1,721,018 research outputs found
The role of tree species richness and identity for herb layer characteristics and next generation tree species establishment in a subtropical forest in China
Full text linkIn recent years, research on Biodiversity ecosystem functioning (BEF) relationships has extended its scope towards to long-lived tree diversity experiments, with their large contribution to ecosystem functions at the global scale. The herbaceous understorey contributes an impressive part to the overall diversity in forests and plays an important role for tree regeneration as well as for ecosystem functioning in general. Vice versa, trees in the overstorey control understorey herb and shrub growth such as through competition for resources. However, despite its importance, our understanding of the interactions between the herbaceous understorey and tree layer diversity is still limited. In the context of the Biodiversity-Ecosystem Functioning project in subtropical China (BEF-China), this thesis investigates the role of tree species richness and identity for herb layer characteristics and next generation tree species establishment. This thesis combines the outcome of three different experimental approaches, including one study on the effect of tree species richness and identity on herb layer composition, richness and productivity and how these relationships across strata change with abiotic environmental conditions and competition intensity. The second study focuses on the temporal aspect and studies the effect of tree species richness and identity on understorey productivity over a period of three years. The third study examines the influence of Janzen-Connell effects on next generation tree species establishment. Results obtained in all three different approaches support light transmittance as major limiting factor at the current state of forest development. While we did encounter additional effects for tree species identities, a similar effect with variations in tree species richness could not be found yet. Moreover, we were able to demonstrate conspecific negative distance-dependent seedling establishment and intraspecific competition being already effective at this early stage of forest development. While the subsequent interplay of the different identified and prospective mechanisms remains to be tested, this thesis adds to our understanding of the current and later phases of forest development and highlights the need to account for confounding effects
Plant invasions in high-UV-B environments: Patterns, mechanisms and projections in the context of Global Change
Full text linkProgressive globalization and continuing human-mediated transports of plant material over long distances facilitated the introduction of a large number of non-native plant species all over the world. The resulting successful plant invasions might have substantial negative impacts on native natural ecosystems and cultural landscapes and, therefore, imply significant ecological and economic harms. To better predict and limit these consequences for humans and ecosystems, a
deeper understanding of mechanisms underlying plant invasions is important. Thus, one central question of invasion ecology research refers to traits and conditions that facilitate plant invasion success of species. Environmental matching of non-native species with novel habitats is known to be strongly dependent on macroclimatic conditions as they predominantly determine the species’ physiological niche. Especially temperature and precipitation have been previously
discussed as most important abiotic filters for species distribution. Radiation, however, displays a so far neglected abiotic environmental factor in the context of plant invasions, although fundamental differences occur at a global scale and plant metabolism strongly relies on light conditions. Nevertheless, biologically active high-energy UV-B radiation might even have negative effects on plant growth and development and, thus, might act as limiting environmental factor during plant invasions.
Local surface UV-B radiation intensities mainly depend on elevation, latitude, daytime and season. Beside the seasonal differences between hemispheres, higher maximum and annual mean UV-B intensities are measured in comparable latitudes of the southern hemisphere. This difference results from the elliptical Earth’s orbit around the sun leading to a smaller Sun-Earth distance during the southern hemisphere summer. Several hotspots of plant invasions are located on the southern hemisphere and consequently offer suitable conditions to investigate plant invasions in high UV-B environments. Moreover, UV-B radiation intensity is also affected by human impact and ongoing climate change that will further change local UV-B levels worldwide.
This thesis investigates the importance of high UV-B radiation for plant invasions based on 27 invasive species of New Zealand grasslands. The conducted studies contain common garden experiments comparing native and invasive origins in different UV-B environments, as well as macroecological approaches including species distribution data, trait data and global UV-B satellite data. All approaches aimed at assessing the importance of UV-B as selective force during plant invasions and tested for potential UV-B preadaptation of native population, as well as local adaptation of invasive populations as consequence of evolutionary processes in the invaded range.
The results revealed no evidence for genetic post-introduction adaptation to UV-B in high-UV-B environments. However, both origins of the study species responded to UV-B with quantifiable physiological and phenotypic changes that hint at a pronounced plasticity maintaining plant growth in response to UV-B. Moreover, higher UV-B intensities in the native range turned out to be of advantage for alien species under UV-B exposure in novel habitats and indicate an existing UV-B preadaptation of some study species by previous UV-B experience. There was only a medium directly limiting effect of UV-B radiation in comparison to other
common environmental stressors of grasslands, e.g. drought. Nevertheless, greater importance of UV-B during plant invasions might originate from the ability to induce physiological crossprotection to oxidative stress caused by other biotic and abiotic environmental factors. This effect constitutes the relevance of UV-B for invasion processes in the context of predicted global and climate change. Therefore, consideration of UV-B radiation in future species distribution
models might be especially recommendable for predictions of potential suitable habitats and associated risk assessment
Pre-adaptation as a driver of plant invasions in agroecosystems: empirical investigation and theoretical discussion with a historical perspective
Full text linkIn human-altered environments like agroecosystems, invasive plants are particularly harmful due to their adaptation to disturbances from livestock and cultivation. Pre-adaptation to these disturbances might explain their success in such systems. The Neolithic Plant Invasion Hypothesis (NPIH) suggests that European invasive plants are pre-adapted to agropastoralism due to its long history in the region. This thesis tests a key prediction of the NPIH: species with longer exposure to agropastoralism should better withstand its disturbances. I selected invasive species from West Asia (~12000 years), Europe (~8000 years), and America (~5000 years) to compare their responses to agropastoral disturbances. The first experiment studied germination and seedling performance of 30 species under various disturbance levels and space occupancy. While moderate disturbance aided germination, no significant differences were found between the groups. The second experiment involved 45 species, focusing on maturity, reproduction, and fitness. Seedlings were transplanted to a field where soil disturbance and clipping were applied. Over two years, growth and reproduction were monitored, followed by a fitness assessment in a lab. Soil disruption improved growth, while clipping reduced it, but species responses did not clearly correlate with long agropastoral exposure. Fitness differences were seen, but no clear link to historical exposure emerged. These results might reflect modern adaptation or convergent evolution, as agropastoralism has existed in many regions worldwide. Additionally, I reviewed literature on pre-adaptation and its role in invasions, discussing the NPIH and the AIAI hypothesis. With mixed support for NPIH, I explored its historical basis and proposed interdisciplinary approaches, including archaeobotanical research, to study the evolution of invasive plants. Future work on plant invasions should integrate ecological, evolutionary, and socioeconomic perspectives
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
In Search of Similarities in Invasive Plant Species: Comparing Native and Invasive Populations of Six Clonal Plant Species in Germany and New Zealand
Full text linkHumans have introduced non-native species into almost every corner of the world. These, often invasive, alien species have caused severely negative ecological and economical consequences and are regarded to be one of the most problematic aspects of human caused global environmental change. This thesis investigates the invasion of six clonal growing plant species in New Zealand: Achillea millefolium L., Hieracium pilosella L., Lotus pedunculatus Cav., Leucanthemum vulgare Lam., Prunella vulgaris L. and Hypericum perforatum L. This thesis combines four comparisons of native (German) and alien (New Zealand) populations. The studies include a biogeographical multi-species comparison of field populations, a multi-species common garden experiment, a germination experiment including three study species and a UV-B experiment focusing on one species. When grown from seeds, alien plants were less tolerant to simulated herbivory on clonal organs. The germination experiment uncovered higher germination rates in alien populations of three species in medium temperature conditions. In another growth chamber experiment, Hieracium pilosella responded to increased UV-B intensities with high phenotypic plasticity and increased the length of foliar hairs. The outcomes of the common garden experiment indicate that local adaptation and contemporary evolution might be responsible for the observed lower tolerance of alien plants to simulated herbivory on clonal organs. In case of the germination and UV-B experiments, alternative explanations such as phenotypic plasticity or maternal effects are more likely to be the source of different responses. Through the different responses found for different types of clonal growth, this thesis also emphasises the need to more explicitly address clonal growth in future studies in invasion science
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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