1,720,981 research outputs found
Comparative evaluation of the effects of gaseous pollutants, acidic deposition and mineral deficiencies on gas exchange of trees
No full textThe effects of gaseous air pollutants, acidic deposition and mineral deficiency on the gas exchange of trees are reviewed, and published data which illustrate important responses are described in detail. Data are presented which show that mixtures of SO2 plus NO2 can decrease photosynthesis in Betula pendula, that ambient pollutant concentrations (especially O3) tend to increase stomatal conductance of Populus deltoides X Populus nigra in the early part of the day for young growing leaves and throughout the day for mature leaves, and that simulated acid rain (pH 2.8) can cause temporary increases of stomatal conductance. Data for forest sites indicate that ambient concentrations of gaseous air pollution are sufficient to alter the gas exchange of woody plants (Fagus sylvatica, Populus deltoides X Populus nigra, Picea sitchensis and Picea abies). Both inhibitions and stimulations of stomatal conductance have been observed, often associated with no effect or small depressions of photosynthesis. The link between stomatal response to gaseous pollutants and growth effects remains unclear. Initial exposure to wet deposited sulphates and nitrates often leads to stomatal opening while longterm exposure results in closure. Soil-mediated effects may be cumulative and operate via effects on nutrient supply and thus mesophyll characteristics and phloem loading
Nitrate supply and the biophysics of leaf growth in salix viminalis
No full textThe influence of nitrogen on leaf area development and the biophysics of leaf growth was studied using clonal plants of the shrub willow, Salix viminalis grown with either optimal (High N) or sub-optimal (Low N) supplies of nitrate. Leaf growth rate and final leaf size were reduced in the sub-optimal treatment and the data suggest that in young rapidly growing leaves, this was primarily due to changes in cell wall properties, since cell wall extensibility (% plasticity) was reduced in the Low N plants.The biophysical regulation of leaf cell expansion also differed with nitrogen treatment as leaves aged. In the High N leaves, leaf cell turgor pressure (P) increased with age whilst in the Low N leaves P declined with age, again suggesting that for young leaves, cell wall plasticity limited expansion in the Low N plants. Measurements of cell wall properties showed that cell wall elasticity (%E) was not influenced by nitrogen treatment and remained constant regardless of leaf age
Particulate pollution capture by urban trees: effect of species and windspeed
No full textParticulate pollution is a serious health problem throughout the world, exacerbating a wide range of respiratory and vascular illnesses in urban areas. The use of trees to reduce the effects of these pollutants has been addressed in the literature, but has rarely been quantified. The aim of the present study was to quantify the effectiveness of five tree species - pine (Pinus nigra var. maritima), cypress ( × Cupressocyparis leylandii), maple (Acer campestre), whitebeam (Sorbus intermedia), poplar (Populus deltoides × trichocarpa 'Beaupré') - in capturing pollutant particles. This was achieved by exposing them to NaCl droplets of approximately 1 µm diameter at a range of windspeeds in two windtunnels. The deposition velocity (Vg) and particle trapping efficiency (Cp) were calculated from these exposures. In addition, a variable dependent on foliage structure [Stokes number (St)] was correlated with Cp to gauge the effect of tree morphology on particle capture. Maximum Cp values ranged from 2.8% for P. nigra, to 0.12% and 0.06% for P. trichocarpa×deltoides and A. campertre, respectively. The finer, more complex structure of the foliage of the two conifers (P. nigra and C. leylandii) explained their much greater effectiveness at capturing particles. The data presented here will be used to model the effectiveness of tree planting schemes in improving urban air quality by capturing pollutant particles
Changes of partitioning and increased root lengths of spruce and beech exposed to ambient pollution concentrations in southern England
No full textThe control of ozone uptake by Picea abies (L.) Karst. and P. sitchensis (Bong.) Carr. during drought and interacting effects on shoot water relations
No full textExposure to O3 alone has not yet been shown to reproduce the symptoms of the various types of spruce decline which have been identified in Europe. However, there is increasing evidence that this pollutant has physiological effects which interact with those of other environmental factors in ways which may be important in determining tree condition and growth. The effects of O3 episodes and drought on O3 uptake, gas exchange and water relations of Picea abies (L.) Karst. and P. sitchensis (Bong.) Carr. were investigated in two experiments. In the first a rapidly drying soil mixture was used, and seedlings of P. abies were exposed to short (2 h) daily episodes of O3 at 80 nl 1?1 on each day of a 5 d drought. Photosynthesis (A) and stomatal conductance (gs) were significantly decreased (P= 0.01) by water deficit and as a consequence, uptake of O3 by the plants was also significantly decreased. Exposure to O3 did not affect A or gs for this species. In the second experiment a soil mixture designed to give a slower development of water deficit was used and 1 + 1 transplants of P. sitchensis were exposed to a single O3 episode (up to 100 nl 1?1 for 3 h) after water had been withheld for 7 or 14 d. Hofler diagrams showed that mild water deficits did not affect shoot water relations. However, O3 significantly increased solute potential (?s) after 7 d of drought, an effect which was lost after 14 d of drought. Flux of O3 to the watered plants was greater than to the unwatered plants at all concentrations, the effect being more marked at higher concentrations. This effect was partly attributable to the greater stomatal conductances recorded for the well watered plants, but was also partly due to stomatal opening caused by O3, an effect which was diminished or reversed for unwatered plants
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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