Reproductive height determines the loss of clonal grasses with nitrogen enrichment in a temperate grassland

Tall clonal grasses commonly display competitive advantages with nitrogen(N)enrichment.However,it is currently unknown whether the height is derived from the vegetative or reproductive module.Moreover,it is unclear whether the height of the vegetative or reproductive system regulates the probability of extinction and colonization,and determines species diversity.In this study,the impacts on clonal grasses were studied in a field experiment employing two frequencies(twice a year vs.monthly)crossing with nine N addition rates in a temperate grassland,China.We found that the N addition decreased species frequency and increased extinction probability,but did not change the species colonization probability.A low frequency of N addition decreased species frequency and colonization probability,but increased extinction probability.Moreover,we found that species reproductive height was the best index to predict the extinction probability of clonal grasses in N-enriched conditions.The low frequency of N addition may overestimate the negative effect from N deposition on clonal grass diversity,suggesting that a higher frequency of N addition is more suitable in assessing the ecological effects of N deposition.Overall,this study illustrates that reproductive height was associated with the clonal species extinction probability under N-enriched environment.

行政检查、行政约谈能提升平台规制的效果吗?--基于网约车平台的“大数据+准实验”研究

平台经济在我国经济社会发展全局中的地位和作用日益凸显,它为公众提供便捷、高效的同时,也蕴含着新的风险。如何防范化解风险,促进平台经济行稳致远的发展,从而更好地保护公众利益,是我国政府高度重视的问题。然而,既有研究尚未对政府如何在平台规制中发挥作用这一问题提供充分的论证与解释。围绕平台经济发展中的社会性风险这一议题,基于回应性规制理论下的威慑与遵从逻辑,研究采用“大数据+准实验”的设计思路,将网约车监管中的行政检查、行政约谈作为政策干预变量,以公众微博文本中挖掘的情感得分为结果变量,利用间断时间序列分析方法对政府规制效果进行检验。研究发现:(1)行政检查基于强制与惩罚对平台产生了威慑效果,行政约谈通过劝服与对话促成了平台的遵从行为,二者均能有效提升平台规制的效果,干预当期政策效应显著但不具有长效性;(2)行政检查、行政约谈的政策效应存在差异,规制手段强制力的高低对规制效果产生了不同程度的影响,威慑性规制的效果更为稳健。研究将全面的回应性规制框架扩展到平台规制这一新兴技术治理领域,丰富了回应性规制理论的本土化研究,为从公众评价的角度研究政策效果提供了新的思路。研究证明,行政约谈与行政检查的协同互补,能够形成更强大的政策合力,从而有利于风险的防范与治理,这为平台监管机制的完善提供了一定的政策启示。

温带半干旱草原克隆禾草叶性状对氮沉降铵硝比输入的响应:叶序不可忽略

叶片是植物光合作用的主要器官。叶片可塑性在很大程度上决定了植物对富氮环境的适应。然而在自然条件和富氮环境中,测定植物叶片性状时需要采集的最佳数量或比例是否一致并不明确;同时大气氮沉降主要组分铵态氮与硝态氮比例变化是否会改变叶片最佳测量数量缺少实验证据支撑。因此本研究依托自2014年在中国北方温带草原开展的铵硝比添加实验(即对照和5:1、1:1和1:5的NH_(4)^(+)-N/NO_(3)^(−)-N比例),于2020年8月(植物群落生物量高峰时期)测量了两个形态相似的优势多年生禾本科物种(即羊草和冰草)在叶序水平的叶面积、叶厚度和叶绿素含量。我们发现在对照处理(自然条件),除羊草叶面积需要测定5片(占单株总叶片数的78.82%),其它性状通过测量两片完全展开叶即可代表整株该叶性状的单叶平均特征。铵硝比添加处理显著改变了叶性状最大值所在叶序,导致需要增加叶片测定数才能代表整株叶片的单叶平均特征。铵硝比添加显著改变了受叶序调控的叶性状间权衡关系,即叶片虽变大却变薄。因此,本研究结果表明,通过测量全部完全展开叶或者提供一个合适的尺度推绎参数方能更好地评估氮沉降效应下植株叶片性状的响应特征。

Autumn nitrogen enrichment destabilizes ecosystem biomass production in a semiarid grassland

Nitrogen(N)deposition decreases the temporal stability of ecosystem aboveground biomass production(ecosystem stability).However,little is known about how the responses of ecosystem stability differ based on seasonal N enrichment.By adding N in autumn,winter,or growing season,from October 2014 to May 2020,in a temperate grassland in northern China,we found that only N addition in autumn resulted in a significantly positive correlation between ecosystem mean aboveground net primary productivity(ANPP)and its standard deviation and significantly reduced ecosystem stability.Autumn N-induced reduction in ecosystem stability was associated with the vanished negative effect of community-wide species asynchrony(asynchronous dynamics among populations to environmental perturbations)on the standard deviation of ecosystem ANPP in combination with the emerged positive effect of dominance(Simpson's dominance index that indicates the relative weight of dominant species in a community).Our findings indicate that autumn N addition might overestimate the negative effect of annual atmospheric N deposition on ecosystem stability,suggesting that to better evaluate the influence of N deposition in temperate grasslands,both field experiments and global modeling should consider not only the annual N load but also its seasonal dynamics.Moreover,further studies should pay more attention to the alteration in the ecosystem temporal deviations,which might be more sensitive to human-induced environmental changes.

The Dynamic Behaviors of a Shape Memory Polymer Membrane

As a kind of popular smart materials, shape memory polymers （SMPs） have a great potential for applications in deployable aerospace structures and other engineering struc- tures. However, the vibration analysis of shape memory polymer structures, which would play an important role in engineering, has not gained much attention. In this study, we propose a dynamic model and establish the governing equations for characterizing the dynamic behavior of a shape memory polymer membrane subjected to time-dependent forces. The derivation of governing equations is based on a well-developed constitutive model of SMPs combined with the Euler-Lagrange equation. With the proposed model, two different loading cases are stud- ied： the equal-biaxial sinusoidal force and the uniaxial sinusoidal force. To analyze the dynamic response of a shape memory polymer membrane and find some effective ways to control vibra- tion, the isothermal amplitude-frequency response, the time-dependent behavior of vibration and the vibration in a variable temperature process are investigated in the numerical simulation. It is observed that temperature, mechanical force and heating rate have significant effects on the dynamic performances of a shape memory polymer membrane. We also investigate the shape memory behavior of SMP membrane involving the dynamic response. The influence of dynamics on shape fixation and shape recovery is discussed. These results and discussion may provide guidance for exploring the vibration and dynamic performances of shape memory polymer in deployable aerospace structures.