Four hundred years in the history of pinniped colonies around Mar del Plata,Argentina

1. Historical records show the area of Mar del Plata (38°05′ S/57°32′ W, Argentina) as inhabited by extensive seal colonies; the present study describes the evolution of their size and location from the 16th century to the present. 2. Southern sea lion (Otaria flavescens Shaw 1800), South American fur seal (Arctocephalus australis Zimmerman 1783) and southern elephant seal (Mirounga leonina Linnaeus 1758) colonies, which consisted of between 80 000 and 165 000 animals, were subject to no commercial harvest, and only small local catches were performed by transient aboriginal groups. 3. During the second half of the 19th century coastal zones were rapidly colonized by man and by the turn of the century, the pinniped rookeries finally disappeared. Such a dramatic decline was not only due to spatial competition with man, but also to the indirect effect of pinniped over-exploitation in other areas of the south-western Atlantic. 4. No seal colonies were recorded in Mar del Plata during the 20th century until the mid sixties, when small non-breeding groups of sea lions and fur seals established themselves in the area. 5. The seal rookeries' decline in Mar del Plata provides an interesting example of how human activity may severely affect the conservation of pinniped colonies, even with no direct action through massive catches. © 1998 John Wiley & Sons, Ltd.     

Scattered shrubs in pearl millet fields in semiarid Niger: Effect on millet production

Various kinds of shrubs species are found in many fields and fallows in semiarid Niger. In order to understand the nature of their interactions with millet (Pennisetum glaucum L.) and their role in soil conservation, experiments were conducted in 1995 and 1996. Three treatments were applied with different shrub densities: cutting all shrubs, cutting half of the shrubs and leaving all shrubs. Millet was harvested per plot and in different circular zones around selected shrubs. In addition, transects across shrubs were investigated for soil parameters and microtopography changes. Millet yield and soil-nutrient status were higher around shrubs compared to the open field. Microtopography was elevated up to 20 cm near shrubs. The positive influence of shrubs on increased millet production extended to 2 m distance from the shrub. Competition between uncut shrubs and millet plants was found within a distance of 1.2 m. An increasing yield of millet was found in plots with shrubs up to a density of about 450 shrubs per hectare. Shrubs showed to be effective for preservation of soil fertility of fields in south-west Niger and thus important for millet production. Optimal for shrub management in farmers' fields would be cutting half of the shrubs and leaving the other half uncut for simultaneous wood production. This revised version was published online in June 2006 with corrections to the Cover Date.     

Weed community characteristics and crop performance: a neighbourhood approach

The presence of weeds in crop fields often causes yield reductions. However, the effects of weed diversity have not been fully examined. This study tested the hypothesis that, holding density constant, increased weed species richness would decrease the effects of competition on spring wheat target plant performance. Measurements were taken from circular neighbourhoods (16.5 cm radius) with a single spring wheat plant surrounded by combinations of Setaria viridis , Avena fatua , Kochia scoparia , Thlaspi arvense and spring wheat plants, representing all combinations of neighbour species at four density levels. Using regression models, we found that species richness had no significant direct effect on spring wheat biomass, yield, or relative growth rate and that there were no significant neighbour species interaction terms. For weedy target plants, the presence of negative interaction terms suggested that increasing species richness had negative effects on growth of individual weed species. Additional analysis suggested that increased species richness may limit competitive ability of dominant weeds. Although we found no evidence of a direct effect of weed species richness on crop performance, increased weed species richness had no negative effect on spring wheat performance. Further, species richness of the weed community appears to influence weedy plant performance, which may offer a future opportunity to influence crop performance.     

Sowing date and nitrogen supply determine the outcome of competition between dallisgrass (Paspalum dilatatum Poir.) and tall fescue (Festuca arundinacea Schreb.) in the Pampas region of Argentina

The effects of sowing date and nitrogen (N) fertilizer on the inter‐specific competition between dallisgrass (Paspalum dilatatum Poir.) and tall fescue (Festuca arundinacea Schreb.) in the humid Pampas of Argentina were investigated in two pot experiments where a constant soil moisture content was maintained. Tall fescue and dallisgrass seeds were sown either in the spring (October 2000) or in the autumn (March 2001) in mixed and mono‐specific stands with 0 or 100 kg N ha^?1. In the spring, competition from tall fescue depressed dry‐matter (DM) yield of dallisgrass from 1·53 to 0·36 g DM per plant and tiller number from 9·4 to 3·7 tillers per plant in mixed and in mono‐specific stands, respectively, while tall fescue had 3–4 times higher DM yields in mixed stands. Leaf extension rate (LER) of tall fescue was higher (1·3 mm d^?1) than that of dallisgrass (0·53 mm d^?1). In the autumn, inter‐specific competition did not affect DM yield of dallisgrass and N fertilizer increased DM yield from 0·53 to 2·07 g DM per plant, tiller number from 6·8 to 14·2 tillers per plant and LER at the beginning of autumn from 1·2 to 2·12 mm d^?1 in both species. As temperature decreased, LER was reduced in both species to 0·31 mm d^?1 by late autumn. The number of leaves per tiller was not affected by treatment. Nitrogen fertilizer increased N concentration of above‐ground tissues of both species (18 g kg^?1 DM in autumn and 20 g kg^?1 DM in spring). It was concluded that a productive mixed pasture of dallisgrass and tall fescue can be obtained by sowing early in the autumn. The application of N fertilizer in this season is essential to ensure a high herbage yield and quality.     

根土复合体力学效应及其模型构建研究进展与展望

[目的] 为探究根土复合体的力学效应机理及模型方法应用。[方法] 采用文献分析和对比分析法,归纳分析根土复合体概念和内涵、根土复合体力学效应及力学模型原理、优缺点以及适用范围。[结果] (1)根土复合体是根系与土体之间力学耦合效应的复合整体,根系在土体中交叉缠绕,起到加固作用;(2)根系与土体力学关系实质上是根土复合体土力、水力和复合力学特性作用的结果,土力特性、水力特性分别侧重研究根系对土体的影响和土中水分对土体和根系的影响,复合力学特性侧重于根土复合体自身特性对植物根系特征以及结构的直接影响,从而通过三者作用使根系与土体的力学关系处于动态平衡之中;(3)根土复合体复合力学模型研究较土力和水力学模型略少,土力学和水力学模型都是根据量化参数,通过参数间对比来衡量固土效果,但复合力学特性同时涉及土力特性和水力特性,考虑全面,应是未来研究的重点方向。[结论] 未来在开展冻融循环、干湿交替、干热循环对不同地区根系与土体相互作用的影响、多类型植物混种抗剪强度、化学作用和微生物作用对土水特性影响机理及复合模型构建等方面有待深入研究。研究结果可为生态脆弱区植被恢复、水土保持和可持续发展等提供重要的理论价值和工程借鉴意义。     

四川省能源和粮食生产用水竞争及与经济关系研究

水-能源-粮食系统存在相互依存、相互制约的复杂关系,量化该系统的相互关系,对于合理配置和协调利用水、能源和粮食资源,促进区域社会经济的可持续发展具有重要意义。本文运用能源和粮食生产水足迹模型、竞争指数模型、脱钩理论及锡尔指数等方法,核算分析了四川省2009—2019年能源和粮食生产水足迹及其时空变化特征,以及能源和粮食生产对水的竞争关系,探究了水足迹和地区生产总值的内在联系。研究结果表明：能源和粮食生产水足迹均呈上升趋势。能源生产蓝水足迹年均值为5.45×109m3,占能源生产水足迹的93.76%;粮食生产蓝水足迹年均值为1.06×1010m3,占粮食生产水足迹的26.02%,绿水足迹的贡献率超过蓝水足迹和灰水足迹之和,占比53.06%。四川省能源和粮食生产对水的竞争指数呈上升趋势,增幅为42.69%。由于川西北生态示范区和攀西经济区等地区兼顾粮食和水电生产,其竞争指数明显高于其他地区。四川省水足迹和地区生产总值的脱钩关系为弱脱钩,处于相对协调发展状态,说明经济发展对用水量的控制状态相对较好;水足迹强度总差异呈扩张趋势,说明四川省用水效率不均衡性逐渐扩大,但研究后期有所下降,发展逐渐趋同。该研究可为四川省水资源调配、能源开发和粮食生产提供建议,也可为该地区优化经济发展方式以及用水效率的协同发展提供参考。     

技能大赛引领师生共建优质教学资源路径探析

为了提高职业院校教学质量,培养满足职业岗位需求的复合应用型人才,开发和建设符合现代教学需求的优质数字化教学资源是必不可少的环节。职业技能大赛是集中展现职业院校学生风采,充分展示职业教育改革丰硕成果的学生职业能力素养的竞赛活动。以国赛中园艺赛项内容转化为教学资源作为切入点,以师生共同设计和录制教学视频以及校企合作制作实操动画的过程为研究对象,探索技能大赛引领,师生共建优质教学资源的模式和路径,以期提升课堂教学效果、增强学生自主学习能力和信息化素质,进而推动教学改革的深入开展。     

西瓜的力学特性及其有限元分析

西瓜的力学特性对减少西瓜贮运损失和设计相关机具等具有重要意义。通过对西瓜进行压缩试验,分析不同加载方式下西瓜的受载特点,获得了西瓜瓜梗垂直和水平受压的弹性模量分别为9.45×105Pa和8.50×105Pa。运用有限元法建立西瓜的压缩力学模型,比较西瓜水平受压的试验值和仿真值,二者最大差异是10%;研究西瓜瓜梗垂直和水平受压的承载特性,验证了仿真数值解的可行性。结果表明:在相同压力作用下,西瓜瓜梗垂直放置时,其受压的应力和应变都小于水平受压的情况。研究结果可为西瓜的贮藏、运输和加工等提供了理论依据。     

水稻谷粒与脱粒元件碰撞过程的接触力学分析

针对中国水稻在机械化收获过程中稻谷损伤严重,脱粒装置的设计仍以经验为主等现状,从碰撞的角度,建立了谷粒与脱粒元件接触过程的位移量和最大压力分布方程,并以钉齿脱粒滚筒为例,求得了稻谷产生应力裂纹或破碎时稻谷与脱粒元件碰撞的临界相对速度,室内台架试验验证了理论分析的正确性,为深入研究水稻谷粒与脱粒元件的相互作用、稻谷的脱粒损伤机理以及脱粒装置的设计提供了理论依据。     

北京市顺义区土地资源竞争与土地利用变化分析

社会对有限土地资源的多样需求导致土地利用竞争的发生,其实质是一种利益竞争,结果是土地利用类型的转换。该文在界定利益相关者和土地利用竞争概念的基础上,结合土地利用变化的热点区域——北京市顺义区,诊断确定了该区土地利用的利益相关者及其土地资源利用目标,系统分析了多种土地资源利用方式对土地资源的竞争及其导所致的土地利用变化。由于对土地利用经济效益的追求,顺义区耕地不断转换为建设用地和园地,其中水田的转移速率大于旱地;农业系统内部经济效益的差异使得粮食作物的种植面积逐渐减少,而经济作物的种植面积扩大。另外,随着人们对居住环境的要求逐步提高,顺义区生态用地的扩大也侵占了耕地。