Ondřej Hájek

Separating habitat invasibility by alien plants from the actual level of invasion.

Habitats vary considerably in the level of invasion (number or proportion of alien plant species they contain), which depends on local habitat properties, propagule pressure, and climate. To determine the invasibility (susceptibility to invasions) of different habitats, it is necessary to factor out the effects of any confounding variables such as propagule pressure and climate on the level of invasion. We used 20 468 vegetation plots from 32 habitats in the Czech Republic to compare the invasibility of different habitats. Using regression trees, the proportion of alien plants, including archaeophytes (prehistoric to medieval invaders) and neophytes (recent invaders), was related to variables representing habitat properties, propagule pressure, and climate. The propagule pressure was expressed as the proportion of surrounding urban and industrial or agricultural land, human population density, distance from a river, and history of human colonization in the region. Urban and industrial land use had a positive effect on the proportion of both archaeophytes and neophytes. Agricultural land use, higher population density, and longer history of human impact positively affected the proportion of archaeophytes. Disturbed human-made habitats with herbaceous vegetation were most invaded by both groups of aliens. Neophytes were also relatively common in disturbed woody vegetation, such as broad-leaved plantations, forest clearings, and riverine scrub. These habitats also had the highest proportion of aliens after removing the effect of propagule pressure and climate, indicating that they are not only the most invaded, but also most invasible. These habitats experience recurrent disturbances and are rich, at least temporarily, in available nutrients, which supports the hypothesis that fluctuating resources are the major cause of habitat invasibility. The least invaded habitats were mires and alpine-subalpine grasslands and scrub. After removing the effect of propagule pressure and climate, some habitats actually invaded at an intermediate level had very low proportions of aliens. This indicates that these habitats (e.g., dry, wet, and saline grasslands, base-rich fens, and broad-leaved deciduous woodlands) are resistant to invasion.

Stratified resampling of phytosociological databases: some strategies for obtaining more representative data sets for classification studies

Abstract Question: The heterogeneous origin of the data in large phytosociological databases may seriously influence the results of their analysis. Therefore we propose some strategies for stratified resampling of such databases, which may improve the representativeness of the data. We also explore the effects of different resampling options on vegetation classification. Methods: We used 6050 plot samples (relevés) of mesic grasslands from the Czech Republic. We stratified this database using (1) geographical stratification in a grid; (2) habitat stratification created by an overlay of digital maps in GIS; (3) habitat stratification with strata defined by traditional phytosociological associations; (4) habitat stratification by numerical classification and (5) habitat stratification by Ellenberg indicator values. Each time we resampled the database, taking equal numbers of relevés per stratum. We then carried out cluster analyses for the resampled data sets and compared the resulting classifications using a newly developed procedure. Results: Random resampling of the initial data set and geographically stratified resampling resulted in similar classifications. By contrast, classifications of the resampled data sets that were based on habitat stratifications (2–5) differed from each other and from the initial data set. Stratification 2 resulted in classifications that strongly reflected environmental factors with a coarse grain of spatial heterogeneity (e.g. macroclimate), whereas stratification 5 resulted in classifications emphasizing fine-grained factors (e.g. soil nutrient status). Stratification 3 led to the most deviating results, possibly due to the subjective nature of the traditional phytosociological classifications. Conclusions: Stratified resampling may increase the representativeness of phytosociological data sets, but different types of stratification may result in different classifications. No single resampling strategy is optimal or superior: the appropriate stratification method must be selected according to the objectives of specific studies. Abbreviations: ASS = Phytosociological association; ELL = Ellenberg indicator values; GEO = Geographical stratification; GIS = Geographical information system; NUM = Numerical classification; RAN = Random resampling.

Semi-dry grasslands along a climatic gradient across Central Europe: Vegetation classification with validation

Abstract Question: What is the variation in species composition of Central European semi-dry grasslands? Can we apply a training-and-test validation approach for identifying phytosociological associations which are floristically well defined in a broad geographic comparison; can we separate them from earlier described associations with only a local validity? Location: A 1200 km long transect running along a gradient of increasing continentality from central Germany via Czech Republic, Slovakia, NE Austria, Hungary to NW Romania. Methods: Relevés with > 25% cover of Brachypodium pinnatum and/or Bromus erectus were geographically selected from a larger database. They were randomly split into two data sets, TRAINING and TEST, each with 422 relevés. Cluster analysis was performed for each data set on scores from significant principal coordinates. Different partitions of the TRAINING data set were validated on the TEST data set, using a new method based on the comparison of % frequencies of species occurrence in clusters. Clusters were characterized by statistically defined groups of diagnostic species and values of climatic variables. Results: Species composition changed along the NW-SE gradient and valid clusters were geographically well separated. Optimal partition level was at 11 clusters, six being valid: two clusters Germany and the Czech Republic corresponded to the Bromion erecti; two clusters from the Czech Republic and Hungary to the Cirsio-Brachypodion, and two clusters were transitional between these two alliances. Conclusion: The training-and-test validation method used in this paper proved to be efficient for discriminating between robust clusters, which are appropriate candidates for inclusion in the national or regional syntaxonomic overviews, and weak clusters, which are specific to the particular classification of the given data set. Nomenclature: Ehrendorfer (1973).

Floristic diversity patterns in the White Carpathians Biosphere Reserve, Czech Republic

The flora of the White Carpathians, a mountain range in the south-east of the Czech Republic, is documented by about 485,000 records of vascular plant occurrences collected since the mid-19th century. A total of 1299 species recorded in 93 grid cells of 2.8 × 3.1 km were used for an analysis of spatial patterns of floristic diversity in the White Carpathians. Multivariate statistical techniques such as ordination and classification were used to reveal the main gradients in floristic composition and species richness, and measured environmental data and Ellenberg indicator values were used to assess underlying environmental factors. There is a striking floristic contrast between the western and eastern part of the study area, which is associated with differences in climate, mean altitude, topographic heterogeneity measured as altitudinal range, and land use. The western part is characterised by thermophilous, continental and calcicolous species of open habitats. In contrast, the more forested eastern part along the state border with Slovakia and the north-eastern part of the area are characterised by acidophilous species with higher moisture requirements. This pattern is consistent with the established phytogeographical division of the Czech Republic into the phytogeographical regions of Thermophyticum and Mesophyticum. The further division of the area into four regions, based on classified grid data, is also similar to the current division into phytogeographical districts, except for the Javorníky district. There are two distinct hot spots of species richness, in the western and the extreme north-eastern part. A poorer flora was found in landscapes with intensive agriculture. Species richness is associated with different environmental factors than species composition, namely with soil types and land-use categories. Alien species are more common in areas with a higher incidence of arable land and built-up areas, and less common in areas dominated by grasslands and forests.

High Plant Diversity of Grasslands in a Landscape Context: A Comparison of Contrasting Regions in Central Europe

Some regions and habitats harbour high numbers of plant species at a fine scale. A remarkable example is the grasslands of the White Carpathian Mountains (Czech Republic), which holds world records in local species richness; however, the causes are still poorly understood. To explore the landscape context of this phenomenon and its relationships to diversity patterns at larger scales, we compared diversity patterns in grasslands and other vegetation types in the White Carpathians with those in nearby regions lacking extremely species-rich grasslands, using data from vegetation plots and flora grid mapping of entire landscapes. Although small-scale species richness of grasslands and ruderal/weed vegetation of the White Carpathians was higher than in the nearby regions, the number of grassland and ruderal/weed species in the regional flora of the White Carpathians was not. Diversity of forests was not higher in this region at any scale. Thus the remarkably high local species richness of the White Carpathian grasslands does not result from a larger grassland species pool in the region, but from the fine-scale co-occurrence of many grassland species in this landscape, which results in the formation of grassland communities that are locally rich but with similar species composition when comparing different sites (i.e. high alpha but low beta diversity). This pattern can be partly attributed to the large total area of these grasslands, which reduces random extinctions of rare species, low geological diversity, which enables many species to occur at many sites across the landscape, and high land-cover diversity, which supports mixing of species from different vegetation types.

Local ranges of phytosociological associations: are they reflected in numerical classification?

In the tradition of European phytosociology, delimitations of vegetation units such as associations are mostly based on data from small areas where more detailed vegetation sampling has been carried out. Such locally delimited vegetation units are often accepted in large-scale synthetic classifications, e.g. national vegetation monographs, and tentatively assigned to a small geographical range, forming groups of similar (vicarious) vegetation units in different small areas. These vicarious units, however, often overlap in species composition and are difficult to recognize from each other. We demonstrate this issue using an example of the classification of dry grasslands (Festuco-Brometea) in the Czech Republic. The standard vegetation classification of the Czech Republic supposes that the majority of accepted associations (66 out of 68) have a restricted distribution in one of the two major regions, Bohemia or Moravia. We compared the classification into traditional associations with the numerical classification of 1440 phytosociological relevés from the Czech Republic, in order to test whether the traditionally recognized associations with small geographical ranges are reflected in numerical classification. In various comparisons, the groups of relevés identified by numerical analysis occupied larger areas than the traditional associations. This suggests that with consistent use of total species composition as the vegetation classification criterion, the resulting classification will usually include more vegetation units with larger geographical ranges, while many of the traditional local associations will disappear.

ABIOTIC STREAM TYPES AND SPECIES ASSEMBLAGES: IS THERE ANY SIMPLE LINKAGE? CZECH STREAMS AND BENTHIC MACROINVERTEBRATES AS AN EXAMPLE

Ecological status assessment of water bodies according to the European Water Framework Directive (WFD) is generally based on the comparison of a reference and observed status. There are two basic approaches to define the reference conditions for a particular water body: a site-specific and a type-specific. For the latter, required by the Directive, it is necessary to create a typology of water bodies. The Directive offers two ways (system A, system B) of their inherence based on abiotic descriptors. Within the assessment based on biological elements, various metrics derived from the species assemblage composition are used. The type-specific approach is associated with a multimetric system, i.e. sets of metrics for selected stressors and specific water body types (AQEM consortium 2002). The site-specific approach, which is independent on typology and deals with abiotic conditions (descriptors) of a particular site or stretch, is associated with predictive models like the British RIVPACS (WRIGHT et al. 1993) or analogous the Czech PERLA (KOKES et al. 2001). More or less detailed knowledge of biotic and abiotic conditions of pristine or near natural sites is necessary for both approaches. Due to the fact that the metrics are generally derived from taxonomic composition of the species assemblages, a correspondence between types and these assemblages is essential for the truthfulness of the type-specific approach. Therefore, the selection of abiotic descriptors should be done very carefully. The correspondence among species assemblages of reference sites and abiotic types was tested on Czech streams and benthic macroinvertebrates.

Spatial Soil Modeling of Organochlorine Pesticides, Their Pools and Volatilization Fluxes

The goal of this study was to use the modeling tools for prediction of environmental concentrations and pools of pesticides (HCB and DDT) in soil. The characterization and quantification of secondary background sources of HCB, were computed using fugacity based deterministic model. Areas with a high potential for deposition and volatilization of HCB were identified. Results of modeling were maps showing spatial distribution of HCB and DDT in the Czech Republic which have been visualized on the web portal GENASIS (Global Environmental Assessment Information System) to provide information on environment contamination.