RESUMEN

Stretched-exponential distributions and relaxation responses are encountered in a wide range of physical systems such as glasses, polymers, and spin glasses. As found recently, this type of behavior occurs also for the distribution function of a certain trap time in a number of coupled dynamical systems. We analyze a one-dimensional mathematical model of coupled chaotic oscillators that reproduces an experimental setup of coupled diode resonators and identify the necessary ingredients for stretched-exponential distributions.

RESUMEN

We investigate the response of a linear chain of diffusively coupled diode resonators under the influence of thermal noise. We also examine the connection between spatiotemporal stochastic resonance and the presence of kink-antikink pairs in the array. The interplay of nucleation rates and kink speeds is briefly addressed. The experimental results are supplemented with simulations on a coupled map lattice. We furthermore present analytical results for the synchronization and signal processing properties of a Phi(4) field theory and explore the effects of various forms of nonlinear coupling. (c) 1998 American Institute of Physics.

RESUMEN

This paper reviews recent advances in the application of chaos control techniques to the stability analysis of two-dimensional dynamical systems. We demonstrate how the system's response to one or multiple feedback controllers can be utilized to calculate the characteristic multipliers associated with an unstable periodic orbit. The experimental results, obtained for a single and two coupled diode resonators, agree well with the presented theory. (c) 1997 American Institute of Physics.

RESUMEN

Stem maintenance respiration rates were measured in five contrasting balsam fir (Abies balsamea (L.) Mill.) stands. At 15 degrees C, average respiration rates for individual stands ranged from 120 to 235 micro mol m(-3) s(-1) when expressed per unit of sapwood volume, from 0.80 to 1.80 micro mol m(-2) s(-1) when expressed per unit of stem surface area, and from 0.50 to 1.00 micro mol g(-1) s(-1) when expressed per unit of nitrogen in the living stem biomass, but differences among stands were not statistically significant. Coefficients of variation ranged from 50 to 100% within stands and were similar for all bases used to express respiration rates. Coefficients of determination for regressions between chamber flux and chamber values of sapwood volume, stem surface area and nitrogen content varied between stands and no one base was consistently higher than the other bases. We conclude that the bases for expressing stem respiration are equally useful. Respiration rates were more closely correlated to stem temperature observed approximately 2 h earlier than to current stem temperature. Among stands, annual stem maintenance respiration per hectare varied from 0.1 to 0.4 Mmol ha(-1) year(-1), primarily because of large differences in sapwood volumes per hectare. Annual stem maintenance respiration per unit of leaf area ranged from 3 to 6 mol m(-2) year(-1), increasing as sapwood volume per hectare increased.

Asunto(s)

RESUMEN

Simulation models of ecosystem processes may be necessary to separate the long-term effects of climate change on forest productivity from the effects of year-to-year variations in climate. The objective of this study was to compare simulated annual stem growth with measured annual stem growth from 1930 to 1982 for a uniform stand of ponderosa pine (Pinus ponderosa Dougl.) in Montana, USA. The model, FOREST-BGC, was used to simulate growth assuming leaf area index (LAI) was either constant or increasing. The measured stem annual growth increased exponentially over time; the differences between the simulated and measured stem carbon accumulations were not large. Growth trends were removed from both the measured and simulated annual increments of stem carbon to enhance the year-to-year variations in growth resulting from climate. The detrended increments from the increasing LAI simulation fit the detrended increments of the stand data over time with an R(2) of 0.47; the R(2) increased to 0.65 when the previous year's simulated detrended increment was included with the current year's simulated increment to account for autocorrelation. Stepwise multiple linear regression of the detrended increments of the stand data versus monthly meteorological variables had an R(2) of 0.37, and the R(2) increased to 0.47 when the previous year's meteorological data were included to account for autocorrelation. Thus, FOREST-BGC was more sensitive to the effects of year-to-year climate variation on annual stem growth than were multiple linear regression models.

RESUMEN

Physiological effects of different nitrate applications were studied using the C(4) plant, Amaranthus powellii Wats. Plants were grown in a controlled environment chamber and watered daily with nutrient solutions containing 45, 10, 5, or 1 millimolar nitrate. Chloride and sulfate were used to keep the cation and phosphate concentrations constant. Total leaf nitrogen concentration, chlorophyll concentration, specific leaf mass, leaf area, relative growth rate, relative leaf growth rate, unit leaf rate (increase of dry mass per unit leaf area per day), net photosynthetic rate, and incident quantum yield decreased with decreasing nitrate concentration. The per cent decrease of unit leaf rate was similar to the decrease of light-saturated net photosynthetic rate; however, the decrease in relative growth rate was less than that of unit leaf rate because leaf area ratio (leaf area per unit dry mass) increased with decreasing nitrate concentration. Essential mineral concentrations per unit leaf area were about equal among all treatments. Leaf expansion, determined by stomatal density, decreased except for the 1 millimolar treatment which showed relatively more cell expansion but less cell division. Decreased nitrate application was correlated with higher osmotic potentials and lower pressure potentials (determined by pressure-volume curves), whereas leaf water potentials were equal among treatments. Even though total leaf area and shoot mass decreased with decreasing applied nitrate, the increase of the leaf area ratio may be related to selection for the highest possible growth rate.

RESUMEN

Optimal allocation of leaf nitrogen maximizes daily CO(2) assimilation for a given leaf nitrogen concentration. According to the hypothesis of optimization, this condition occurs when the partial derivative of assimilation rate with respect to leaf nitrogen concentration is constant. This hypothesis predicts a linear increase of assimilation rate with leaf nitrogen concentration under constant conditions. Plants of Amaranthus powellii Wats. were grown at 1, 5, 10, or 45 millimolar nitrate to obtain leaves with different nitrogen concentrations. Assimilation rate at 340 microbar CO(2)/bar, stomatal conductance, CO(2)- and light-saturated net photosynthetic rate, the initial slope of the CO(2) response of photosynthesis, ribulose-1,5'-bisphosphate carboxylase activity, and phosphoenolpyruvate carboxylase activity were linearly related to estimated or actual leaf nitrogen concentration. The data are consistent with the optimal use of leaf nitrogen. This hypothesis and the hypothesis of optimal stomatal conductance were combined to determine the relationship between conductance and leaf nitrogen concentration. The slope of conductance versus leaf nitrogen concentration was not significantly different than the slope predicted by the combination of the two hypotheses. Stomatal conductance was linearly related to leaf nitrogen in the field and the slope decreased with lower xylem pressure potentials in a manner consistent with the hypotheses. Finally, apparent maximum stomatal aperture of isolated abaxial epidermal strips was linearly related to leaf nitrogen suggesting stomatal conductance and assimilation rate are controlled in parallel by leaf nitrogen concentration or some factor correlated with leaf nitrogen.

RESUMEN

The hypothesis of optimal stomatal conductance predicts conductance should vary with changes of the vapor pressure difference between leaf and air (VPD) to keep the partial derivative of transpiration rate (E) with respect to assimilation rate (A) constant ( partial differentialE/ partial differentialA = lambda). Stomatal conductance of Amaranthus powellii Wats. decreased strongly with increasing VPD for leaves with high total leaf nitrogen concentrations; whereas, it decreased slightly with increasing VPD for leaves with low leaf nitrogen concentrations. The calculated value of partial differentialE/ partial differentialA was constant for leaves with high leaf nitrogen concentrations but was not constant for leaves with low leaf nitrogen concentrations. However, the predicted values of stomatal conductance, transpiration rate, and assimilation rate for a constant lambda = 220 moles H(2)O/mole CO(2) and the measured values fit the y = x line for all nitrate treatments. These data extend the experimental support for the optimal water use efficiency hypothesis for a C(4) plant grown under different nitrate concentrations.

Asunto(s)

RESUMEN

Treatment of pregnancy toxaemia in ewes by induction of parturition is reported. The efficacy of 3 formulations of dexamethasone, at different dosages, is recorded and discussed.

Asunto(s)

Asunto(s)

Asunto(s)