1,721,111 research outputs found
Weekly carbon and oxygen isotope dynamics in black spruce: A case study in the northeastern boreal forest of Quebec, Canada
Full text linkThe stable isotopic composition of carbon (δ13C) and oxygen (δ18O) in tree rings is widely used to explore tree eco-physiological dynamics across various time scales. However, interpreting these isotopic signals is challenging due to multiple interacting factors, including gas exchange at the leaf level, stored carbohydrate reserves, and xylem water, whose timing and interactions during the growing season remain poorly understood. In this study, weekly δ13C and δ18O signals were tracked within the cambial region and forming xylem of black spruce (Picea mariana (Mill.) BSP.) in boreal forests of Quebec, Canada. The study covered three consecutive growing seasons (2019–2021) at two forest sites with differing temperature and soil water content. Weekly isotopic profiles were developed for the cambial region (δ13Ccam and δ18Ocam) and developing xylem cellulose (δ13Cxc and δ18Oxc). Strong positive correlations were observed between δ13Ccam and δ18Ocam, with an increasing trend along the growing season. Conversely, negative relationships were observed between δ13Cxc and δ18Oxc, characterized by an increasing trend in δ13Cxc and a decreasing trend in δ18Oxc. The results illustrated that stomatal conductance is the dominant physiological factor controlling seasonal fractionation of δ13Ccam and δ18Ocam. Increasing proportional exchanges between xylem water and sugars at the sites of cellulose synthesis (i.e., Pex effect) are thought to be strong enough to completely blur the observed trends in δ18Ocam during the growing season. This suggests that δ18Oxc signals differ from those originating in the earlier cambium sink. These findings highlight the need to carefully consider the processes influencing isotopic signals to avoid misinterpretations in dendroclimatological studies
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
Role of mixed-species stands in attenuating the vulnerability of boreal forests to climate change and insect epidemics
Full text linkWe investigated whether stand species mixture can attenuate the vulnerability of eastern Canada’s boreal forests to climate change and insect epidemics. For this, we focused on two dominant boreal species, black spruce [Picea mariana (Mill.) BSP] and trembling aspen (Populus tremuloides Michx.), in stands dominated by black spruce or trembling aspen (“pure stands”), and mixed stands (M) composed of both species within a 36 km2 study area in the Nord-du-Québec region. For each species in each stand composition type, we tested climate-growth relations and assessed the impacts on growth by recorded insect epidemics of a black spruce defoliator, the spruce budworm (SBW) [Choristoneura fumiferana (Clem.)], and a trembling aspen defoliator, the forest tent caterpillar (FTC; Malacosoma disstria Hübn.). We implemented linear models in a Bayesian framework to explain baseline and long-term trends in tree growth for each species according to stand composition type and to differentiate the influences of climate and insect epidemics on tree growth. Overall, we found climate vulnerability was lower for black spruce in mixed stands than in pure stands, while trembling aspen was less sensitive to climate than spruce, and aspen did not present differences in responses based on stand mixture. We did not find any reduction of vulnerability for mixed stands to insect epidemics in the host species, but the non-host species in mixed stands could respond positively to epidemics affecting the host species, thus contributing to stabilize ecosystem-scale growth over time. Our findings partially support boreal forest management strategies including stand species mixture to foster forests that are resilient to climate change and insect epidemics
Greater tree species diversity and lower intraspecific competition attenuate impacts from temperature increases and insect epidemics in boreal forests of western Quebec, Canada
Full text linkWe investigated how the surrounding environment influences the growth of dominant trees and their responses to temperature and insect epidemics in boreal forests of eastern Canada. We focused on 82 black spruce and jack pine focal trees in stands spanning a double gradient of species diversity and soil texture within a 36 km2 area of western Quebec. For these trees, we compared their diameter at breast height, growth rates, temperature–growth relations, and growth during insect defoliator epidemics. We used linear models to study how surrounding tree attributes and soil properties affected the growth of focal trees. Models showed that tree growth responses and responses to temperature and insect epidemics were generally negative with higher intraspecific competition and positive with greater tree species diversity. Growth of both species benefitted from lower soil sand content. Our research offers novel insights on the potential role of the surrounding environment, notably tree competition and species diversity, in mitigating the vulnerability of eastern Canada's boreal trees to anthropogenic climate change and insect epidemics
Large-scale estimation of xylem phenology in black spruce through remote sensing
No full textThere is a growing need for understanding the timing of wood formation in relation to the dynamics of bud phenology at wide geographical scale. This study analysed the relationships between long-term chronologies of xylem growth and the timing of plant phenology detected by Normalized Difference Vegetation Index (NDVI) in five permanent stands across the latitudinal distribution of black spruce [Picea mariana (Mill.) BPS] in the boreal forest of Quebec, Canada. Xylogenesis was studied weekly from April to October for thirteen years (2002–2014) on anatomical sections derived by wood microcores. The timing of the growing season detected by remote sensing was extracted from MODerate resolution Imaging Spectroradiometer (MODIS) 250 m 16-days NDVI data. The NDVI time-series were fitted using a double-logistic curve. Phenological chronologies from remote sensing and xylem phenology showed a latitudinal trend. The models correlating the data inferred from satellite sensors and the spring observations of xylem phenology were significant (p < 0.0001). The length of NDVI growing season and the duration of xylogenesis showed a close correlation. This study demonstrated that the timing of xylem formation could be suitably estimated at wide geographical scale using remote sensing data. However, the inter-annual phenological variability remained unexplained, which might limit the application of the models only to the years considered by our chronologies
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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