Effects Of Antecedent Conditions Research Articles

Quantifying ecological memory in plant and ecosystem processes.

The role of time in ecology has a long history of investigation, but ecologists have largely restricted their attention to the influence of concurrent abiotic conditions on rates and magnitudes of important ecological processes. Recently, however, ecologists have improved their understanding of ecological processes by explicitly considering the effects of antecedent conditions. To broadly help in studying the role of time, we evaluate the length, temporal pattern, and strength of memory with respect to the influence of antecedent conditions on current ecological dynamics. We developed the stochastic antecedent modelling (SAM) framework as a flexible analytic approach for evaluating exogenous and endogenous process components of memory in a system of interest. We designed SAM to be useful in revealing novel insights promoting further study, illustrated in four examples with different degrees of complexity and varying time scales: stomatal conductance, soil respiration, ecosystem productivity, and tree growth. Models with antecedent effects explained an additional 18-28% of response variation compared to models without antecedent effects. Moreover, SAM also enabled identification of potential mechanisms that underlie components of memory, thus revealing temporal properties that are not apparent from traditional treatments of ecological time-series data and facilitating new hypothesis generation and additional research.

Effect of antecedent conditions and fixed rock fragment coverage on soil erosion dynamics through multiple rainfall events

The effect of antecedent conditions and specific rock fragment coverage on precipitation-driven soil erosion dynamics through multiple rainfall events was investigated using a pair of 6-m × 1-m flumes with 2.2% slope. Four sequential experiments – denoted E1, E2, E3 and E4, involved 2-h precipitation (rates of 28, 74, 74 and 28 mm h-1, respectively) and 22 h without rainfall – were conducted. In each experiment, one flume was bare while the other had 40% rock fragment coverage. The soil was hand-cultivated and smoothed before the first event (E1) only, and left untouched subsequently. Sediment yields at the flume exit reached steady-state conditions over time scales that increased with sediment size. Experiments were designed such that both steady and non-steady effluent sediment yields were reached at the conclusion of E1. Results from subsequent experiments showed that short-time soil erosion was dependent on whether steady-state erosion was achieved during the preceding event, although consistent steady-state effluent sediment yields were reached for each sediment size class. Steady-state erosion rates were, however, dependent on the rainfall intensity and its duration. If steady-state sediment yields were reached for a particular size class, that class’s effluent sediment yield peaked rapidly in the next rainfall event. The early peak was followed by a gradual decline to the steady-state condition. On the other hand, for size classes in which steady state was not reached at the end of the rainfall event (i.e., E1), in the following event (E2), the sediment yields for those classes increased gradually to steady state, i.e., the sharp peak was not observed. The effect of rock fragment cover (40%) on the soil surface was also found to be significant in terms of the time to reach steady state, i.e., their presence reduced the time for steady conditions to be attained. Effluent sediment yields for the bare and rock fragment-covered flumes (E1) showed steady conditions were reached for the latter, in contrast to the former. We used the Hairsine-Rose (H-R) model to simulate the experimental data as it explicitly models soil particle size classes. Experiments E1 and E2 involved soil compaction by raindrops, and in this case the model predictions were found to be unsatisfactory. However, compaction was effectively completed by the end of experiment E2, and the model provided reasonable predictions for experiments E3 and E4.

Effect of Antecedent Conditions on Prediction of Pore-Water Pressure using Artificial Neural Networks

The effect of antecedent conditions on the prediction of soil pore-water pressure (PWP) using Artificial Neural Network (ANN) was evaluated using a multilayer feed forward (MLFF) type ANN model. The Scaled Conjugate Gradient (SCG) training algorithm was used for training the ANN. Time series data of rainfall and PWP was used for training and testing the ANN model. In the training stage, time series of rainfall was used as input data and corresponding time series of PWP was used as the target output. In the testing stage, data from a different time period was used as input and the corresponding time series of pore-water pressure was predicted. The performance of the model was evaluated using statistical measures of root mean square error and coefficient of determination. The results of the model prediction revealed that when antecedent conditions (past rainfall and past pore-water pressures) are included in the model input data, the prediction accuracy improves significantly.

Can America Nation-Build?

Post-9/11 security concerns and the U.S.-led invasion of Iraq have renewed scholarly interest in nation-building as a form of externally fostered democratization. The selected works assess Iraq and its precursors, seeking general lessons for establishing new democracies. They principally conclude that successful nation-building depends on sustained commitments of time, materiel, and manpower. Although this thesis improves upon earlier studies of democracy promotion, which often treated intentions as determinative, it does not fully reckon with the effect of antecedent conditions on external intervention. As this review addresses, American efforts at nation-building have historically been enabled or constrained by local political institutions. Rather than autonomously reengineering the target society, nation-builders have buttressed bureaucracies and parliaments where they were already available (Germany, Japan) and foundered in countries that lacked such institutions (Somalia, Haiti). In sum, nation-building has been most effective when pursued least ambitiously, amid functioning states with prior experience in constitutional government.

Informational reception apprehension, educational motivation, and achievement

This study explored the relationship among informational reception apprehension (IRA) factors, student motivation, and student achievement. Based on Wheeless, Preiss, and Gayle's (1997) recently developed construct, the authors hypothesized that IRA scores for listening and inflexibility would predict educational motivation scores and measures of achievement. Furthermore, this study extended existing research on student motivation and the effects of antecedent conditions, such as trait‐like IRA. Results indicated that informational reception apprehension can be taken into consideration in order to account more adequately for student motivation and achievement.

The effects of antecedent moisture conditions on the relationship of hydrology to hydrochemistry in a small forested watershed

The relationship between stream discharge and the concentration of different solutes reflects the hydrological processes occurring in a catchment, but is also strongly affected by antecedent moisture conditions. In this study, the effects of antecedent conditions were examined for a small forested watershed near Montréal (Québec) by comparing data from sequences of fall events for two years of contrasting antecedent moisture conditions. The relationship between discharge and stream water chemistry was also quantified using cross-correlation analysis. Results show that concentrations of dissolved organic carbon, nitrate and calcium increased during storm events with dry antecedent conditions but that the relationships were weaker under wet conditions. For both years, concentrations of sulfate, silicon and sodium and the pH were negatively correlated with discharge (cross-correlations from −0·20 to −0·53). With dry antecedent conditions, there was a general decrease in the concentration of all solutes with time, whereas concentrations remained at about the same level under wet conditions. The concentration–discharge relationships for sulfate shifted frequently from higher concentrations during the rising limb to higher concentrations during the falling limb from one event to another as a result of changing antecedent conditions. Although the contrast in antecedent moisture conditions between the two years was moderate in comparison with that reported in other studies (25% dryer than average in 1995 and 12% wetter than average in 1996), the hydrochemistry of the stream was markedly different. Copyright © 1999 John Wiley & Sons, Ltd.

Scale dependence of the episodic hydrochemical response of nested catchments at Plynlimon

Abstract. Previous monitoring of the surface water chemistry of the forested catchments at Plynlimon has concentrated on weekly measurements. Hence, peak values and details of the rapid changes in chemistry occurring during events tend to be missed, particularly for smaller catchments. Knowledge of the detailed hydrochemical response is necessary to predict effects on stream ecology and to quantify processes occurring within the catchment. This information is vital if the likely impacts of changes in landuse and pollutant input on episodic hydrochemical variations are to be modelled. A summary of daily and three-hourly chemistry for the River Severn headwaters at Plynlimon, central Wales is presented. Data are from the summer and autumn of 1995 and the winter and summer of 1996. The effects of antecedent conditions, soiltype, landuse and groundwater and tributary inflow on stream episodic hydrochemical response are discussed for a range of spatial scales, including first, second and third order streams. Detailed information on the chemical changes due to a discrete groundwater inflow into the bed of one stream shows that, during episodes, its influence on stream chemistry is overridden by that of event water. A comparison of drainage ditch chemistry with main stream chemistry during storm events is used to consider the importance of tributary inflow at these times. The contrast in results between sites is assessed, to evaluate the effects of clearfelling and soil type on stream event chemistry.