Effects of processes at the population and community level on carbon dynamics of an ecosystem model

Ecological processes at the population and community level are often ignored in biogeochemical models, however, the effects of excluding these processes at the ecosystem level is uncertain. In this study we analyzed the set of behaviors that emerge after introducing population and community processes into an ecosystem carbon model. We used STANDCARB, a hybrid model that incorporates population, community, and ecosystem processes to predict carbon dynamics over time. Our simulations showed that at the population level, colonization and mortality rates can limit the maximum biomass achieved during a successional sequence. Specifically, colonization rates control temporal lags in the initiation of carbon accumulation, and mortality rates can have important effects on annual variation in live biomass. At the community level, differences in species traits and changes in species composition over time introduced significant changes in carbon dynamics. Species with different set of parameters, such as growth and mortality rates, introduce patterns of carbon accumulation that could not be reproduced using a single species with the average of parameters of multiple species or by simulating the most abundant species (strategies commonly employed in terrestrial biogeochemical models). We conclude that omitting population and community processes from biogeochemical models introduces an important source of uncertainty that can impose important limitations for predictions of future carbon balances

Gamma-Ray Observations of GRO J1655-40

The bright transient X-ray source GRO J1655-40 = XN Sco 1994 was observed by the OSSE instrument on the Compton Gamma Ray Observatory (GRO). Preliminary results are reported here. The initial outburst from GRO J1655-40 was detected by BATSE on 27 Jul 1994. OSSE observations were made in five separate viewing periods starting between 4 Aug 1994 and 4 Apr 1995. The first, third, and fifth observations are near the peak luminosity. In the second observation, the source flux had dropped by several orders of magnitude and we can only set an upper limit. The fourth observation is a weak detection after the period of maximum outburst. In contrast with other X-ray novae such as GRO J0422+32, the spectrum determined by OSSE is consistent with a simple power law over the full range of detection, about 50 - 600 keV. The photon spectral index is in the range of -2.5 to 2.8 in all of the observations. We set an upper limit on fractional rms variation \u3c5% in the frequency range 0.01 – 60 Hz. No significant narrow or broad line features are observed at any energy

Effective Bandgap Shrinkage in GaAs

Electrical measurements of the equilibrium np product (n2ie) in heavily doped n‐ and p‐GaAs were performed. The n2ieDproduct (where D is the diffusivity) was measured by fitting the collector current‐voltage characteristic of a homojunction bipolar transistor to an ideal diode equation modified to account for transport in thin base transistors.The n2ie product was then extracted from n2ieD by utilizing diffusivity results obtained with the zero‐field time‐of‐flight technique. Our results show significant effective band‐gap shrinkage in heavily doped p‐GaAs, and very little effective band‐gap shrinkage in heavily doped n‐GaAs. At extremely heavy dopings, an effective band‐gap widening is observed for both n‐ and p‐GaAs and is attributed to the effects of degeneracy

Logs as sites of tree regeneration in Picea sitchensis-Tsuga heterophylla forests of coastal Washington and Oregon

Logs are seedbeds for trees in many Picea sitchensis-Tsuga heterophylla forests of Washington and Oregon. Factors affecting this close association, including seed retention and seedling establishment on logs and successional changes in log characteristics, are explored In this study. Field and laboratory experiments indicated that competition with vegetation on the forest floor favored tree seedling development on logs. Reciprocal transplants of soil and log blocks revealed differences in substrate quality and not position effects, such as standing water, produced the "nurse-log" phenomenon. Clearing vegetation significantly increased survival of planted and natural conifer seedlings above survival on uncleared plots. Experiments on the effects of predation and soil pathogens indicated these interactions were of minor importance. Logs are therefore sites where competitive effects are sufficiently small to allow abundant seedling recruitment. Log surfaces vary widely in their ability to retain seeds and needles. Moss- and litter-covered surfaces retained 48-98% of seeds and needles, whereas sound and rotten wood and bare bark retained 0-8%. A model of seedling establishment on log surfaces in Picea-Tsuga forests, which incorporated the effects of seed retention and seedling survival rates, indicated 1.3% of the seed crop would produce one-year-old seedlings on logs but only 0.02-0.18% on undisturbed forest floor. Surficial litter accumulations enable Picea and Tsuga seedlings to establish and grow rapidly on slightly decayed logs. Survival rates for the first two years of both species increased asymptotically with litter biomass with a maximum of 62% for Picea and 34% for Tsuga. Seedling growth was fastest when canopy openness and litter biomass were high, but slow when either factor was low. Successional development influences tree seedling recruitment and survival on logs. Changes in bark, bryophytes, humus and trees were examined for a chronosequence of Picea, Pseudotsuga and Tsuga logs. Bark fragmentation was a critical process; it removed plants and humus, reinitiated succession and was responsible for major differences in successional patterns among log species. Although a large proportion of tree seedlings establish on logs in Picea-Tsuga forests, high mortality rates caused by competition, bark fragmentation and toppling from logs indicated long-term survival on logs was very low and possibly equivalent to the forest floor

Heterotrophic respiration in disturbed forests : a review with examples from North America

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 116 (2011): G00K04, doi:10.1029/2010JG001495.Heterotrophic respiration (RH) is a major process releasing carbon to the atmosphere and is essential to understanding carbon dynamics in terrestrial ecosystems. Here we review what is known about this flux as related to forest disturbance using examples from North America. The global RH flux from soils has been estimated at 53–57 Pg C yr−1, but this does not include contributions from other sources (i.e., dead wood, heart-rots). Disturbance-related inputs likely account for 20–50% of all RH losses in forests, and disturbances lead to a reorganization of ecosystem carbon pools that influences how RH changes over succession. Multiple controls on RH related to climate, the material being decomposed, and the decomposers involved have been identified, but how each potentially interacts with disturbance remains an open question. An emerging paradigm of carbon dynamics suggests the possibility of multiple periods of carbon sinks and sources following disturbance; a large contributing factor is the possibility that postdisturbance RH does not always follow the monotonic decline assumed in the classic theory. Without a better understanding and modeling of RH and its controlling factors, it will be difficult to estimate, forecast, understand, and manage carbon balances of regions in which disturbance frequency and severity are changing. Meeting this challenge will require (1) improved field data on processes and stores, (2) an improved understanding of the physiological and environmental controls of RH, and (3) a more formal analysis of how model structure influences the RH responses that can be predicted.Support was provided by the U.S. Geologic Survey and the Kaye and Ward Richardson Endowment

Thermal velocity limits to diffusive electron transport in thin‐base np+n GaAs bipolar transistors

We present experimental evidence that minority electron transport across a thin, quasineutral p+ GaAs region is limited by the thermal velocity of the electrons rather than by conventional diffusive transport. A set of GaAs homojunction np+n transistors with base widths of 4000, 2000, 1000, and 500 Å was fabricated and characterized. The diffusive modelpredicts that the dc collector current of the 500‐Å base width transistors should be eight times larger than the collector current of transistors with a 4000‐Å‐wide base. The experimental results, however, show only a factor of ~3.5 increase in collector current. The measured collector current versus base width characteristic agrees well with theoretical treatments of thin‐base transport. These new results present evidence of quasiballistic electron transport in p+ GaAs and have important implications for GaAs transistor design

Experimental determination of the effects of degenerate Fermi statistics on heavily p‐doped GaAs

The effects of degenerate Fermi statistics on electron injection currents for p+‐GaAs grown by molecular beam epitaxy are presented. To achieve Be dopant concentrations of greater than 8×1019 cm−3, the substrate temperature during growth was reduced to approximately 450 °C from the usual 600 °C. In this heavily doped material, we measure unexpectedly large electron injectioncurrents which are interpreted in terms of an effective narrowing of the band gap. At extremely heavy doping densities, the Fermi level pushes into the valence band and degenerate Fermi statistics must be taken into account. For doping concentrations greater than 1×1020 cm−3, effects due to degenerate Fermi statistics oppose the band‐gap shrinkage effects; consequently, a reduction in the electron injection currents is observed. The result is a substantial reduction in gain for AlGaAs/GaAs heterostructure bipolar transistors when the base is doped above 1020 cm−3

Plant Species Rather Than Climate Greatly Alters the Temporal Pattern of Litter Chemical Composition During Long-Term Decomposition

A feedback between decomposition and litter chemical composition occurs with decomposition altering composition that in turn influences the decomposition rate. Elucidating the temporal pattern of chemical composition is vital to understand this feedback, but the effects of plant species and climate on chemical changes remain poorly understood, especially over multiple years. In a 10-year decomposition experiment with litter of four species (Acer saccharum, Drypetes glauca, Pinus resinosa, and Thuja plicata) from four sites that range from the arctic to tropics, we determined the abundance of 11 litter chemical constituents that were grouped into waxes, carbohydrates, lignin/tannins, and proteins/peptides using advanced 13C solid-state NMR techniques. Decomposition generally led to an enrichment of waxes and a depletion of carbohydrates, whereas the changes of other chemical constituents were inconsistent. Inconsistent convergence in chemical compositions during decomposition was observed among different litter species across a range of site conditions, whereas one litter species converged under different climate conditions. Our data clearly demonstrate that plant species rather than climate greatly alters the temporal pattern of litter chemical composition, suggesting the decomposition-chemistry feedback varies among different plant species

Technique for measurement of the minority carrier mobility with a bipolar junction transistor

A simple technique to measure the minority carrier mobility using a bipolar junction transistor is demonstrated. By fixing the base-emitter voltage, the carrier injection into the base is constant. The collector current is then monitored as a function of a magnetic field applied perpendicular to the current transport across the base. The magnetic field leads to an increase in base transit time and a corresponding decrease in collector current. From the resulting fractional change in collector current, the minority carrier mobility in the base can be determined. For narrow base transistors, quasiballistic transport across the base must be taken into account when determining the bulk minority carrier mobility