648 research outputs found
Random phase approximation for the anisotropic Heisenberg ferromagnet
Anisotropic Heisenberg ferromagnets in random phase and spin wave approximatio
Potential net primary productivity in South America: application of a global model
We use a mechanistically based ecosystem simulation model to describe and analyze the spatial and temporal patterns of terrestrial net primary productivity (NPP) in South America. The Terrestrial Ecosystem Model (TEM) is designed to predict major carbon and nitrogen fluxes and pool sizes in terrestrial ecosystems at continental to global scales. Information from intensively studies field sites is used in combination with continentalâscale information on climate, soils, and vegetation to estimate NPP in each of 5888 nonâwetland, 0.5° latitude °0.5° longitude grid cells in South America, at monthly time steps. Preliminary analyses are presented for the scenario of natural vegetation throughout the continent, as a prelude to evaluating human impacts on terrestrial NPP. The potential annual NPP of South America is estimated to be 12.5 Pg/yr of carbon (26.3 Pg/yr of organic matter) in a nonâwetland area of 17.0 ° 106 km2. More than 50% of this production occurs in the tropical and subtropical evergreen forest region. Six independent model runs, each based on an independently derived set of model parameters, generated mean annual NPP estimates for the tropical evergreen forest region ranging from 900 to 1510 g°mâ2°yrâ1 of carbon, with an overall mean of 1170 g°mâ2°yrâ1. Coefficients of variation in estimated annual NPP averaged 20% for any specific location in the evergreen forests, which is probably within the confidence limits of extant NPP measurements. Predicted rates of mean annual NPP in other types of vegetation ranged from 95 g°mâ2°yrâ1 in arid shrublands to 930 g°m@?yrâ1 in savannas, and were within the ranges measured in empirical studies. The spatial distribution of predicted NPP was directly compared with estimates made using the Miami mode of Lieth (1975). Overall, TEM predictions were °10% lower than those of the Miami model, but the two models agreed closely on the spatial patterns of NPP in south America. Unlike previous models, however, TEM estimates NPP monthly, allowing for the evaluation of seasonal phenomena. This is an important step toward integration of ecosystem models with remotely sensed information, global climate models, and atmospheric transport models, all of which are evaluated at comparable spatial and temporal scales. Seasonal patterns of NPP in South America are correlated with moisture availability in most vegetation types, but are strongly influenced by seasonal differences in cloudiness in the tropical evergreen forests. On an annual basis, moisture availability was the factor that was correlated most strongly with annual NPP in South America, but differences were again observed among vegetation types. These results allow for the investigation and analysis of climatic controls over NPP at continental scales, within and among vegetation types, and within years. Further model validation is needed. Nevertheless, the ability to investigate NPPâenvironment interactions with a high spatial and temporal resolution at continental scales should prove useful if not essential for rigorous analysis of the potential effects of global climate changes on terrestrial ecosystems
Characterization of Central Iowa Forests with Permanent Plots
We describe a standard methodology for establishing and inventorying the woody vegetation in 0.1-ha permanent forest plots and we utilize the data from 86 plots to characterize the mature forest vegetation of central Iowa. Obvious differences existed between bottomland hardwoods and upland forest stands, but disturbed uplands contained species typical of bottomlands. Little-disturbed upland forest plots included a variety of species associations, but variation among plots was continuous. The trees dominating the mature upland forests of central Iowa grow in a wide variety of habitats, with the result that predictability of stand structure at any location is limited. Stands on northerly and southerly aspects did differ consistently from one another, but other aspects were not intermediate in character. The overall structure of the forest stands we inventoried was typical of old secondary forests. Even the oldest forests of central Iowa are still relatively young, and their structure and composition is likely to change in the coming decades. The permanent plots we established will allow for documentation and analysis of this change
Evaluating the Sensitivity of Mortality Attributable to Pollution to Modeling Choices: A Case Study for Colorado
We evaluated the sensitivity of estimated PM2.5 and NO2 health impacts to
varying key input parameters and assumptions including: 1) the spatial scale at
which impacts are estimated, 2) using either a single concentration-response
function (CRF) or using racial/ethnic group specific CRFs from the same
epidemiologic study, 3) assigning exposure to residents based on home, instead
of home and work locations. This analysis was carried out for the state of
Colorado. We found that the spatial scale of the analysis influences the
magnitude of NO2, but not PM2.5, attributable deaths. Using county-level
predictions instead of 1 km2 predictions of NO2 resulted in a lower estimate of
mortality attributable to NO2 by ~ 50% for all of Colorado for each year
between 2000-2020. Using an all-population CRF instead of racial/ethnic group
specific CRFs results in a higher estimate of annual mortality attributable to
PM2.5 by a factor 1.3 for the white population and a lower estimate of
mortality attributable to PM2.5 by factors of 0.4 and 0.8 for Black and
Hispanic residents, respectively. Using racial/ethnic group specific CRFs did
not result in a different estimation of NO2 attributable mortality for white
residents, but led to lower estimates of mortality by a factor of ~ 0.5 for
Black residents, and by a factor of 2.9 for to Hispanic residents. Using NO2
based on home instead of home and workplace locations results in a smaller
estimate of annual mortality attributable to NO2 for all of Colorado by ~0.980
each year and 0.997 for PM2.5.Comment: 24 pages, 6 figures, 2 table
Beam Measurement Systems for the CERN Antiproton Decelerator (AD)
The new, low-energy antiproton physics facility at CERN has been successfully commissioned and has been delivering decelerated antiprotons at 100 MeV/c since July 2000. The AD consists of one ring where the 3.5 GeV/c antiprotons produced from a production target are injected, rf manipulated, stochastically cooled, decelerated (with further stages involving additional stochastic and electron cooling and rf manipulation) and extracted at 100 MeV/c. While proton test beams of sufficient intensity could be used for certain procedures in AD commissioning, this was not possible for setting-up and routine operation. Hence, special diagnostics systems had to be developed to obtain the beam and accelerator characteristics using the weak antiproton beams of a few 10E7 particles at all momenta from 3.5 GeV/c down to 100 MeV/c. These include systems for position measurement, intensity, beam size measurements using transverse aperture limiters and scintillators and Schottky-based tools. This paper gives an overall view of these systems and their usage
Seasonal patterns and controls on net ecosystem CO2 exchange in a boreal peatland complex
We measured seasonal patterns of net ecosystem exchange (NEE) of CO2 in a diverse peatland complex underlain by discontinuous permafrost in northern Manitoba, Canada, as part of the Boreal Ecosystems Atmosphere Study (BOREAS). Study sites spanned the full range of peatland trophic and moisture gradients found in boreal environments from bog (pH 3.9) to rich fen (pH 7.2). During midseason (JulyâAugust, 1996), highest rates of NEE and respiration followed the trophic sequence of bog (5.4 to â3.9 ÎŒmol CO2 mâ2 sâ1) \u3c poor fen (6.3 to â6.5 ÎŒmol CO2 mâ2 sâ1) \u3c intermediate fen (10.5 to â7.8 ÎŒmol CO2 mâ2 sâ1) \u3c rich fen (14.9 to â8.7 ÎŒmol CO2mâ2 sâ1). The sequence changed during spring (MayâJune) and fall (SeptemberâOctober) when ericaceous shrub (e.g., Chamaedaphne calyculata) bogs and sedge (Carex spp.) communities in poor to intermediate fens had higher maximum CO2 fixation rates than deciduous shrubâdominated (Salix spp. and Betula spp.) rich fens. Timing of snowmelt and differential rates of peat surface thaw in microtopographic hummocks and hollows controlled the onset of carbon uptake in spring. Maximum photosynthesis and respiration were closely correlated throughout the growing season with a ratio of approximately 1/3 ecosystem respiration to maximum carbon uptake at all sites across the trophic gradient. Soil temperatures above the water table and timing of surface thaw and freezeâup in the spring and fall were more important to net CO2 exchange than deep soil warming. This close coupling of maximum CO2 uptake and respiration to easily measurable variables, such as trophic status, peat temperature, and water table, will improve models of wetland carbon exchange. Although trophic status, aboveground net primary productivity, and surface temperatures were more important than water level in predicting respiration on a daily basis, the mean position of the water table was a good predictor (r2 = 0.63) of mean respiration rates across the range of plant community and moisture gradients. Q10 values ranged from 3.0 to 4.1 from bog to rich fen, but when normalized by above ground vascular plant biomass, the Q10 for all sites was 3.3
Relationship between ecosystem productivity and photosynthetically-active radiation for northern peatlands
We analyzed the relationship between net ecosystem exchange of carbon dioxide (NEE) and irradiance (as photosynthetic photon flux density or PPFD), using published and unpublished data that have been collected during midgrowing season for carbon balance studies at seven peatlands in North America and Europe. NEE measurements included both eddy-correlation tower and clear, static chamber methods, which gave very similar results. Data were analyzed by site, as aggregated data sets by peatland type (bog, poor fen, rich fen, and all fens) and as a single aggregated data set for all peatlands. In all cases, a fit with a rectangular hyperbola (NEE = α PPFD Pmax/(α PPFD + Pmax) + R) better described the NEE-PPFD relationship than did a linear fit (NEE = ÎČ PPFD + R). Poor and rich fens generally had similar NEE-PPFD relationships, while bogs had lower respiration rates (R = â2.0ÎŒmol mâ2sâ1 for bogs and â2.7 ÎŒmol mâ2sâ1 for fens) and lower NEE at moderate and high light levels (Pmax = 5.2 ÎŒmol mâ2sâ1 for bogs and 10.8 ÎŒmol mâ2sâ1 for fens). As a single class, northern peatlands had much smaller ecosystem respiration (R = â2.4 ÎŒmol mâ2sâ1) and NEE rates (α = 0.020 and Pmax = 9.2ÎŒmol mâ2sâ1) than the upland ecosystems (closed canopy forest, grassland, and cropland) summarized by Ruimy et al. [1995]. Despite this low productivity, northern peatland soil carbon pools are generally 5â50 times larger than upland ecosystems because of slow rates of decomposition caused by litter quality and anaerobic, cold soils
Variation in Soil Respiration across Soil and Vegetation Types in an Alpine Valley.
BACKGROUND AND AIMS: Soils of mountain regions and their associated plant communities are highly diverse over short spatial scales due to the heterogeneity of geological substrates and highly dynamic geomorphic processes. The consequences of this heterogeneity for biogeochemical transfers, however, remain poorly documented. The objective of this study was to quantify the variability of soil-surface carbon dioxide efflux, known as soil respiration (Rs), across soil and vegetation types in an Alpine valley. To this aim, we measured Rs rates during the peak and late growing season (July-October) in 48 plots located in pastoral areas of a small valley of the Swiss Alps.
FINDINGS: Four herbaceous vegetation types were identified, three corresponding to different stages of primary succession (Petasition paradoxi in pioneer conditions, Seslerion in more advanced stages and Poion alpinae replacing the climactic forests), as well as one (Rumicion alpinae) corresponding to eutrophic grasslands in intensively grazed areas. Soils were developed on calcareous alluvial and colluvial fan deposits and were classified into six types including three Fluvisols grades and three Cambisols grades. Plant and soil types had a high level of co-occurrence. The strongest predictor of Rs was soil temperature, yet we detected additional explanatory power of sampling month, showing that temporal variation was not entirely reducible to variations in temperature. Vegetation and soil types were also major determinants of Rs. During the warmest month (August), Rs rates varied by over a factor three between soil and vegetation types, ranging from 2.5 ÎŒmol m-2 s-1 in pioneer environments (Petasition on Very Young Fluvisols) to 8.5 ÎŒmol m-2 s-1 in differentiated soils supporting nitrophilous species (Rumicion on Calcaric Cambisols).
CONCLUSIONS: Overall, this study provides quantitative estimates of spatial and temporal variability in Rs in the mountain environment, and demonstrates that estimations of soil carbon efflux at the watershed scale in complex geomorphic terrain have to account for soil and vegetation heterogeneity
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