1,721,022 research outputs found
Partitioning direct and indirect effects reveals the response of water-limited ecosystems to elevated CO2
No full textIncreasing concentrations of atmospheric carbon dioxide are expected to affect carbon assimilation and evapotranspiration (ET), ultimately driving changes in plant growth, hydrology and the global carbon balance. Direct leaf biochemical effects have been widely investigated, while indirect effects, although documented, elude explicit quantification in experiments. Here, we used a mechanistic model to investigate the relative contributions of direct (through carbon assimilation) and indirect (via soil moisture savings due to stomatal closure, and changes in leaf area index, LAI) effects of elevated CO2 across a variety of ecosystems. We specifically determined which ecosystems and climatic conditions maximise the indirect effects of elevated CO2. The simulations suggest that the indirect effects of elevated CO2 on net primary productivity are large and variable, ranging from less than 10% to more than 100% of the size of direct effects. For ET, indirect effects were on average 65% of the size of direct effects. Indirect effects tended to be considerably larger in water-limited ecosystems. As a consequence, the total CO2 effect had a significant, inverse relationship with the wetness index and was directly related to vapor pressure deficit. These results have major implications for our understanding of the CO2-response of ecosystems and for global projections of CO2 fertilization because, while direct effects are typically understood and easily reproducible in models, simulations of indirect effects are far more challenging and difficult to constrain. Our findings also provide an explanation for the discrepancies between experiments in the total CO2 effect on net primary productivity
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
The Effects of Seabirds, Rats, and Ecosystem Restoration on Invertebrate Food Webs
No full textBurrowing seabirds that nest on islands transfer nutrients from the sea, disturb the soil through burrowing, damage tree foliage when landing, and thereby modify the surface litter. One of the greatest effects seabirds may have on their recipient ecosystems may be via the nutrient subsidies they transfer onto islands from the sea. How these nutrients effect their recipient ecosystems however, depends on many factors such as water availability. However, seabirds are in decline worldwide, as are their community- and ecosystem-level impacts, primarily due to invasive predatory mammals. Seabird islands are vulnerable to the invasion of predatory mammals such as rats, which can have lasting effects even after these pests are eradicated. Once these islands are restored and seabirds start to return the ecosystems can recover quickly, returning to a pre-disturbance state within as little as 20 years. However, legacy effects of the invasive mammals may occur meaning ecosystems may revert to alternate stable states. The direct and indirect effects of seabirds, their decline and recolonisation on ecosystems are inherently complex. I employed network analysis of invertebrate food webs, as a means of simplifying ecological complexity, to better understand the effects seabirds, their loss, and recolonization, may have on island invertebrate communities. I found that on rat-invaded islands the invertebrate food webs were smaller and less complex than on their seabird-dominated counterparts, likely due to the suppression of seabird derived nutrients and consequent effects on trophic cascades. There was also an interplay between nutrient subsidies and water availability, where invertebrate food webs were larger and more complex as litter water increased and soil C: N slightly decreased. When comparing a restored island to invaded islands and those never invaded I found that the restored island supported some areas that were virtually indistinguishable from an invaded island and it demonstrated strong environmental gradients indicative of a recovering island. Finally when comparing the family richness and missing common families between islands I found that the restored islands had a similar number of missing families to invaded islands and were missing more family groups than islands that had never been invaded when controlling for covariates. Seabird and rat effects on island ecosystems are manifested throughout entire food webs. As seabirds spread across restored islands the areas similar to invaded islands will become fewer as the island starts to fully resemble a burrowing seabird island ecosystem. A key finding was the resilience of the invertebrate food webs, which shrunk to a fraction of their full potential complexity during arid periods then reconstructed themselves with increased water availability. However, the invertebrate food webs were unable to reconstruct fully on restored islands due to the legacy effects of invasive mammal suppression. This may have had negative effects on the nutrient cycling of at least one of the restored islands. I finally conclude that more effort is needed to understand and integrate invertebrate communities into ecosystem restoration in the future
The Role of Honeybee Pollination in Native New Zealand Plants
No full textHoneybees (Apis mellifera) fill a keystone role in the pollination of many plant species. Studies have found that honeybees visit native New Zealand plants, however, with exception to Metrosideros excelsa, little data on honeybee pollination has been collected. Therefore, the aim of my study was to gain an understanding of the role of honeybee pollination in native New Zealand plants which may aid conservation efforts. This was done by measuring components of pollinator performance, which encompassed flower visitation, visitor volume and pollen collection. Flower visitation was counted and timed from recordings. Visitor volume was measured live. Honeybee samples were collected whereby the number of pollen grains and pollen species was determined via acetolysis. My results showed that honeybees are likely good pollinators of many native plants. Honeybees often visited an ample number of flowers and consistently spent more time foraging than not. Furthermore, honeybees usually collected a large number of pollen grains in which most were conspecific. My study found that small plants with grouped flowers are likely to benefit the most from honeybee pollination. In contrast, plants with miniscule flowers, plants in cold, windy environments and the threatened Muehlenbeckia astonii are unlikely to benefit from honeybee pollination. My research fills part of the knowledge gap of honeybee pollination in native New Zealand plants which has implications in conservation. Research on the remainder of the honeybee pollinator performance components and cross-pollination by honeybees, particularly in natural environments, is needed. In addition, the pollinator performance of other pollinators needs to be investigated
Nor any drop to drink: Water relations of the mangrove avicennia marina (Forssk.) vierh
No full textEssential players within the planet’s water cycle, plants are themselves sensitive to ongoing global change. Future shifts in water availability are predicted to change the face of the earth’s forests. In the last decades there have been many advances in our understanding of water relations of plants and their response to environmental conditions. However, most of these studies are based on relatively few North-American and European tree species, with a notable underrepresentation of southern-hemisphere or tropical tree species. This kind of knowledge is needed, not only to reach a representative understanding of eco-physiological diversity of plant function, but also to establish the full spectrum of forest responses to changing environmental conditions.
In my project, I study the water relations of the grey mangrove, Avicennia marina (Forssk.) Vierh., in the temperate mangrove forests of New Zealand. Mangroves are enigmatic species, which live in the harsh conditions imposed by brackish water and periodically flooded soils. Although they have elicited much scientific curiosity throughout history, much of our knowledge of mangrove water-related physiology is based on manipulative experiments with seedlings. Over the course of two years, I employed a diverse array of ecophysiological techniques to closely monitor different aspects of adult A. marina water relations in the stem and crown.
When investigating the seasonal and monthly courses of water-induced stem radial changes with a variety of stem cycle analysis techniques, I found that these were highly heterogeneous even within the same individual. Despite this heterogeneity however, it was still possible to correlate the amplitude of stem radius changes with environmental conditions. Presence or absence of precipitation events was an important driver of stem swelling periods, whilst atmospheric water availability indicators, such as VPD explained stem shrinkage periods. I found that Avicennia marina shows unusual daytime refilling of stem elastic water storage tissues, which deviates from most terrestrial trees and is hypothesized to be due to endogenous osmotic adjustment. This theory is reinforced by my finding that light sum, a proxy for photosynthesis, is a prominent driver of stem swelling amplitudes. In order to understand the “peristaltic” depletion of internally stored water within the tree stem, I studied the seasonal changes in the timing and time-lags of peak stem swelling at different tiers. I discovered an annual switch in the direction of the peristaltic water depletion wave along the stem, potentially related to seasonal changes crown photosynthesis and influence of non-structural carbohydrate dynamics on stem radial change caused by the onset of the growing season. Daytime stem swelling was studied further by performing measurements directly on xylem tissue, revealing the inner-bark turgor driven signal behind whole-stem daytime swelling. However, due to A. marina’s unique wood structure, consisting of multiple phloem-xylem layers, the use of this methodology yielded heterogeneous and highly variable results. Lastly, I found that leaf turgor pressure-probes proved a reliable source of information on leaf turgor dynamics. The study of leaf turgor confirmed the importance of fresh water inputs for leaf hydration. My results also suggested that osmoregulation behind daytime stem swelling momentarily overrides the water demands of transpiring leaves, causing delayed recovery of leaf turgor in the evenings. The time-lags between stem water storage mobilization and leaf turgor recovery followed changes in atmospheric water demand, and also showed differences in the sensitivity of the upper and lower stem to leaf water demands. My work is a significant contribution not only to the understanding of mangrove ecophysiology in particular, but also adds to our increasingly complex map of plant physiological diversity which goes far beyond the established paradigms of plant-water relations.
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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