address cross-scale hypotheses using statistics and simulations designed for nested hierarchies; these analyses will benefit from geographic extension of treeline research. © 2011 Arctic, Antarctic, and Alpine Research. Source

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