1,720,958 research outputs found
Climate Warming-Related Growth Decline Affects Fagus sylvatica, But Not Other Broad-Leaved Tree Species in Central European Mixed Forests
No full textClimate warming is predicted to extend the duration and enhance the severity of summer droughts in Central Europe, which may pose a serious risk to forest productivity and forest health. Fagus sylvatica (European beech), the most abundant tree species of Central Europe’s natural forest vegetation and one of the key species in forestry, is thought to be particularly vulnerable to drought. Here, we present a dendrochronological analysis in three mixed temperate broad-leaved forests along a precipitation gradient with the aim of comparing the climatic response of radial growth of F. sylvatica with the performance of four co-existing species (Acer pseudoplatanus, A. platanoides, Quercus petraea, Fraxinus excelsior). We hypothesized that Fagus is the most drought sensitive of the five species, which implies that it could lose its competitive advantage at drier sites in the course of climate warming. In support of this hypothesis, we found that F. sylvatica in all stands exhibited an increase in the number of negative pointer years and a decrease in radial increment in the driest stand since about 1980, in parallel to increasing summer temperatures and drought intensity. Such a response was missing in the other four species and may point to shifts in the competitive hierarchy in these mixed forests under a future warmer climate. We conclude that Central Europe’s forestry sector should consider carefully the risk of failure of beech in regions with relatively low and decreasing summer precipitation
Rapid recovery of stem increment in Norway spruce at reduced SO2 levels in the Harz Mountains, Germany
No full textTree-ring width of Picea abies was studied along an altitudinal gradient in the Harz Mountains, Germany, in an area heavily affected by SO2-related forest decline in the second half of the 20th century. Spruce trees of exposed high-elevation forests had earlier been shown to have reduced radial growth at high atmospheric SO2 levels. After the recent reduction of the SO2 load due to clean air acts, we tested the hypothesis that stem growth recovered rapidly from the SO2 impact. Our results from two formerly damaged high-elevation spruce stands support this hypothesis suggesting that the former SO2-related spruce decline was primarily due to foliar damage and not to soil acidification, as the deacidification of the (still acidic) soil would cause a slow growth response. Increasing temperatures and deposited N accumulated in the topsoil are likely additional growth-promoting factors of spruce at high elevations after the shortfall of SO2 pollution. (C) 2012 Elsevier Ltd. All rights reserved
60-year record of stem xylem anatomy and related hydraulic modification under increased summer drought in ring- and diffuse-porous temperate broad-leaved tree species
No full textThe vascular architecture plays a crucial role in the productivity and drought tolerance of broadleaf trees, but it is not yet fully understood how the hydraulic system is acclimating to a warmer and drier climate. Because vessel features may record temporal and spatial variability in climatic signals of the past better than tree-ring width, we combined dendrochronological time-series analysis with the calculation of stem hydraulic properties derived from radial vessel features. We aimed to reconstruct the development and sensitivity of the hydraulic system over six decades and to identify climatic control of xylem anatomy for five co-existing broad-leaved diffuse- and ring-porous tree species (genera Acer, Fagus, Fraxinus and Quercus) across three sites covering a precipitation gradient from 548 to 793 mm. We observed a significant influence of the climatic water balance (CWB) on the vessel features of all species, but the time lag, magnitude and direction of the response was highly species-specific. All diffuse-porous species suffered a decline in vessel diameter in dry years, and increase in vessel density in dry years and the year following. However, F. sylvatica was the only species with a significant long-term change in anatomical traits and a significant reduction in potential hydraulic conductivity (Kp) after dry winters and in dry summers, accompanied with the largest long-term decline in tree-ring width and the largest growth reduction in and after years with a more negative CWB. In contrast, the comparison across the precipitation gradient did not reveal any significant vessel-climate relationships. Our results revealed considerable plasticity in the hydraulic system especially of F. sylvatica, but also evidence of the drought-sensitivity of this species in accordance with earlier dendroecological and physiological studies. We conclude that the long-term reconstruction of hydraulic properties can add substantially to the understanding of the acclimation potential of different tree species to climate change
Auxin is a long-range signal that acts independently of ethylene signaling on leaf abscission in Populus
No full textTiming of leaf abscission is an important trait for biomass production and seasonal acclimation in deciduous trees. The signaling leading to organ separation, from the external cue (decreasing photoperiod) to ethylene-regulated hydrolysis of the middle lamellae in the abscission zone, is only poorly understood. Data from annual species indicate that the formation of an auxin gradient spanning the abscission zone regulates the timing of abscission. We established an experimental system in Populus to induce leaf shedding synchronously under controlled greenhouse conditions in order to test the function of auxin in leaf abscission. Here, we show that exogenous auxin delayed abscission of dark-induced leaves over short and long distances and that a new auxin response maximum preceded the formation of an abscission zone. Several auxin transporters were down-regulated during abscission and inhibition of polar auxin transport delayed leaf shedding. Ethylene signaling was not involved in the regulation of these auxin transporters and in the formation of an abscission zone, but was required for the expression of hydrolytic enzymes associated with cell separation. Since exogenous auxin delayed abscission in absence of ethylene signaling auxin likely acts independently of ethylene signaling on cell separation
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Full text linkWe 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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