1,720,977 research outputs found
Fundamental tradeoffs generating the worldwide leaf economics spectrum
Get PDFRecent work has identified a worldwide “economic” spectrum of correlated leaf traits that affects global patterns of nutrient cycling and primary productivity and that is used to calibrate vegetation–climate models. The correlation patterns are displayed by species from the arctic to the tropics and are largely independent of growth form or phylogeny. This generality suggests that unidentified fundamental constraints control the return of photosynthates on investments of nutrients and dry mass in leaves. Using novel graph theoretic methods and structural equation modeling, we show that the relationships among these variables can best be explained by assuming (1) a necessary trade-off between allocation to structural tissues versus liquid phase processes and (2) an evolutionary trade-off between leaf photosynthetic rates, construction costs, and leaf longevity.Shipley, Bill; Lechowicz, Martin J; Wright, Ian; Reich, Peter B. (2006). Fundamental tradeoffs generating the worldwide leaf economics spectrum. Retrieved from the University Digital Conservancy, 10.1890/05-1051
Comparative ecophysiology of North American spruce species
No full textAn ecophysiological comparison among species was conducted to investigate the possible factors controlling the distribution of North American spruce (Picea) species, especially with regard to the possible influence of global change. The seedlings of 8 North American spruce species were grown in a growth chamber, with half of them being given an episodic 'increased temperature and drought' stress treatment. Trait values among species, particularly growth analysis components, were compared and related to climatic variables associated with the geographical range of the species. Relative growth rate (RGR) and specific leaf area (SLA) were positively correlated with latitude, and the leaf weight ratio (LWR) variation negatively with the dryness of species' natural ranges. All these relationships hold with both messed and unstressed seedlings, even thou seedling response to the stress was significant. The SLA-latitude and LWR-dryness relationships are likely to have ecological significance, and this indicates that foliage stricture (SLA) and allocation (LWR) play important roles determining a species range along temperature and dryness gradients
Functional ecology of hardwood trees under chronic drought
No full textI compared the interrelationships among traits affecting interspecific variation in the growth of seedlings of 22 hardwood tree species subjected to chronic intermittent drought. Leaf traits were the most responsive to contrasting water regimes; canopy architecture, hydraulic conductivity and leaf phenology were less affected. Relative growth rate (RGR) under drought, however, was most closely related to variation in canopy duration: species in leaf longer had greater RGR. Secondarily, maintaining higher stomatal conductance (g) under drought further enhanced RGR. Higher g is associated with a higher photosynthetic rate (A), but the relationship between A and g varied with internal CO concentration (c), foliar nitrogen and specific leaf mass. Species with weaker homeostatic control over c had higher water use efficiency. While photosynthetic characteristics were related primarily to species' shade tolerance, water use efficiency was most closely related to habitat moisture preference
Ecology of American beech and sugar maple in an old-growth forest
No full textThe dynamics in Fagus-Acer forest have been frequently investigated, a particular interest being the replacement patterns and coexistence of the two dominant species, Fagus grandifolia Ehrh. and Acer saccharum Marsh. This thesis examines whether the community pattern and dynamics in a Fagus-Acer forest at Mont. St. Hilaire, Quebec, are consistent with predictions made by the disturbance hypothesis. This conceptual model explains the mechanisms underlying the coexistence and replacement patterns of the two dominant species. While the hypothesis suggests disturbance frequency and interspecific differences in growth under variable light conditions to be the key factors that determine the dynamics in Fagus-Acer forests, I demonstrate that edaphic factors and regeneration of Fagus by root sprouts can also play a significant role. Based on the findings, I propose an alternative model for the coexistence and replacement patterns in Fagus-Acer forest to better account for the influence of edaphic factors and of Fagus root sprouts
Ecological separation among fern species in an old-growth forest
No full textThe validity of niche-assembly rules in explaining community structure is revisited by testing for ecological separation among ferns. An intensive, fine-scale survey of fern abundance and environmental variation was done in 1-ha of old-growth forest. Three methods of detecting niche segregation (canonical correspondence analysis, detrended correspondence analysis and GIS mapping) suggested that most fern species at the site are distributed according to distinct environmental preferences. The most important gradients separating fern species are first, the amount of soil moisture and second, soil nitrate concentration. Contrary to other findings, pH had little influence on controlling fern distribution. Spatial autocorrelation, detected by partialled ordinations, obscured the presence of niche partitioning. As well, sampling grain changed the apparent location of some species on environmental gradients and their ecological similarity to other species. Finer-scaled environmental heterogeneity or dispersal-mediated processes may account for the unexplained variation in fern species abundance of this site
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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