二维伊辛模型自旋状态图样的蒙特卡罗模拟

以Ｍｅｔｒｏｐｏｌｉｓ蒙特卡罗模拟方法考察了在２０×２０正方格子上的二维伊辛自旋模型，得到了该模型不同温度下自旋状态的图样，符合统计力学分析。     

森林可持续发展的动态优化模型

运用动态规划方法讨论森林可持续发展问题，建立了森林自然发展模型和森林可持续发展的优化开发模型，给出了相应的迭代公式，并选用青冈－夏蜡梅群落进行模拟计算。结果表明经过一段时间的自然发展，森林群落可达到稳定态，夏蜡梅群落必将过渡到青冈－夏蜡梅稳定群落。其后，各阶段各树种的数量有微小变化，但总体稳定，且算法快速收敛于稳定点。图3表2参4。     

中草药添加剂对绿壳蛋鸡蛋品质的影响

试验选取4000只绿壳蛋鸡,随机分为4组(A组、B组、C组、D组),每组2个重复,每个重复500只鸡。对照组(A组)饲喂基础饲粮,各试验组分别在基础日粮基础上添加500、1000、1500 mg/kg中草药添加剂。结果表明,各试验组的蛋形指数,蛋壳强度,蛋壳厚度,蛋黄颜色,蛋黄比率,哈氏单位,血、肉斑率较对照组均得到一定改程度改善,但差异不显著(P>0.05);各试验组蛋比重均显著高于对照组(P<0.05)。由此可见中草药添加剂可一定程度改善绿壳蛋鸡的蛋品质。     

天然次生林中乔木树种的种间关系

创建了局部联结度指数构造模型，构造了一种与密度相关的联结度指数及相应的显著性检验统计量，证明了经典联结度检验统计量是本模型的特例。应用联结度定义研究了帽儿山林场的固定标准地数据10个树种的联结关系，得到了几个有显著联结度的树种关系，并捡测到白桦与山杨间随年龄而变化的联结关系。     

Responses of gross primary productivity to different sizes of precipitation events in a temperate grassland ecosystem in Inner Mongolia,China

Changes in the sizes of precipitation events in the context of global climate change may have profound impacts on ecosystem productivity in arid and semiarid grasslands. However, we still have little knowledge about to what extent grassland productivity will respond to an individual precipitation event. In this study, we quantified the duration, the maximum, and the time-integrated amount of the response of daily gross primary productivity(GPP) to an individual precipitation event and their variations with different sizes of precipitation events in a typical temperate steppe in Inner Mongolia, China. Results showed that the duration of GPP-response(τR) and the maximum absolute GPP-response(GPPmax) increased linearly with the sizes of precipitation events(Pes), driving a corresponding increase in time-integrated amount of the GPP-response(GPPtotal) because variations of GPPtotal were largely explained by τR and GPPmax. The relative contributions of these two parameters to GPPtotal were strongly Pes-dependent. The GPPmax contributed more to the variations of GPPtotal when Pes was relatively small(20 mm), whereas τR was the main driver to the variations of GPPtotal when Pes was relatively large. In addition, a threshold size of at least 5 mm of precipitation was required to induce a GPP-response for the temperate steppe in this study. Our work has important implications for the modeling community to obtain an advanced understanding of productivity-response of grassland ecosystems to altered precipitation regimes.     

Modelling population dynamics of Sinapis arvensis in organically grown spring wheat production systems

Although we know that alterations in crop density, crop spatial pattern and inclusion of more selective weed control can improve weed suppression for organic growers, it is unknown whether these result in changes to the weed seedbank that increase cropping system profitability over time. Data collected from field trials conducted in 2009 and 2010 in Maine, USA, comparing regional grower practices (Standard) with management that aimed to (i) facilitate better physical weed control through the use of wide row spacing and inter‐row cultivation (Wide) or (ii) enhance crop–weed competition through increased seeding rate and narrow inter‐row spacing (Narrow HD), were used to construct a matrix population model with an economic sub‐model. Using field measurements of grain yield and weed survival and fecundity, we investigated the lasting implications of employing alternative organic spring wheat (Triticum aestivum) production practices on Sinapis arvensis population dynamics. In most scenarios, the model indicated that regional production practices were not sufficient to prevent an increase in the weed seedbank, even with excellent weed control. The two alternative methods, on the other hand, were able to limit weed population growth when initial densities were low or cultivation efficacy was >80%. Due to higher seed costs in the Narrow HD system, net returns were still lower after 10 years of simulation in this system compared with wide rows with cultivation, despite a lower weed seedbank.     

城市居民茶叶消费行为影响因素实证分析——基于广州调查数据

为了研究当前哪些因素影响城市居民茶叶消费行为,为茶产业产学研提供理论依据,本文通过采纳城市居民茶叶消费行为的调查数据,使用计量经济软件Eviews3．1,运用线性模型,进行普通最小二乘法估计。实证研究结果表明：月总收入、家庭人口数、饮茶年份与消费者的茶叶消费支出成正相关,教育水平与茶叶消费支出成负相关,收入是影响茶叶消费支出弹性的主要因素。     

股票收益独立性对可转换债券定价的影响

在我国利用布莱克-休斯期权模型对可转换债券中的期权价值进行定价得到了广泛应用,但股票收益率独立性是模型成立的必要条件。笔者给出了收益率与股价存在相关的情形下布莱克-休斯期权模型的修正形式,通过对样本股票的进一步实证研究表明,直接应用布莱克-休斯期权模型会高估中国可转换债券的价格。     

A probability model for root and butt rot in Picea abies derived from Norwegian national forest inventory data

Root rot in Norway spruce (Picea abies (L.) Karst.) causes substantial economic losses to the forestry sector. In this study, we developed a probability model for decay at breast height utilizing 18,141 increment cores sampled on temporary plots of the Norwegian National Forest Inventory. The final model showed a good fit to the data and retained significant relationships between decay and a suite of tree, stand and site variables, including diameter at breast height, stand age, altitude, growing season temperature sum (threshold 5°C), and vegetation type. By comparing model predictions with recorded decay at stump height in an independent data set, we estimated a proportionality function to adjust for the inherent underestimation of total rot that will be obtained by applying a probability model derived from increment cores sampled at breast height. We conclude that the developed model is appropriate for national and regional scenario analyses in Norway, and could also be useful as a tool for operational forestry planning. This would however require further testing on independent data, to assess how well the new model predicts decay at local scales.