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The structure of Aspergillus flavus communities associated with south Texas cottonseed was determined by analysing samples from 178 truckloads of commercial cottonseed from 35 gins, extending from Fort Bend County in the north to the Rio Grande Valley in the south, from September 1999 to October 2001. The number of colony-forming units (CFU) of A. flavus on the cottonseed, and the percentage of S strain (%S) were both correlated with aflatoxin contamination of cottonseed. The number of CFU differed between both regions and seasons, while %S differed only between regions. Comparison of maps of CFU and %S revealed that CFU shows a higher variation across years, while %S shows higher spatial variation. The Rio Grande Valley had significantly lower CFU and %S strain than the Coastal Bend and Upper Coast regions. Cottonseed produced in 1999 had significantly more A. flavus than that produced in either 2000 or 2001. Identification of factors dictating geographical variation in S-strain incidence may provide insights that will lead to improved aflatoxin management. 相似文献
13.
采用颍次分布X~2检验、Iwao平均拥挤度m~*=a+βx和Taylor幂法则S~2=am~b等8种方法,分析了七块麦田中七星瓢虫幼虫的空间分布型,认为其分布型为负二项分布,且聚集的原因是由于个体之间存在着相互吸引,并确定了代表性较强的棋盘式抽样技术,最佳抽样数量为:Q=28.7088/+0.7552。 相似文献
14.
Change in spatial characteristics of forest openings in the Klamath Mountains of northwestern California,USA 总被引:4,自引:0,他引:4
Carl N. Skinner 《Landscape Ecology》1995,10(4):219-228
Change in the spatial characteristics of forest openings was investigated in three forested watersheds in north-western Siskiyou County, California totalling approximately 24,600 hectares. Watersheds with minimal human disturbance were chosen for study. However, fire suppression has been pervasive throughout. Characteristics of forest openings (area, perimeter, distance between neighboring openings) were measured on aerial photographs taken 41 years apart. An index of regional form was determined for the landscape. Shape complexity for each opening was calculated using two indices based upon fractals. Significant differences were found using the Kolmogorov-Smirnov two-sample test between the perimeters, areas, distance from sample point to nearest opening, and distance between neighboring openings. The perimeters and areas became smaller, and the distances from the sample point to the nearest opening and between neighboring openings became greater over the 41 years between aerial photo sets. The estimated area occupied by openings decreased from 25.8 % to 15.6 % of the study area. No significant difference was found in the shape of the openings except as the shape indices were influenced by changes in size of the openings.This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. 相似文献
15.
阜康绿洲土壤盐渍化特征及其与肥力的相关性分析 总被引:9,自引:0,他引:9
本文以电导、可溶性总盐含量和pH作为指标,将阜康绿洲的盐渍化类型划分为9种,其中以强盐渍土和盐土为主,在纬度梯度上从南至北,即从洪积扇下部到沙漠按照中盐渍土-强盐渍土-盐土-强盐渍土-盐土-非盐渍土的顺序依次分布。而土壤含水率却是从洪积下部至扇缘溢出带逐渐升高,在细土平原地区差异较大,在沙漠中又迅速降低。通过土壤剖面中有机质、全量N、P、K和速效N、P、K的分析,结果表明阜康荒漠自然状态下土壤肥力很低,各养分含量在空间上呈现规律性的变化,在纬度梯度上逐渐升高又在沙漠中降低。通过水盐与土壤肥力的相关性分析,结果表明:有机质、全N、P和速P与含水率、电导、总盐均具有显著和极显著正相关,而与pH成负相关,但相关性不显著。由于盐生植物的多样性,在盐渍土和盐土土壤肥力并无显著降低,只是土壤溶液的高盐浓度限制了植物对养分的吸收,因此防止和治理土壤盐渍化是发挥土地生产潜力的重要手段。 相似文献
16.
基于不同尺度的干旱区城市景观生态研究--以陕西榆林市为例 总被引:6,自引:0,他引:6
从区域与城市两个尺度水平 ,对毛乌素沙漠南缘的榆林市城市景观进行研究认为 :在大尺度水平上 ,区域景观生态特征对城市景观特征具有一定的控制作用 ,而城市景观强大的功能流对区域景观产生干扰作用。城市景观的空间扩展与演化 ,既受区域景观特征的影响 ,也是区域景观演化的一个有机构成部分 ;在小尺度空间上 ,景观功能区、景观斑块与廊道的格局与功能 ,对城市景观格局动态以及维持城市景观多样的生态功能具有重要意义。在不同的利益追求下形成的景观文化 ,将深刻地影响到区域与城市景观演化的方向与过程。 相似文献
17.
A. van Maanen X.-M. Xu 《European journal of plant pathology / European Foundation for Plant Pathology》2003,109(7):669-682
An epidemic is the progress of disease in time and space. Each epidemic has a structure whose temporal dynamics and spatial patterns are jointly determined by the pathosystem characteristics and environmental conditions. One of the important objectives in epidemiology is to understand such spatio-temporal dynamics via mathematical and statistical modelling. In this paper, we outline common methodologies that are used to quantify and model spatio-temporal dynamics of plant diseases, with emphasis on developing temporal forecast models and on quantifying spatial patterns. Several examples of epidemiological models in cereal crops are described, including one for Fusarium head blight. 相似文献
18.
新疆农田土壤有机质含量及组成特征 总被引:3,自引:0,他引:3
目前新疆农田土壤有机质含量在5.30~24.21g/kg之间,平均14.65g/kg,与第二次土壤普查相比,新疆农田土壤有机质含量呈上升趋势;胡敏素碳含量占到有机质碳量的53%~70%,平均61%,腐殖酸碳量只占39%。 相似文献
19.
旅游地空间竞争与弱势旅游地的发展研究 总被引:19,自引:1,他引:19
本文在介绍旅游地空间竞争的基础上 ,导入弱势旅游地的概念。并以游客吸引半径为尺度 ,对西安和宝鸡的客源市场竞争进行分析。最后总结了发展弱势旅游地的契机和方法。 相似文献
20.