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81.
Urban trees provide numerous ecosystem goods and services by providing shade, habitat for wildlife, removal of air pollutants and the removal and storage of atmospheric CO2. Carbon removal services provided by Canadian urban trees have previously been assessed using an IPCC 2006 guidelines approach based on the percentage of urban area covered by tree canopy (UTC) for the 2012 time period (Pasher et al., 2014). That work however provided only a single point in time assessment of the national scale UTC and carbon removal services. The research undertaken for this study was a continuation of this earlier work focusing on a 1990 national scale UTC assessment and carbon sequestration estimates for 1990. UTC estimates for 1990 were developed using a point sampling approach with circa 1990 air photos covering a large portion of Canadian urban areas. In total almost 179,000 points were sampled for the 1990 time period, reassessing 83% of the points used for the previous 2012 assessment. Based on the urban area boundary layers for 1991 and 2011, Canada’s urban areas grew by an estimated 6% for this time period. Most of this growth occurred through conversion of agricultural and forested lands to urban. At the national scale the UTC for 1990 was estimated to be 27.6%, as compared to the 2012 UTC estimate of 26.1%, the difference between estimates for the two time periods fell within the uncertainty range. Carbon removal estimates based on the UTC estimates were also very similar for the two dates with 660.2 kt C removed in 1990 and 662.8 kt C removed in 2012. It was noted that urban development in the Prairie regions resulted in an increase in tree cover as compared to the pre-conversion agricultural and natural landscapes and also that in most urban areas across the country UTC increases through time as tree cover matured in newly developed urban areas. These two assessments provide a time series of urban trees for 22 year time period, which will be useful for further studies and analysis. 相似文献
82.
Prattley DJ Morris RS Cannon RM Wilesmith JW Stevenson MA 《Preventive veterinary medicine》2007,81(4):225-235
Our BSurvE spreadsheet model estimates the BSE prevalence in a national cattle population, and can be used to evaluate and compare alternative strategies for a national surveillance program. Each individual surveillance test has a point value (based on demographic and epidemiological information) that reflects the likelihood of detecting BSE in an animal of a given age leaving the population via the stated surveillance stream. A target sum point value for the country is calculated according to a user-defined design prevalence and confidence level, the number of cases detected in animals born after the selected starting date and the national adult-herd size. Surveillance tests carried out on different sub-populations of animals are ranked according to the number of points gained per unit cost, and the results can be used in designing alternative surveillance programs. 相似文献
83.
Maggi Kelly Desheng Liu Brice McPherson David Wood Richard Standiford 《Journal of Forest Research》2008,13(5):312-319
Sudden oak death is a disease affecting coastal forests in California and southern Oregon. The spatial pattern of disease
dynamics is important for forest and landscape pathology; in this work we investigated the interaction across landscape scales
of disease symptomology in coast live oaks, Quercus agrifolia, (trunk bleeding, presence of beetles, and presence of the fungus Hypoxylon thouarsianum) and tree mortality through time. We used two-dimensional spatial analysis tools with data gathered in point-centered-quarter
format in 2001 and 2004 to quantify the population density of the disease through time; to examine the spatial pattern of
tree mortality across scales through time; and to examine the spatial co-occurrence of disease symptoms with crown mortality
through time. Early in the study period dead trees were strongly clustered at smaller scales (~300 m) and after three years
this clustering was less pronounced. Bleeding on trees occurred in clusters away from dead trees, particularly in 2004, likely
indicating a new cohort of infected trees. The presence of H. thouarsianum was strongly related to overstory mortality through time. Beetle-infested trees co-occurred with mortality in 2001. By 2004,
they occurred throughout the forest, and were less strongly correlated with overstory tree mortality, suggesting a future
peak of tree mortality. 相似文献
84.
在水温 2 4 .4~ 2 6.0℃时对斑鱼种 (尾重 1.12~ 4 .61g )进行了耗氧率与窒息点测定。结果表明 :耗氧率与窒息点均随鱼体体重的增加而降低。平均尾重 4 .61g的斑耗氧率为0 .1580 mg/g.h ,窒息点为 0 .12 92 mg/L ;平均尾重 1.12 g的斑耗氧率为 0 .2 636mg/g.h ,窒息点为 0 .30 4 0 mg/L。白昼和夜晚耗氧率的差异不大 相似文献
85.
Arunas Juska Lourdes Gouveia Jackie Gabriel Kathleen P. Stanley 《Agriculture and Human Values》2003,20(1):3-19
This article outlines aconceptual framework for examining recentoutbreaks of E. coli O157:H7 infectionassociated with the consumption of beef in theUnited States. We argue that beef produced inthis country is generally safer frombacteriological contamination than in the past.Paradoxically, increasing intensification andconcentration in the meat subsector since theearly 1980s has (a) altered agro-food ecology,including characteristics of foodborne bacteriaand human physiology; (b) created conditionsfavorable for the rapid amplification of lowconcentrations of pathogens; and (c) reducedthe beef industry's flexibility to introducechanges necessary to preclude and/or controlthe rapid spread of pathogens in meat and meatproducts. As a result, beef industry currentlyis capable of producing large quantities ofbacteriologically safe meat whilesimultaneously becoming more vulnerable to foodcontaminations that can be fatal in some cases.The limitations and effectiveness of a newregulatory regime, the HACCP (Hazard Analysisand Critical Control Points) system as well asother efforts to decontaminate the beef supplyare discussed. 相似文献
86.
结合光伏电池常用的等效电路和太阳能电池的数学模型,基于Matlab/Simulink建立了光伏电池的仿真模型,得到光伏电池的P-U曲线,并对仿真结果进行了分析。与传统的光伏电池模型相比,本文考虑了环境温度与电池温度之间的关系,使得光伏电池的仿真结果能反映实际环境与电池温度的变化;提出了改进扰动观察的最大功率点跟踪(MPPT)控制方法,并建立了带有MPPT控制功能的光伏发电系统仿真模型,仿真结果表明,该系统能较好地实现最大功率点的跟踪,提高了光伏电池的发电效率。 相似文献
87.
88.
89.
由于三聚氰胺事件给消费者造成的损害至今未能根除,同时,也对我国乳品产业造成了巨大冲击,乳制品安全成为我国食品安全的热点问题。本文探讨了对乳品质量安全进行有效管理的方法,提倡采用全过程管理模式,以haccp控制体系为基础,着力提升我国乳与乳制品的质量安全水平,提高公共危机管理能力。 相似文献
90.
Forestry around the world has been experiencing a paradigm shift towards more nature-oriented forest management leading foresters to emulate natural disturbances by their silvicultural treatments. Important characteristics of all disturbances are their size, severity, temporal and spatial distribution. This study focuses on the spatial distribution of gaps in the forest canopy which are typically caused by small-scale, low intensity disturbances. 相似文献