全球植被碳储量的时空格局动态   总被引：1，自引：0，他引：1  

孙晓芳  岳天祥  范泽孟  张昊 《资源科学》2013,35(4):782-791

植被碳储量对生态系统碳平衡具有重要调节作用,对植被碳储量进行模拟并分析其对气候变化的响应是全球陆地生态系统碳循环和气候变化研究的重要内容.基于全球植被动态模型模拟的全球植被碳储量,分析植被碳储量在1901年-2000年间的时空变化趋势,及其与气温和降水的时空关系.将LPJ-DGVM (Lund-Potsdam-Jena Dynamic Global Vegetation Model)模型对植被碳储量的模拟结果与国际上被广泛接受的其他研究结果进行了对比后得出,该模型对植被碳储量具有较好的模拟能力.对植被碳储量的模拟结果表明:全球过去100年的植被碳储量在整体上呈增加趋势,增加率为0.0016kgC/(m2·a),通过分段线性回归方法得出植被碳储量时间变化趋势存在显著的转折点,20世纪50年代初以前,植被碳储量具有下降趋势,线性斜率为-0.0014kgC/(m2·a),之后植被碳储量呈现显著的增加趋势,增加率为0.0055kgC/(m2·a),是整体增加率的3倍多,因此整体增加率显著低估了近半个世纪以来植被碳储量的实际增加率.在空间栅格尺度上的分析表明,植被碳储量对气候变化的响应具有很大的区域差异,在不同地区,温度和降水的变化对植被碳储量的变化趋势贡献不同,并与各地区植物生长所受的环境条件约束的状况有关.研究结论进一步说明LPJ能够较好的模拟植物生长与气候之间的相关关系、揭示植被碳储量变化规律和特征,另外为了更好地研究气候变化对植被碳储量的影响,下一步需要对LPJ进行改进,增强其对人类活动如土地利用变化和农业灌溉行为等的模拟能力.  相似文献   

基于最优多属性约束的IPV6动态路由协议优化     

王军 《科技通报》2013,(10)

提出一种基于最优多属性约束的IPV6动态路由协议优化方法。根据IPV6动态路由协议中数据属性的约束条件,建立最优多属性约束模型,用来描述不同属性约束条件需要符合的要求。详细描述了IPV6动态路由协议的设计方式,从而实现大范围网络通信。实验结果表明,利用这种方法进行网络通信,能够避免传统算法的缺陷,提高了通信效率,取得了令人满意的结果。  相似文献   

Sprint mechanical differences at maximal running speed: Effects of performance level     

Giorgios P Paradisis  Athanassios Bissas  Panagiotis Pappas  Elias Zacharogiannis  Apostolos Theodorou  Olivier Girard 《Journal of sports sciences》2013,31(17):2026-2036

ABSTRACT

As the effect of performance level on sprinting mechanics has not been fully studied, we examined mechanical differences at maximal running speed (MRS) over a straight-line 35 m sprint amongst sprinters of different performance levels. Fifty male track and field sprinters, divided in Slow, Medium and Fast groups (MRS: 7.67 ± 0.27 m?s^?1, 8.44 ± 0.22 m?s^?1, and 9.37 ± 0.41 m?s^?1, respectively) were tested. A high-speed camera (250 Hz) recorded a full stride in the sagittal plane at 30–35 m. MRS was higher (p < 0.05) in Fast vs. Medium (+11.0%) and Slow (+22.1%) as well as in Medium vs. Slow (+10.0%). Twelve, eight and seven out of 21 variables significantly distinguished Fast from Slow, Fast from Medium and Medium from Slow sprinters, respectively. Propulsive phase was signi?cantly shorter in Fast vs. Medium (?17.5%) and Slow (?29.4%) as well as in Medium vs. Slow (?14.4%). Fast sprinters had significantly higher vertical and leg stiffness values than Medium (+44.1% and +18.1%, respectively) and Slow (+25.4% and +22.0%, respectively). MRS at 30–35 m increased with performance level during a 35-m sprint and was achieved through shorter contact time, longer step length, faster step rate, and higher vertical and leg stiffness.  相似文献   

Perceiving the affordance of string tension for power strokes in badminton: Expertise allows effective use of all string tensions     

Qin Zhu 《Journal of sports sciences》2013,31(11):1187-1196

Abstract

Affordances mean opportunities for action. These affordances are important for sports performance and relevant to the abilities developed by skilled athletes. In racquet sports such as badminton, different players prefer widely different string tension because it is believed to provide opportunities for effective strokes. The current study examined whether badminton players can perceive the affordance of string tension for power strokes and whether the perception of affordance itself changed as a function of skill level. The results showed that string tension constrained the striking performance of both novice and recreational players, but not of expert players. When perceptual capability was assessed, perceptual mode did not affect perception of the optimal string tension. Skilled players successfully perceived the affordance of string tension, but only experts were concerned about saving energy. Our findings demonstrated that perception of the affordance of string tension in badminton was determined by action abilities. Furthermore, experts could adjust the action to maintain a superior level of performance based on the perception of affordance.  相似文献   

Resource constraints as triggers of radical innovation: Longitudinal evidence from the manufacturing sector     

Marcus Matthias Keupp  Oliver Gassmann 《Research Policy》2013

Despite the significant role of radical innovation as a driver of firm growth and performance, the consequences of resource constraints for radical innovation outcomes remain unknown. Our paper addresses this gap. We combine arguments from entrepreneurship theory and the theory of recombinative innovation to construct an overarching theoretical framework that argues why resource constraints can promote, rather than impede, radical innovation. We then build hypotheses on two specific resource constraints, knowledge and financial, and test these by a lagged-variable random-effects Tobit model with longitudinal data from an exceptionally large and detailed innovation survey. Controlling for absorptive capacity, firm age, and firm growth, we find full support for the hypothesis that knowledge constraints spur radical innovation and partial support for the hypothesis that financial constraints spur radical innovation. We discuss the theoretical significance of these findings and point to managerial implications and paths for future research.  相似文献   

企业家人口统计特征对中小企业融资约束的影响机制研究          下载免费PDF全文

陈晓红  高阳洁 《科研管理》2013,34(12):110-119

基于高层梯队理论,以2010年我国530家中小上市企业为样本,采用层级回归方法,分析企业家人口统计特征对中小企业融资约束的影响及其作用机制。研究结果表明,企业家不同的人口统计特征对中小企业融资约束产生不同的影响,具体来说,企业家受教育程度和职称对缓解中小企业融资约束产生积极影响,企业家年龄和任期对中小企业融资约束产生显著的U型影响,而女性企业家对缓解中小企业融资约束产生消极影响。研究结果不仅丰富了企业家人口统计特征与中小企业融资约束之间的相关理论,并为缓解我国中小企业融资约束提供了实践支持。  相似文献   

Is movement variability important for sports biomechanists?     

Roger Bartlett  Jon Wheat  Matthew Robins 《Sports biomechanics / International Society of Biomechanics in Sports》2013,12(2):224-243

This paper overviews the importance for sports biomechanics of movement variability, which has been studied for some time by cognitive and ecological motor skills specialists but, until quite recently, had somewhat been overlooked by sports biomechanists. The paper considers biomechanics research reporting inter- and intra-individual movement variability in javelin and discus throwing, basketball shooting, and locomotion. The overview does not claim to be comprehensive and we exclude such issues as the theoretical background to movement and coordination variability and their measurement. We overview evidence, both theoretical and empirical, of inter-individual movement variability in seeking to achieve the same task goal, in contrast to the concept of “optimal” movement patterns. Furthermore, even elite athletes cannot reproduce identical movement patterns after many years of training, contradicting the ideas of motor invariance and “representative” trials. We contend that movement variability, far from being solely due to neuromuscular system or measurement “noise” – as sports biomechanists may have previously supposed – is, or could be, functional. Such functionality could allow environmental adaptations, reduce injury risk, and facilitate changes in coordination patterns. We conclude by recommending that sports biomechanists should focus more of their research on movement variability and on important related topics, such as control and coordination of movement, and implications for practice and skill learning.  相似文献   

Analyzing Mathematical Teaching-Learning Situations — the Interplay of Communicational and Epistemological Constraints     

Heinz Steinbring 《Educational Studies in Mathematics》2005,59(1-3):313-324

This is a commentary paper in the volume on “Teachings situations as object of research: empirical studies within theoretical perspectives”. An essential object of mathematics education research is the analysis of interactive teaching and learning processes in which mathematical knowledge is mediated and communicated. Such a research perspective on processes of mathematical interaction has to take care of the difficult relationship between mathematics education theory and everyday mathematics teaching practice. In this regard, the paper tries to relate the development in mathematics education research within the theory of didactical situations to developments in interaction theory and in the epistemological analysis of mathematical communication.  相似文献   

线性不等式约束优化问题的强信赖域算法     

聂涛  王廷春 《合肥联合大学学报》2006,16(3):1-4,17

对一类带有非负边界约束的线性不等式约束优化问题进行了研究，提出了一种新的信赖域算法．该算法在内点法的基础上，把非负边界约束从一般的不等式约束中分离出来，化为信赖域约束的一部分，得到一个简单易解的子问题．在一定的条件下证明了该算法具有强收敛性，并给出了数值结果．  相似文献   

Multimedia learning environments: Issues of learner control and navigation   总被引：1，自引：0，他引：1  

LAWLESS  KIMBERLY A.  BROWN  SCOTT W. 《Instructional Science》1997,25(2):117-131

Using schema theory as a framework, we view learning as an active, constructive process. It is affected not only by learners' internal knowledge structures, but by the external constraints of the learning environment as well (Kozma, 1991). This article examines how different internal learner characteristics (e.g., prior knowledge, self efficacy and interest) and different external constraints (e.g., learner control, instructional design and level of control) influence the learning process. Specifically, we address learning from a variety of multimedia environments such as video, hypertexts, kiosks and other hypermedia within a schema theoretic approach that incorporates a constructivist view.  相似文献