Environmental change in grasslands: Assessment using models

Modeling studies and observed data suggest that plant production, species distribution, disturbance regimes, grassland biome boundaries and secondary production (i.e., animal productivity) could be affected by potential changes in climate and by changes in land use practices. There are many studies in which computer models have been used to assess the impact of climate changes on grassland ecosystems. A global assessment of climate change impacts suggest that some grassland ecosystems will have higher plant production (humid temperate grasslands) while the production of extreme continental steppes (e.g., more arid regions of the temperate grasslands of North America and Eurasia) could be reduced substantially. All of the grassland systems studied are projected to lose soil carbon, with the greatest losses in the extreme continental grassland systems. There are large differences in the projected changes in plant production for some regions, while alterations in soil C are relatively similar over a range of climate change projections drawn from various General Circulation Models (GCM's). The potential impact of climatic change on cattle weight gains is unclear. The results of modeling studies also suggest that the direct impact of increased atmospheric CO₂ on photosynthesis and water use in grasslands must be considered since these direct impacts could be as large as those due to climatic changes. In addition to its direct effects on photosynthesis and water use, elevated CO₂ concentrations lower N content and reduce digestibility of the forage.     

Evaluation and comparison of spatial interpolators II

The performance of several variations on ordinary kriging and inverse distance estimators is evaluated. Mean squared errors (MSE) were calculated for estimates made on multiple resamplings from five exhaustive data bases representing two distinctly different types of estimation problem. Ordinary kriging, when performed with variograms estimated from the sample data, was more robust than inverse-distance methods to the type of estimation problem, and to the choice of estimation parameters such as number of neighbors.Notice: Although the research described in this article has been funded in part by the United States Environmental Protection Agency through Cooperative Agreement CR818526 to the Harry Reid Center for Environmental Studies, University of Nevada-Las Vegas, it has not been subjected to Agency review. Therefore it does not necessarily reflect the views of the Agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.     

Geochemical constraints on magma processes in a peralkaline system: The Paisano volcano,west Texas

Major and trace element data for a sequence of peralkaline silicic lavas and pyroclastic flows, exposed in the caldera wall of the Paisano volcano, west Texas, document systematic fractional crystallization during magmatic evolution and an open system, magma mixing event in the upper parts of the sequence. Stratigraphically lowest flows are comendite and comenditic quartz trachyte lavas and ash flow tufts. Overlying these units is a trachyte with compositional, textural and mineralogical features indicating that it is the product of magma-mixing; similar flows occur in other parts of the volcano at the same stratigraphic level. This composite trachyte is considered to be a mixture of mugearitic or mafic trachytic magma, derived from a similar source region which yielded the earlier caldera wall flows. Trace element concentrations of the post-trachyte comenditic quartz trachyte lavas suggest they were erupted from a chamber whose magma was diluted by an influx of mugearitic or mafic trachytic magma during a magma mixing event.Rayleigh fractionation calculations show that the comendites and comenditic quartz trachytes can be derived from a parental mugearite magma by 88% to 93% fractionation of dominantly plagioclase and alkali feldspar, with lesser amounts of clinopyroxene, magnetite and apatite. Zircon was not a significant fractionating phase. The composition, mineralogy and depth of the source region(s) which generated these magmas cannot be constrained from the present data set.     

Forecasting errors: The importance of the decision-making context

This article demonstrates the importance of context on forecast errors. It de J.scribes the development of an unrealistic transportation planning forecast for Denver, Colorado, noting its influence on air quality planning for Denver. The importance of two contextual features, procedural rationality and conflict of interest, to the introduction of error is discussed. The case described implies that different decision-making contexts define different rules of rationality, and substantive rationality may not fit the rules. The type of context and not necessarily a code of ethics will determine this.     

Sources of magnetic susceptibility in a slate

Magnetic susceptibility and its anisotropy in the Borrowdale Volcanic slates at Kentmere in the English Lake District are attributed largely to preferred orientation of a paramagnetic chlorite of diabantite-ripidolite composition. In units of 10⁻⁶ cgs/g, the principal susceptibilities for the slates are 9.61; 9.42; 8.69 and for the chlorite grains the minimum anisotropy is represented by principal susceptibilities of 11.57; 11.22 and 9.15. Because the magnetic susceptibility is carried by a tightly packed, matrix-forming mineral that has recrystallised during the deformation it is not possible to imagine simple grain rotation as being responsible for the anisotropy of susceptibility.     

Intra-field variability of scalar flux densities across a transition between a desert and an irrigated potato field

This paper reports on measurements of sensible and latent heat and CO₂ fluxes made over an irrigated potato field, growing next to a patch of desert. The study was conducted using two eddy correlation systems. One measurement system was located within the equilibrium boundary layer 800 m downwind from the edge of the potato field. The other measurement system was mobile and was placed at various downwind positions to probe the horizontal transition of vertical scalar fluxes. Latent (LE) and sensible (H) heat fluxes, measured at 4 m above the surface, exhibited marked variations with downwind distance over the field. Only after the fetch to height ratio exceeded 75 to 1 didLE andH become invariant with downwind distance. When latent and sensible heat fluxes were measured upwind of this threshold, significant advection of humidity-deficit occurred, causing a vertical flux divergence ofH andLE.The measured fluxes of momentum, heat, and moisture were compared with predictions from a second-order closure two-dimensional atmospheric boundary layer model. There is good agreement between measurements and model predictions. A soil-plant-atmosphere model was used to examine nonlinear feedbacks between humidity-deficits, stomatal conductance and evaporation. Data interpretation with this model revealed that the advection of hot dry air did not enhance surface evaporation rates near the upwind edge of the potato field, because of negative feedbacks among stomatal conductance, humidity-deficits, andLE. This finding is consistent with results from several recent studies.     

Short polarity intervals within the Matuyama: transitional field records from hydraulic piston cored sediments from the North Atlantic

Detailed sampling of two short magnetozones within the Matuyama Chronozone recorded at DSDP Site 609 (49.86°N, 335.77°E) confirms that one, the Cobb Mountain Subchronozone (1.12 Ma), is a very short, full normal polarity interval and that the other, the older interval, is a record of a geomagnetic excursion which occurred at approximately 1.55 Ma. The Cobb Mountain Subchron lasted approximately 25,000 years, one third the duration of the Jaramillo Subchron. The normal polarity interval is bounded by two transition zones which document an antisymmetry in the sequence of directions in the reverse to normal and normal to reverse polarity transitions. We interpret the antisymmetry as reflecting a dependence upon the sense of the reversal, without significant changes in the relative contributions of non-dipole terms. The polarity interval recorded at 1.55 Ma lasted only 8,800 years with what may be regarded as full polarity directions observed across only 3 cm of stratigraphic section. This feature is interpreted as an excursion of the geomagnetic field and appears to be correlative with the Gilsa Subchron. Similarities between the transition bounding these two magnetozones suggest that these features occur as the result of the same process or triggering mechanisms in the earth's outer core.     

Community structure and productivity of the Cornwallis Estuary, Minas Basin

Preliminary results of a seasonal study of the pelagic community at a station on the outer edge of the Cornwallis Estuary suggest that the seasonal variation in plankton community respiration (PCR) is related to organic inputs from nearby salt marshes. Annual phytoplankton production is low (<30 g C m⁻² y⁻¹) and exhibits a seasonal cycle very different from PCR. There is no indication that resuspension of benthic diatoms is an important energy input to the pelagic system. PCR, however, is quite high and exhibits a seasonal trend similar to the export of salt marsh detritus. Zooplankton densities (5–200 l⁻¹) and biomass (<0.4 g m⁻³) appear to be much greater than could be supported by phytoplankton alone. The Cornwallis Estuary may be an estuarine system exhibiting a net export of organic matter to nearby offshore waters.