Aim: Formalized classifications synthesizing vegetation data at the continental scale are being attempted only now, although they are of key importance for nature conservation planning. Therefore, we aim to provide a vegetation classification and to describe the main biogeographical patterns of floodplain forests and alder carrs in Europe. Location: Europe.Methods: A database of more than 40 000 vegetation plots of floodplain forests and alder carrs across Europe was compiled. After geographic stratification, 16 392 plots were available for classification, which was performed using the supervised method Cocktail. We also searched for new associations using semi-supervised Kmeans classification. The main biogeographic patterns and climate-related gradients in species composition were determined using detrended correspondence analysis and cluster analysis.Results: Thirty associations of floodplain forests and alder carrs were distinguished, which belong to five alliances. The Alnion incanae includes riparian, seepage and hardwood floodplain forests in the nemoral and hemiboreal zones (dominated by Alnus glutinosa and Fraxinus excelsior) and in the boreal zone (dominated by A. incana). The Osmundo-Alnion represents oceanic vegetation dominated by Alnus glutinosa, Fraxinus angustifolia and F. excelsior distributed mostly on the Iberian Peninsula and composed of species with Atlantic distribution and Iberian endemics. The Populion albae comprises floodplain forests frequently dominated by Fraxinus angustifolia, Populus alba and P. nigra that are widespread in floodplains of large rivers under summer-dry climates in the Mediterranean region. The Platanion orientalis represents eastern Mediterranean floodplain forests dominated by Platanus orientalis. The Alnion glutinosae includes forest swamps dominated by Alnus glutinosa distributed mostly in the nemoral and hemiboreal zones. The main biogeographic patterns within European floodplain forests and alder carrs reflect the climatic contrasts between the Mediterranean, nemoral, boreal and mountain regions. Oceanic floodplain forests differ from those in the rest of Europe. The hydrological regime appears to be the most important factor influencing species composition within regions.Conclusions: This study is the first applying a formalized classification at the association level for a broad vegetation type at the continental scale. The proposed classification provides the scientific basis for the necessary improvement of the habitat classification systems used in European nature conservation.

Questions: How have the historical frequency and severity of natural disturbances in primary Picea abies forests varied at the forest stand and landscape level during recent centuries? Is there a relationship between physiographic attributes and historical patterns of disturbance severity in this system? Location: Primary P. abies forests of the Eastern Carpathian Mountains, Romania; a region thought to hold the largest concentration of primary P. abies forests in Europe's temperate zone.Methods: We used dendrochronological methods applied to many plots over a large area (132 plots representing six stands in two landscapes), thereby providing information at both stand and landscape levels. Evidence of past canopy disturbance was derived from two patterns of radial growth: (1) abrupt, sustained increases in growth (releases) and (2) rapid early growth rates (gap recruitment). These methods were augmented with non-metric multidimensional scaling to facilitate the interpretation of factors influencing past disturbance.Results: Of the two growth pattern criteria used to assess past disturbance, gap recruitment was the most common, representing 80% of disturbance evidence overall. Disturbance severities varied over the landscape, including stand-replacing events, as well as low-and intermediate-severity disturbances. More than half of the study plots experienced extreme-severity disturbances at the plot level, although they were not always synchronized across stands and landscapes. Plots indicating high-severity disturbances were often spatially clustered (indicating disturbances up to 20 ha), while this tendency was less clear for lowand moderate-severity disturbances. Physiographic attributes such as altitude and land form were only weakly correlated with disturbance severity. Historical documents suggest windstorms as the primary disturbance agent, while the role of bark beetles (Ips typographus) remains unclear. Conclusions:The historical disturbance regime revealed in this multi-scale study is characterized by considerable spatial and temporal heterogeneity, which could be seen among plots within stands, among stands within landscapes and between the two landscapes. When the disturbance regime was evaluated at these larger scales, the entire range of disturbance severity was revealed within this landscape.

It has been suggested that a heterogeneous environment enhances species richness and allows for the coexistence of species. However, there is increasing evidence that environmental heterogeneity can have no effect or even a negative effect on plant species richness and plant coexistence at a local scale. We examined whether plant species richness increases with local heterogeneity in the water table depth, microtopography, pH and light availability in a swamp forest community at three local spatial scales (grain: 0.6, 1.2 and 11.4 m). We also used the variance partitioning approach to assess the relative contributions of niche-based and other spatial processes to species occurrence. We found that heterogeneity in microtopography and light availability positively correlated with species richness, in accordance with the habitat heterogeneity hypothesis. However, we recorded different heterogeneity-diversity relationships for particular functional species groups. An increase in the richness of bryophytes and woody plant species was generally related to habitat heterogeneity at all measured spatial scales, whereas a low impact on herbaceous species richness was recorded only at the 11.4 m scale. The distribution of herbaceous plants was primarily explained by other spatial processes, such as dispersal, in contrast to the occurrence of bryophytes, which was better explained by environmental factors. Our results suggest that both niche-based and other spatial processes are important determinants of the plant composition and species turnover at local spatial scales in swamp forests.

The stress-gradient hypothesis predicts a switch from competition to facilitation, under increasing environmental stress. However, it is unclear how important is the change in competition-facilitation balance (i.e., the net outcome of plant-plant interactions) along the stress gradient in the regulation of community temporal stability (i.e., the inverse of temporal variability in total biomass). Increasing environmental stress may enhance community temporal stability by reduced competition or eventually by leading to facilitative interactions between the dominant and subordinate species. Here, we present the results of a 5-yr mesocosm experiment that demonstrates the effects of interspecific interactions on the temporal stability of a riparian community across different drought-stress scenarios. We constructed artificial communities of dominant species (Carex elata) and three subordinate species to simulate the independent effects of environmental stress and interspecific interactions. Using removal of the dominant species, we evaluated the interplay of various mechanisms regulating the temporal stability of the subordinate species (competition-facilitation balance, species asynchrony, and dominant species stability). By simultaneous testing of these stabilizing mechanisms, we show their importance differs depending on environmental variability and harshness. The predominant role is taken by species asynchrony in a seasonally dry environment, whereas in a permanently dry environment, the importance of reduced competition increases. Reduced competition was stabilizing, in particular through increased total community biomass, whereas species asynchrony increased total community biomass and decreased biomass variation. These results suggest experiments and simulations that exclude interspecific interactions may not offer realistic predictions of the effects of changing hydrological regimes on ecosystem functioning.

No abstract