The simultaneous analysis of mixed discrete and continuous outcomes using nonlinear threshold models     

Todd Coffey  Chris Gennings 《Journal of Agricultural, Biological & Environmental Statistics》2007,12(1):55-77

Mixed discrete and continuous outcomes are commonly measured on each experimental unit in dose-response studies in toxicology. The dose-response relationships for these outcomes often have dose thresholds and nonlinear patterns. In addition, the endpoints are typically correlated, and a statistical analysis that incorporates the association may result in improved precision. We propose an extension of the generalized estimating equation (GEE) methodology to simultaneously analyze binary, count, and continuous outcomes with nonlinear threshold models that incorporates the intra-subject correlation. The methodology uses a quasi-likelihood framework and a working correlation matrix, and is appropriate when the marginal expectation of each outcome is of primary interest and the correlation between endpoints is a nuisance parameter. Because the derivatives of threshold models are not continuous at each point of the parameter space, we describe the necessary modifications that result in asymptotically normal and consistent estimators. Using dose-response data from a neurotoxicity experiment, the methodology is illustrated by analyzing five outcomes of mixed type with nonlinear threshold models. In this example, the incorporation of the intra-subject correlation resulted in decreased standard errors for the threshold parameters.  相似文献   

Effects of nitrogen fertilizer application rates on nitrate nitrogen distribution in saline soil in the Hai River Basin,China     

Fei Yin  Bojie Fu  Renzhao Mao 《Journal of Soils and Sediments》2007,7(3):136-142

Background and Objectives  

Soil nitrate nitrogen (NO₃ ⁻-N) accumulation is related closely to NO₃ ⁻ leaching, which is an important issue in groundwater pollution, especially in intensive agricultural areas with saline soils where volumes of water are used in irrigation to avoid salt accumulation in the root zone. However, in the saline environment in Hai River Basin, China, the importance of detailed research into NO₃ ⁻-N distribution in the root zone has not been adequately recognized. Considering the impacts of eco-environmental system N and crop production, the present study aimed at contributing to an understanding of the effects of N application rate on soil NO₃ ⁻-N distribution, NO₃ ⁻-N residue, N loss, and maize (Zea mays L.) yield in this region.  相似文献   

Large-sample hypothesis tests for stratified group-testing data     

Joshua M. Tebbs  Melinda H. McCann 《Journal of Agricultural, Biological & Environmental Statistics》2007,12(4):534-551

Insect-vectored plant diseases impact the agricultural community each year by affecting the economic value, the quantity, and the quality of crops. Controlling the spread of disease is an important area in risk assessment, and understanding the dynamics of vector populations helps researchers to develop effective treatments. In this article, we consider an experimental design commonly used by researchers who study plant disease and examine large-sample, likelihood-based hypothesis tests that can be used to characterize disease-transmission behavior in a stratified population. Small-sample size and power results along with design considerations are provided. We illustrate our testing procedures using two real data examples and provide recommendations for plant-disease researchers in the field.  相似文献   

Florida manatees, Trichechus manatus latirostris, respond to approaching vessels     

Stephanie M Nowacek  Randall S Wells  Edward C.G Owen  Richard O Flamm 《Biological conservation》2004,119(4):517-523

Florida manatees inhabit shallow coastal and estuarine waters of the southeast US, a range that brings them into frequent contact with vessels. More than 30% of documented annual mortalities are attributed to vessel collisions, and most living animals bear the scars of multiple, non-lethal encounters. To document the behavior of manatees in the presence of vessels, we recorded their movements with an overhead video system. We scored six aspects of behavior during 170 vessel approaches, and compared their behavior with 187 control segments when no boats were present. Manatees in shallow waters and at the edge of the channel responded to approaches by orienting towards the nearest deep water, a boat channel, and increasing their swimming speed. Close boat approaches and shallow water depths exacerbated these responses. Our results indicate that manatees detect and respond to approaching vessels with an apparent flight response, a response which includes movement towards deeper water. If given sufficient time, i.e., approached or passed slowly, the manatees may then be able to reach deeper water and safe depths.  相似文献   

An application of the mixed linear model and the dirichlet process prior in veterinary medicine research     

Abrie J. van der Merwe  Albertus L. Pretorius 《Journal of Agricultural, Biological & Environmental Statistics》2003,8(3):328-343

This article applies Bayesian nonparametric techniques of analysis to the mixed linear model. The distribution of the random effects is specific as a nonparametric prior. A Dirichlet process prior is specified on the space of prior distributions. A modified Dirichlet process is described and applied using a Gibbs sampler. The approach is demonstrated in an investigation of the changes over time of packed cell volume in two breeds of cattle.  相似文献   

Seasonal changes in periphyton nitrogen fixation in a protected tropical wetland     

Rodrigo Vargas  Eberto Novelo 《Biology and Fertility of Soils》2007,43(3):367-372

Cyanobacteria are important for global nitrogen cycle and often form complex associations referred to as cyanobacterial mats or periphyton that are common in tropical, limestone-based wetlands. The objective of this study was to monitor the nitrogen fixation rate using the acetylene reduction assay of these cyanobacterial mats in a tropical, unfertilized, and protected wetland. To account for temporal and spatial variation of nitrogenase activity, we were interested in seasons in a hydrological cycle (dry, rains, and end of rains), sites with different vascular vegetation, and rates of nitrogenase activity in a 24-h cycle. The annual average of nitrogenase activity was 22 nmol C₂H₄ cm⁻² h⁻¹, with a range of <6 to 35 nmol C₂H₄ cm⁻² h⁻¹, and the annual nitrogen fixation rate of our study site (9.0 g N m⁻² year⁻¹) is higher than similar estimates from other freshwater wetlands. There was a clear temporal pattern in nitrogenase activity with a maximum rate occurring during the rainy season (August) and a maximum nitrogenase activity occurring between 0600 and 1200 hours. We found spatial differences in nitrogenase activity among the four sites that could be attributed to variations in species composition within the periphyton.  相似文献   

Nitrogen and carbon mineralization of semi-arid shrubland soil exposed to long-term atmospheric nitrogen deposition     

George L. Vourlitis  Gypsi Zorba 《Biology and Fertility of Soils》2007,43(5):611-615

Anthropogenic N-deposition represents a significant input of N into semi-arid chaparral and coastal sage scrub (CSS) shrublands of southern California. High levels of atmospheric N deposition have the potential to increase soil C and N mineralization, and we hypothesize that semi-arid shrubland soil exposed to long-term (decades) high N deposition will have significantly higher C and N mineralization potentials. This hypothesis was tested in a laboratory incubation where the inorganic N (NH₄+NO₃) and CO₂ production of soils maintained at a constant temperature of 25°C and a soil moisture of 0.25 g H₂O/g (65% water-filled pore space) were sampled sequentially over a 50-week period. The temporal trend in cumulative C and N mineralization was well described by a first- and zero-order model, respectively. Long-term atmospheric N deposition significantly increased potential N mineralization but not C mineralization, and both the rate and total N mineralization were significantly positively correlated with the surface (0–10 cm) soil δ ¹⁵N natural abundance and negatively correlated with the surface soil C:N ratio. While the incubation techniques used here do not provide realistic estimates of in situ C or N mineralization, these assays indicate that atmospheric N deposition has significantly altered ecosystem N storage and cycling.  相似文献   

Changes in soil chemistry associated with the establishment of forest gardens on eroded,acidified grassland soils in Sri Lanka     

Ilyas Siddique  Caroline Gutjahr  Gamini Seneviratne  Broder Breckling  Sudheera W. Ranwala  Ian J. Alexander 《Biology and Fertility of Soils》2007,44(1):163-170

Topsoil properties were determined in forest gardens established about 20 years ago on eroded grassland soils (abandoned tea lands) in the wet zone of the Sri Lankan highlands. They were compared with adjacent, eroded grasslands (abandoned tea lands) on strongly weathered soils vs soils at earlier stages of pedogenic development in a two-way analysis of variance. Soil pH in forest gardens was, on average, 6.1, nearly one unit higher than in the adjacent grasslands. In the garden soils, the cation exchange capacity (CEC measured at pH 4.8) was nearly double, exchangeable calcium concentrations five times and exchangeable magnesium three times as high as in the grasslands soils. Total soil N content was found to be nearly 40% higher in the gardens. Topsoil gravel contents in the gardens were less than half as high as in the grasslands. The increases in exchangeable bases and N in gardens, relative to grasslands, were attributed to increased nutrient retention and acquisition. Higher retention was partly due to the higher CEC_pH4.8, and probably to reduced erosion and increased, continuous fine root density in the garden topsoils. Higher field CEC in gardens was likely to result from generally higher C contents and from the reversal of acidification, presumably caused by base accumulation and decomposition processes. Our results suggest that forest garden establishment on degraded grasslands can lead to accumulation of mobile nutrients in the topsoil, probably due to increased nutrient retention, subsoil uptake and litter input exceeding nutrient uptake by the standing biomass.  相似文献   

Repellent and stocking guards reduce mammal browsing in eucalypt plantations     

Alison M. Miller  Julianne M. O’Reilly-Wapstra  Brad M. Potts  Clare McArthur 《New Forests》2011,42(3):301-316

Mammalian herbivores regularly browse plantation seedlings. In many areas, seedlings require some form of protection if they are to survive and grow into a productive plantation. Two general approaches for reducing browsing damage to seedlings are to apply chemical repellents and tree guards. Both methods have existed for a long time, and new variations are constantly being developed. Seedling stocking guards, a type of tree guard, are being used operationally in Tasmania, Australia, but there is limited data quantifying their effectiveness and concerns with negative effects on tree performance. Conversely, although proven effective, repellents are not being used, but are potentially cheaper and less problematic. We therefore aimed to determine which is more effective under operational conditions, whether this effectiveness can be improved or extended, and if either treatment has any effects on seedling form or survival. We planted Eucalyptus nitens seedlings with combinations of repellent and stocking guards in six operational plantations to examine and compare their effectiveness. Seedlings were monitored for 12 months to examine treatment longevity. We found that both stocking guards and repellent significantly reduced and delayed browsing severity, with their effects being additive. No negative effects on growth were evident after 12 months, but adverse effects on seedling form warrant further investigation. Both of these methods can be easily and relatively cheaply applied in the nursery before planting, making them appealing options to reduce browsing. However, the ideal method for a given site will depend upon local browsing intensity.  相似文献   

Editorial     

Jules Bayala  Niéyidouba Lamien  Innocent Butare  Sibiri Jean Ouédraogo  Oscar Eyog-Matig 《New Forests》2011,41(3):277-279

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