Abstract Variability of species density and species composition on different spatial scalesexemp... more Abstract Variability of species density and species composition on different spatial scalesexemplary results from dry grasslands and consequences for the sampling setup in biodiversity studies. The article deals with three questions:(1) In which manner do species ...
A general understanding of grazing effects on plant diversity in drylands is still missing, despi... more A general understanding of grazing effects on plant diversity in drylands is still missing, despite an extensive theoretical background. Cross-biome syntheses are hindered by the fact that the outcomes of disturbance studies are strongly affected by the choice of diversity measures, and the spatial and temporal scales of measurements. The aim of this study is to overcome these weaknesses by applying a wide range of diversity measures to a data set derived from identical sampling in three distinct ecosystems. We analyzed three fence-line contrasts (heavier vs. lighter grazing intensity), representing different degrees of aridity (from arid to semiarid) and precipitation regimes (summer rain vs. winter rain) in southern Africa. We tested the impact of grazing intensity on multiple aspects of plant diversity (species and functional group level, richness and evenness components, alpha and beta diversity, and composition) at two spatial scales, and for both 5-yr means and interannual variability. Heavier grazing reduced total plant cover and substantially altered the species and functional composition at all sites. However, a significant decrease in species alpha diversity was detected at only one of the three sites. By contrast, alpha diversity of plant functional groups responded consistently across ecosystems and scales, with a significant decrease at heavier grazing intensity. The cover-based measures of functional group diversity responded more sensitively and more consistently than functional group richness. Beta diversity of species and functional types increased under heavier grazing, showing that at larger scales, the heterogeneity of the community composition and the functional structure were increased. Heavier grazing mostly increased interannual variability of alpha diversity, while effects on beta diversity and cover were inconsistent. Our results suggest that species diversity alone may not adequately reflect the shifts in vegetation structure that occur in response to increased grazing intensity in the dryland biomes of southern Africa. Compositional and structural changes of the vegetation are better reflected by trait-based diversity measures. In particular, measures of plant functional diversity that include evenness represent a promising tool to detect and quantify disturbance effects on ecosystems.
Forest openings (clearings) are particular ecological settings harbouring various plant species g... more Forest openings (clearings) are particular ecological settings harbouring various plant species groups, i.e. grassland, forest, and edge species. In the low-elevated regions of the Southeastern Carpathians, patches of grassy vegetation enclosed within woodlands have been maintained by an extensive grazing regime, which has been exerted once trees have been artificially cleared. Since these openings are rich in species and display high floristical turnover, we considered them as appropriate models to test the behaviour of various diversity measures proposed in the literature. Our sampling was conducted in 40 openings of different sizes, ranging between 15 and 50 m in width, and using transects with four 1-m 2 plots. We measured canopy openness using hemispherical photos (for each 1-m2 plot), heat load index, altitude, and recorded tree litter cover and bedrock type. Species richness at 1 m2 was modelled as a function of these variables using generalized linear mixed models (GLMMs). β-diversity, as calculated with five distinct measures, was modelled against the means and ranges of the variables along each transect, using multiple linear regressions. Species richness at 1 m2 was explained mostly by canopy openness and tree litter cover. The additive and multiplicative measures of β-diversity behaved differently when it came to their relationships with the predictors, as the first was explained by canopy openness range and the second by altitude. The multiplicative β-diversity was similar to the Sørensen-based multiple-site dissimilarity. Neither β-diversity expressed by the slope of distance decay nor Simpson-based multiple-site dissimilarity did have significant relationships with the variables. Our discussion is based on possible ecological explanations for these contrasting relationships, while taking into account also the mathematical constraints of the diversity measures used. We articulate the necessity of simultaneously considering multiple diversity measures when seeking underlying causes of variation in community structure.
We present the plans for a comprehensive, national dry grassland database for Germany, in which v... more We present the plans for a comprehensive, national dry grassland database for Germany, in which vegetation-plot records from different databases will be collected. These data will particularly be used for a plot-based, consistent classification of dry grassland vegetation in Germany. The results of the classification project will then be published in a special feature contribution to Applied Vegetation Science and in the series Synopsis der Pflanzengesellschaften Deutschlands. We warmly invite persons who have dry grassland relevés to participate in this project by contributing data and/or helping with data analysis.
Abstract Variability of species density and species composition on different spatial scalesexemp... more Abstract Variability of species density and species composition on different spatial scalesexemplary results from dry grasslands and consequences for the sampling setup in biodiversity studies. The article deals with three questions:(1) In which manner do species ...
A general understanding of grazing effects on plant diversity in drylands is still missing, despi... more A general understanding of grazing effects on plant diversity in drylands is still missing, despite an extensive theoretical background. Cross-biome syntheses are hindered by the fact that the outcomes of disturbance studies are strongly affected by the choice of diversity measures, and the spatial and temporal scales of measurements. The aim of this study is to overcome these weaknesses by applying a wide range of diversity measures to a data set derived from identical sampling in three distinct ecosystems. We analyzed three fence-line contrasts (heavier vs. lighter grazing intensity), representing different degrees of aridity (from arid to semiarid) and precipitation regimes (summer rain vs. winter rain) in southern Africa. We tested the impact of grazing intensity on multiple aspects of plant diversity (species and functional group level, richness and evenness components, alpha and beta diversity, and composition) at two spatial scales, and for both 5-yr means and interannual variability. Heavier grazing reduced total plant cover and substantially altered the species and functional composition at all sites. However, a significant decrease in species alpha diversity was detected at only one of the three sites. By contrast, alpha diversity of plant functional groups responded consistently across ecosystems and scales, with a significant decrease at heavier grazing intensity. The cover-based measures of functional group diversity responded more sensitively and more consistently than functional group richness. Beta diversity of species and functional types increased under heavier grazing, showing that at larger scales, the heterogeneity of the community composition and the functional structure were increased. Heavier grazing mostly increased interannual variability of alpha diversity, while effects on beta diversity and cover were inconsistent. Our results suggest that species diversity alone may not adequately reflect the shifts in vegetation structure that occur in response to increased grazing intensity in the dryland biomes of southern Africa. Compositional and structural changes of the vegetation are better reflected by trait-based diversity measures. In particular, measures of plant functional diversity that include evenness represent a promising tool to detect and quantify disturbance effects on ecosystems.
Forest openings (clearings) are particular ecological settings harbouring various plant species g... more Forest openings (clearings) are particular ecological settings harbouring various plant species groups, i.e. grassland, forest, and edge species. In the low-elevated regions of the Southeastern Carpathians, patches of grassy vegetation enclosed within woodlands have been maintained by an extensive grazing regime, which has been exerted once trees have been artificially cleared. Since these openings are rich in species and display high floristical turnover, we considered them as appropriate models to test the behaviour of various diversity measures proposed in the literature. Our sampling was conducted in 40 openings of different sizes, ranging between 15 and 50 m in width, and using transects with four 1-m 2 plots. We measured canopy openness using hemispherical photos (for each 1-m2 plot), heat load index, altitude, and recorded tree litter cover and bedrock type. Species richness at 1 m2 was modelled as a function of these variables using generalized linear mixed models (GLMMs). β-diversity, as calculated with five distinct measures, was modelled against the means and ranges of the variables along each transect, using multiple linear regressions. Species richness at 1 m2 was explained mostly by canopy openness and tree litter cover. The additive and multiplicative measures of β-diversity behaved differently when it came to their relationships with the predictors, as the first was explained by canopy openness range and the second by altitude. The multiplicative β-diversity was similar to the Sørensen-based multiple-site dissimilarity. Neither β-diversity expressed by the slope of distance decay nor Simpson-based multiple-site dissimilarity did have significant relationships with the variables. Our discussion is based on possible ecological explanations for these contrasting relationships, while taking into account also the mathematical constraints of the diversity measures used. We articulate the necessity of simultaneously considering multiple diversity measures when seeking underlying causes of variation in community structure.
We present the plans for a comprehensive, national dry grassland database for Germany, in which v... more We present the plans for a comprehensive, national dry grassland database for Germany, in which vegetation-plot records from different databases will be collected. These data will particularly be used for a plot-based, consistent classification of dry grassland vegetation in Germany. The results of the classification project will then be published in a special feature contribution to Applied Vegetation Science and in the series Synopsis der Pflanzengesellschaften Deutschlands. We warmly invite persons who have dry grassland relevés to participate in this project by contributing data and/or helping with data analysis.
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Papers by Jürgen Dengler