Biomedical subjects

D D Breshears

Publications and source records attributed to D D Breshears.

5 recordsLinked to original sources

PubMed1998

Drought-induced shift of a forest-woodland ecotone: rapid landscape response to climate variation.

In coming decades, global climate changes are expected to produce large shifts in vegetation distributions at unprecedented rates. These shifts are expected to be most rapid and extreme at ecotones, the boundaries between ecosystems, particularly those in semiarid landscapes. However, current models do not adequately provide for such rapid effects-particularly those caused by mortality-largely because of the lack of data from field studies. Here we report the most rapid landscape-scale shift of a woody ecotone ever documented: in northern New Mexico in the 1950s, the ecotone between semiarid ponderosa pine forest and pinon-juniper woodland shifted extensively (2 km or more) and rapidly (<5 years) through mortality of ponderosa pines in response to a severe drought. This shift has persisted for 40 years. Forest patches within the shift zone became much more fragmented, and soil erosion greatly accelerated. The rapidity and the complex dynamics of the persistent shift point to the need to represent more accurately these dynamics, especially the mortality factor, in assessments of the effects of climate change.

Journal Article↗

PubMed1990

Estimation of radionuclide ingestion: the "PATHWAY" food-chain model.

This paper describes the structure of the dynamic food-chain model PATHWAY and its utility for estimating radionuclide ingestion after fallout deposition from nuclear testing in Nevada. Model input requirements are described and output examples are provided. The basic output of PATHWAY is the time-integrated radionuclide ingestion by humans per unit fallout deposition (Bq per Bq m-2). Output specific to sex, age, life-style (diet), location (agricultural practice), event (calendar date), and radionuclide may be generated. Uncertainties of model predictions, based on "Monte Carlo" simulations using parameter value distributions, are described. Results of a sensitivity analysis, based on a ranking of partial correlation coefficients, are reviewed to illustrate the relative importance of parameters and associated transport pathways. Output data for 131I and 137Cs in milk are compared with predictions from several well known food-chain models. Preliminary efforts to validate PATHWAY results with real data sets are described.

Adolescent↗

PubMed1989

Uncertainty in predictions of fallout radionuclides in foods and of subsequent ingestion.

Uncertainty in predictions from the PATHWAY food-chain model was estimated using Monte Carlo simulation. Uncertainty estimates, measured by the geometric standard deviation (GSD), were obtained for median values of time-integrated concentrations of 131I, 136Cs, and 137Cs in foods and for the corresponding time-integrated intakes resulting from ingestion of all foods. The GSDs associated with a given food for the two short-lived radionuclides, 131I and 136Cs, were not significantly different, but they differed from the GSDs for the longer-lived radionuclide. The GSDs for integrated concentrations of radionuclides in milk varied with the time of year fallout was deposited, but uncertainty for nondairy products was relatively independent of the date of fallout deposition. The estimated GSDs were applied to other radionuclides of interest based on physical half-life and ranged from 1.7 to 2.7 for time-integrated intake across all foods for radionuclides with physical half-lives less than 30 d, from 1.8 to 2.3 for half-lives ranging from 30 to 500 d, and from 1.9 to 2.1 when half-lives were greater than 500 d.

Animal Feed↗

PubMed1988

Genetic variability in white-tailed deer.

Genetic variability at 36 loci was studied in white-tailed deer (Odocoileus virginianus) populations, on the Savannah River Plant (SRP) in South Carolina. Mean multilocus heterozygosity (H), percentage of polymorphic loci (P) and average number of alleles per locus (A) for white-tailed deer were calculated and compared with values for white-tailed deer from several locations (SRP deer: H = 9.9 per cent, P0.05 = 30.6 per cent, A = 1.89; white-tailed deer overall: H = 10.4 per cent, P0.05 = 32.3 per cent, A = 1.94). Frequency distributions for single locus heterozygosity values (h) and the number of alleles per locus for white-tailed deer were found to be significantly different from those of mammals in general. Analysis of single-locus data based on quaternary structure and functional groups of proteins failed to demonstrate expected differences as predicted from the literature. White-tailed deer have a high level of heterozygosity, but they do not exhibit many of the life history and environmental characteristics associated with high heterozygosity in other animals.

Animals↗

PubMedDate not supplied

Measuring total soil carbon with laser-induced breakdown spectroscopy (LIBS).

Improving estimates of carbon inventories in soils is currently hindered by lack of a rapid analysis method for total soil carbon. A rapid, accurate, and precise method that could be used in the field would be a significant benefit to researchers investigating carbon cycling in soils and dynamics of soil carbon in global change processes. We tested a new analysis method for predicting total soil carbon using laser-induced breakdown spectroscopy (LIBS). We determined appropriate spectral signatures and calibrated the method using measurements from dry combustion of a Mollisol from a cultivated plot. From this calibration curve we predicted carbon concentrations in additional samples from the same soil and from an Alfisol collected in a semiarid woodland and compared these predictions with additional dry combustion measurements. Our initial tests suggest that the LIBS method rapidly and efficiently measures soil carbon with excellent detection limits (approximately 300 mg/kg), precision (4-5%), and accuracy (3-14%). Initial testing shows that LIBS measurements and dry combustion analyses are highly correlated (adjusted r2 = 0.96) for soils of distinct morphology, and that a sample can be analyzed by LIBS in less than one minute. The LIBS method is readily adaptable to a field-portable instrument, and this attribute--in combination with rapid and accurate sample analysis--suggests that this new method offers promise for improving measurement of total soil carbon. Additional testing of LIBS is required to understand the effects of soil properties such as texture, moisture content, and mineralogical composition (i.e., silicon content) on LIBS measurements.

Automation↗