Vegetation samples from 15 successional seres in various disturbed habitats in the western part of the Czech Republic were analysed to detect possible trends. For particular seres, data on species cover were available from the onset to 10-76 yr of succession. All seres started on bare ground. Species which attained at least 1% cover in any sere in any year were used as input data for Canonical Correspondence Analysis, assessing the effect of time as the environmental variable, for Detrended Correspondence Analysis and TWINSPAN classification. Two distinct groups of seres were distinguished: 'ruderal', occurring in agricultural, industrial or urban landscapes altered by men, usually on fertile sites; and 'non-ruderal', occurring in less altered, mostly forested landscapes, usually on acid, nutrient-poor and wetter soils. The former type of succession starts with ruderal annuals, being followed by ruderal perennials. In the latter case non-ruderal clonal perennials prevail from the onset of succession. The landscape frame is emphasized, beside site environmental conditions, as influencing the type of succession. The character of species attaining dominance in succession, participation of dominant woody plants and the character of late successional stages, i.e. features important from the point of view of potential restoration of human-disturbed habitats, are discussed.

AimThe response of L. salicaria and P. australis plants originating from a broad latitudinal gradient to mineral nutrient and water supply was studied. We suggest implications for mechanisms possibly underlying the invasiveness of these two plant species.Location A common garden tub experiment was carried out at T r ebo ˜ , Czech Republic (49 ° N 14 ° 47 ′ E, altitude 433 m). MethodsPlants of 30 L. salicaria and 6 P. australis populations originating from a wide geographical area in Europe and in Israel (60 °− 32 ° N latitude, 6 ° W − 20 ° E longitude) were cultivated for one growing season at two water levels and three ( L. salicaria ) or two ( P. australis ) nutrient doses, respectively. At the end of cultivation, basic morphological and growth characteristics were recorded. ResultsThe latitude of the original geographical location was the most significant factor, affecting both the phenology and most of the plant morphological and growth characteristics measured in both plant species, with the characteristics related to plant size being negatively correlated with latitude. The effect of nutrient dose was very strong in both species, while the effect of water level was more pronounced in P. australis than in L. salicaria . Main conclusionsThe results confirmed the existence of a large phenotypic variability in both plant species within their native ranges of occurrence. In both plant species, the latitudinal variation in plant morphological and growth characteristics most probably indicates the differentiation of genotypes adapted to local geographical conditions. The plants of both species originating from all geographical locations tested responded to the respective nutrient treatments in a similar manner. Exceptions were found in growth characteristics related to reproduction in L. salicaria , indicating that better nutrient supply could enhance the reproductive ability of certain genotypes. Based on the results obtained, we suggest implications for mechanisms possibly underlying the invasiveness of the plant species studied.

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Pinus rotundata dominated peatbog (Žofinka Nature Reserve) in the Třeboň Basin, Czech Republic, was affected by "natural" disturbances: wind damage (1984), followed by a bark beetle attack, and fire (1994, 2000). Phytosociological relevés were used to document vegetation. Soil water chemistry was compared in three differently affected stands: (1) an undisturbed Pinus rotundata bog forest, (2) a windthrow -bark beetle affected stand and (3) a site burned by wildfire in 2000. The species composition of the windthrow -bark beetle affected sites and the undisturbed P. rotundata bog forest differed mainly in the shrub and tree layers. Burned sites were partly colonized by anemochorous species (e.g. Taraxacum sp. div.) that disappeared within two or three years after colonization. Bare peat was colonized by bryophytes (e.g. Marchantia polymorpha and Funaria hygrometrica) typical of the disturbed sites, and by Polytrichum sp. div. and Aulacomnium palustre. Most plant species characteristic of the P. rotundata bog forest occurred at the burned sites eight years after the fire, but in different abundances. The edificator of the former community-P. rotundata-was mostly absent. Compared with windthrow followed by the bark beetle attack, fire promoted rapid expansion of Molinia caerulea. Soil water in both the undisturbed P. rotundata bog forest and the windthrow -bark beetle affected sites had a similar composition: very low pH values, high P concentrations, low concentrations of cations (Ca 2+ , Mg 2+ and K + ) and inorganic nitrogen. The concentrations of soluble reactive phosphorus (SRP) and NH þ 4 À N were negatively correlated with the groundwater table. Total P, SRP and NH þ 4 À N concentrations in the soil water at the burned site were by one order of magnitude higher than those in the P. rotundata bog forest, while concentrations of K + , Mg 2+ and Ca 2+ were only about two times higher. High concentrations of P and N in the soil water found three years after the fire indicated a long-term elevated nutrient content in the soil water.

Plants of Lythrum salicaria and Phragmites australis originating from localities across the European northsouth geographical gradient were cultivated in parallel in an outdoor tub experiment. A strong correlation was found between growth and morphometric characteristics related to plant size (plant height, basal diameter, aboveground-and belowground plant biomass, etc.) and the position of the respective populations along the north-south gradient. Plants of both L. salicaria and P. australis from the southern localities grew taller and more vigorously and flowered later than plants from relatively more northern localities. From this point of view, the plants originating from south European populations were comparable to invasive North American plants. Our study indicates that explanation of the competitive success of populations invading new geographical areas may involve the role of geographic gradients within the species native range.