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Species richness, resource availability, and disturbance are the primary factors considered in assessing the invasibility of plant communities. Nonetheless, the density of individuals in a community is a common and easy trait to measure. The ecological significance of the density of both native and invasive tree species was assessed using a systematic review and formal meta-analysis. The densities of recipient communities and invasive exotic tree species in novel ranges were identified in the published literature. In addition, we compared by means of a meta-analysis: (i) densities of invasive versus native species in invaded communities; (ii) densities of native species in invaded versus uninvaded communities; and (iii) densities of invasive species along distance gradients from initial locus of invasion. Invasive trees were found at higher densities than native species in recipient communities. Invasions by woody species were also recorded in communities with relatively low densities of natives suggesting that (i) low density forests may be more susceptible to invasion and/or (ii) density of the recipient community may be reduced during the invasion process. In addition, comparison of native species densities between invaded and uninvaded stands from the same community suggests that invasive trees negatively affect density of native trees once established. Therefore, the widely reported low density and often richness of native plants in invaded communities cannot be directly linked to ecosystem susceptibility to invasion without considering concomitant impacts. These findings suggest that density is a key preliminary determinant or factor which should be considered when assessing tree invasion dynamics.

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. 58 1 418 Dobson, J. (1991): Climate change - a bibliography of recent literature July 1990-February 1991. - Freshwater Biological Association, 14

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literature. Vol. 4. Ed. 2 . – Regnum Vegetabile 110. Bohn, Scheltema and Holkema , Utrecht , 1214 pp.

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Species richness in an assemblage is frequently used as a measure of biological diversity. However, observed species richness is strongly dependent on sample size. If more samples are collected, then more species are observed. Non-parametric species richness estimators, such as the jackknife 1 and 2 and the Chao 1 and 2, are indicated in the literature as potential solutions to the problem of dependence of observed species richness on sampling effort. These methods are intended to estimate the total species richness in an area or assemblage with small sampling effort. Non-parametric estimators are based on the number of species observed, and the number of rare species in a sample, i.e., that occurred in one and/or two sampling units, or with one and/or two individuals. High estimates are produced when samples contain large proportions of rare species. Using a range of real datasets, I show that estimates produced by non-parametric methods are generally dependent on observed species richness. An implicit assumption of these non-parametric techniques is that the rare species curve should present high values at small sample sizes and decreasing values as sampling effort is increased. This assumption was observed in only one out of eight datasets presented. Instead, the rare species curve generally flattens off around a constant value as sampling effort increases. I conclude that non-parametric estimators are not reliable to estimate species richness in an assemblage when the rare species curve does not show a decreasing trend. Comments are made on the possibilities of using non-parametric estimators in the comparison of species assemblages.

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Community patterns in species-by-site matrices provide valuable clues for inferring ecological processes at work. One such pattern is the occupancy frequency distribution (OFD) depicting the frequency distribution of row sums (i.e., occupancy) with a quarter OFDs of bimodal forms. Another pattern that also reflects the structure of row sums is the ranked species occupancy curve (RSOC), and has been shown to imply a 50% of bimodality in OFDs. The use of RSOCs has been advocated in literature over the OFD based on two conclusions from a 6-model inference using only 24 matrices: (i) RSOCs have two general forms, with half representing bimodal OFDs; (ii) there are no effects of spatial and study scales on RSOCs of different forms. Using a much more representative dataset of 289 matrices, I cast doubt on these two conclusions. A missing but dominant RSOC model (the truncated power law) is added. The number of species and the nestedness of the community differ significantly among matrices of different RSOC forms; however, the number of sites and the taxa in the studies do not differ among RSOC or OFD forms. The quarter OFDs of bimodal forms is reassured, with the least frequent occupancy consistent with Raunkiaer’s law of frequency. Importantly, a RSOC is mathematically transferrable to an OFD, with the derivative of the occupancy ranking curve being equal to the negative reciprocal of the occupancy frequency. Based on the type of the community (null versus interactive) and site environment (homogenous versus heterogeneous), four scenarios are needed to identify pre-inferring assemblage mechanisms. The results highlight the need for shifting research from the emphasis of marginal sums to the analysis of matrix structure for an in-depth understanding of the community assemblage patterns and mechanisms.

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Forest biomes have expanded and contracted in response to past climate fluctuations, but it is not clear how they will respond to human-induced atmospheric change. We provide a review of the literature, describing historical links between biogeographical and atmospheric patterns, comparing characteristics of forest biomes and describing expected changes in climate forcings from observed range shifts. Over the geological history, climate fluctuations prompted changes in forest distribution that, in turn, stabilized the atmosphere. Over the past century, warming-induced stress has caused widespread declines of mature forests, but new forests have expanded into open areas of boreal, tropical and temperate regions. Historically, forest expansion happened at much faster rates in cold than in warm regions. Across biomes, species interactions control the use of limiting resources, regulating community dynamics and expansion rates in response to climate variability. Modern impacts of land use change on the distribution of forest biomes are well understood, but the expansion of new forests and their role in stabilizing the atmosphere are yet to be accounted for in global models. Expansion of tropical and temperate forests would yield a negative climate forcing through increased carbon sequestration and evaporative cooling, but in the boreal region forest expansion could amplify climate warming due to changes in albedo. Although qualitative descriptions of forest-atmosphere interactions are possible based on existing records, the net climate forcing from forest range shifts remains uncertain. Three critical gaps in knowledge hinder rigorous evaluations of causality necessary to probe for linkages between climatic and biogeographical patterns: (i) reconstructions of vegetation dynamics have not sufficiently represented warm biomes; (ii) climate and vegetation dynamics are typically assessed at non-comparable scales; and (iii) single-proxies are normally used to simultaneously infer changes in climate and vegetation distribution, leading to redundancy in interpretation. Addressing these issues would improve our ability to decipher past and predict future outcomes of forest-atmosphere interactions.

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Community Ecology
Authors: G. Bonanomi, G. Incerti, A. Stinca, F. Cartenì, F. Giannino, and S. Mazzoleni

Ring shaped patches of clonal plants fascinated plant ecologists since long time. In this work we review the reports on the occurrence of ring pattern in different environmental conditions, the growth forms of ring-forming plants, the mechanisms underlying ring formation, and the consequences for species diversity at community scale. Rings formed by 83 species of clonal vascular plants have been found in grasslands, deserts, bare substrates of lava flow, harvested peat lands, salt marshes, and sand dunes. Four causal hypotheses have been proposed for the emergence of ring patterns: i. occurrence of architectural constraints for ramets development; ii. induction by fire, drought, trampling or overgrazing; iii. nutrient and water depletion by competition inside the ring; and iv. onset of species-specific negative plant-soil feedback in the inner zone of the clone. Since almost all the available studies are observations of ring structure or modelling exercises, none of the putative mechanisms for ring formation emerged from the literature as either generally applicable or suitable for rejection. Therefore, long-term field experiments are needed to investigate the relative prevalence of different mechanisms in different environments. Ring formation bears important consequences at community scale, because ring forming plants often act as “nurses”, enhancing the recruitment and development of different plant species. In fact, ring establishment modifies above- and below-ground environmental conditions, providing specialized safe sites for beneficiaries in the inner zone of the clones. Such interspecific facilitation by ring forming plants, particularly in chronically stressed environments, contributes to increase plant species richness and can locally promote the successional dynamics.

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The sugar composition of floral nectar was studied in 17 sour cherry cultivars in Újfehértó in 1997-2000. All samples contained the three most frequent sugar components: glucose, fructose and sucrose, similarly to our previous data. The ranking of the floral nectar in sour cherry cultivars based on sucrose content was the following: 'Újfehértói fürtös' and 'Pándy 48' reached the threshold value of bee visitation in at least three seasons, and a significant amount of sucrose was detected in the flowers of 'Érdi jubileum AB' and 'Érdi bőtermő' as well. The above cultivars proved to be the most valuable in Újfehértó from the viewpoint of apiculture. According to the ranking based on fructose content, which considers human sensation of taste, the most favourable cultivars were 'Újfehértói fürtös', 'Meteor USA' and 'Korai pipacs'. Based on total sugar content the secretory products preferred by bees were those of 'Újfehértói fürtös' (in three seasons); 'Korai pipacs', 'Érdi nagygyümölcsű', 'Sárándi S/Gy', 'Debreceni bőtermő', 'Kántorjánosi 3', 'Montmorency' and 'Meteor USA' (in two seasons). The ratio of nectar sugars, based on the Baker-quotient, S/(G+F), was sucrose-dominant at least in one season in 'Érdi jubileum AB', 'Érdi nagygyümölcsű' and 'Pándy 48'; hexose-rich in 'Korai pipacs', 'Kántorjánosi 3' and 'Montmorency'; all other cultivars had a sucrose-rich nectar. The nectar of all studied sour cherry cultivars possessed a composition preferred by bees. The basis of bees' nectar preference is the ratio, quantity and concentration of nectar sugar components, which were influenced by the effects of season to a high degree, differing from data in literature. From the viewpoint of nectar composition and concentration the most favourable temperature was around 20 °C.

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In the 19th century, it was found that attraction of bees to light was controlled by light intensity irrespective of colour, and a few critical entomologists inferred that vision of bees foraging on flowers was unlike human colour vision. Therefore, quite justly, Professor Carl von Hess concluded in his book on the Comparative Physiology of Vision (1912) that bees do not distinguish colours in the way that humans enjoy. Immediately, Karl von Frisch, an assistant in the Zoology Department of the same University of Münich, set to work to show that indeed bees have colour vision like humans, thereby initiating a new research tradition, and setting off a decade of controversy that ended only at the death of Hess in 1923. Until 1939, several researchers continued the tradition of trying to untangle the mechanism of bee vision by repeatedly testing trained bees, but made little progress, partly because von Frisch and his legacy dominated the scene. The theory of trichromatic colour vision further developed after three types of receptors sensitive to green, blue, and ultraviolet (UV), were demonstrated in 1964 in the bee. Then, until the end of the century, all data was interpreted in terms of trichromatic colour space. Anomalies were nothing new, but eventually after 1996 they led to the discovery that bees have a previously unknown type of colour vision based on a monochromatic measure and distribution of blue and measures of modulation in green and blue receptor pathways. Meanwhile, in the 20th century, search for a suitable rationalization, and explorations of sterile culs-de-sac had filled the literature of bee colour vision, but were based on the wrong theory.

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Cereal Research Communications
Authors: A. Nyéki, G. Milics, A.J. Kovács, and M. Neményi

This paper reviews the works related to the effect of soil compaction on cereal yield and focuses on research of field experiments. The reasons for compaction formation are usually a combination of several types of interactions. Therefore one of the most researched topics all over the world is the changes in the soil’s physical and chemical properties to achieve sustainable cereal production conditions. Whether we are talking about soil bulk density, physical soil properties, water conductivity or electrical conductivity, or based on the results of measurements of on-line or point of soil sampling resistance testing, the fact is more and more information is at our disposal to find answers to the challenges.

Thanks to precision plant production technologies (PA) these challenges can be overcome in a much more efficient way than earlier as instruments are available (geospatial technologies such as GIS, remote sensing, GPS with integrated sensors and steering systems; plant physiological models, such Decision Support System for Agrotechnology Transfer (DSSAT), which includes models for cereals etc.). The tests were carried out first of all on alteration clay and sand content in loam, sandy loam and silt loam soils. In the study we examined especially the change in natural soil compaction conditions and its effect on cereal yields.

Both the literature and our own investigations have shown that the soil moisture content changes have the opposite effect in natural compaction in clay and sand content related to cereal yield. These skills would contribute to the spreading of environmental, sustainable fertilizing devoid of nitrate leaching planning and cereal yield prediction within the framework of the PA to eliminate seasonal effects.

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