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Long-term analyses of biodiversity data highlight a ‘biodiversity conservation paradox’: biological communities show substantial species turnover over the past century 1 , 2 , but changes in species richness are marginal 1 , 3 – 5 . Most studies, however, have focused only on the incidence of species, and have not considered changes in local abundance. Here we asked whether analysing changes in the cover of plant species could reveal previously unrecognized patterns of biodiversity change and provide insights into the underlying mechanisms. We compiled and analysed a dataset of 7,738 permanent and semi-permanent vegetation plots from Germany that were surveyed between 2 and 54 times from 1927 to 2020, in total comprising 1,794 species of vascular plants. We found that decrements in cover, averaged across all species and plots, occurred more often than increments; that the number of species that decreased in cover was higher than the number of species that increased; and that decrements were more equally distributed among losers than were gains among winners. Null model simulations confirmed that these trends do not emerge by chance, but are the consequence of species-specific negative effects of environmental changes. In the long run, these trends might result in substantial losses of species at both local and regional scales. Summarizing the changes by decade shows that the inequality in the mean change in species cover of losers and winners diverged as early as the 1960s. We conclude that changes in species cover in communities represent an important but understudied dimension of biodiversity change that should more routinely be considered in time-series analyses. Time-series data including 1,794 plant species from 7,738 vegetation plots in Germany between 1927 and 2020 reveal patterns of change in biodiversity, and suggest that more species declined than increased in abundance during this period.

We studied the impact of the neophyte tree Fraxinus pennsylvanica on the diversity of beetles in floodplain forests along the river Elbe in Germany in 2016, 2017 and in 2020, where 80% of all Fraxinus excelsior trees had died following severe droughts. Beetles were collected by insecticidal knock-down from 121 trees (64 F. excelsior and 57 F. pennsylvanica) and identified to 547 species in 15,214 specimens. The trees sampled in 2016 and 2017 showed no signs of drought stress or ash dieback and serve as a reference for the comparison with the 2020 fauna. The data proved that F. excelsior harbours the most diverse beetle community, which differed also significantly in guild composition from F. pennsylvanica. Triggered by extremely dry and long summer seasons, the 2020 ash dieback had profound and forest-wide impacts. Several endangered, red-listed beetle species of Saxonia Anhalt had increased in numbers and became secondary pests on F. excelsior. Diversity decreased whilst numbers of xylobionts increased on all trees, reaching 78% on F. excelsior. Proportions of xylobionts remained constant on F. pennsylvanica. Phytophages were almost absent from all trees, but mycetophages increased on F. pennsylvanica. Our data suggest that as a result of the dieback of F. excelsior the neophyte F. pennsylvanica might become a rescue species for the European Ash fauna, as it provides the second-best habitat. We show how difficult it is to assess the dynamics and the ecological impact of neophytes, especially under conditions similar to those projected by climate change models. The diversity and abundance of canopy arthropods demonstrates their importance in understanding forest functions and maintenance of ecosystem services, illustrating that their consideration is essential for forest adaptation to climate change.

Floodplain habitat area and quality have decreased significantly over the last decades, mainly due to anthropogenic changes. Conservation efforts targeting floodplain species must consider changing climate when choosing suitable areas for restoration, especially for sessile organisms. Species distribution models, based on data from national and international databases on species’ occurrences and various environmental predictors, allow forecasting changes in species’ spatial distributions and facilitate planning at the catchment scale. Modelled predictions for floodplain organisms and communities suggest that current protected floodplains in Switzerland do not provide sufficient habitat and refugia for typical floodplain species and that climate-adapted conservation planning is needed which includes new areas. Similarly, visualisations of habitat which might potentially be occupied in the future can help to distinguish refugia from short-term sanctuaries. Temporal changes in water availability, for example, during periods of droughts, are likely to further lead to local habitat decrease of floodplain plant communities, as can be shown in hydrological models at reach scale for the Rhine in Germany. In fragmented landscapes along rivers, protected areas can provide refuge for specialised terrestrial species and promote species conservation, as we report for a floodplain which was protected for 30 years. Future restoration projects at the reach scale should, therefore, include planning at the catchment level, as well as consider hydrological regimes at the reach scale, especially for sessile floodplain species’ conservation under changing climate. Highlights Floodplain communities are threatened, especially sessile species; Climate-driven species distribution models distinguish refugia and sanctuaries; Hydrological predictions under changing climate show shifts of habitat for riverine plants at the local scale; A case study from a protected area shows that it promotes floodplain biodiversity; Prioritisation of local restoration projects should be based on planning at the catchment scale.

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 K‐means 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.

Floodplains provide a multitude of ecosystem functions and services with water purification being one of them. For this study we modelled the retention of total phosphorous (TP) in the floodplains of the river Rhine and the river Elbe, looking at sediment deposition as the main process responsible for removing TP from rivers during inundation events. We applied two different approaches: a proxy-based approach (PBA) and a one-dimensional model based approach (MBA). We used both to calculate the yearly TP retention and compared it with the annual TP load in the rivers. Compared to the transported river load the Elbe floodplains investigated retained approx. 4.9% TP resp. 1.4% (PBA vs. MBA) while in the floodplains of the river Rhine about 1.8% vs. 0.3% TP was retained. We found that the greatest difficulty in quantifying TP retention in floodplains is due to the lack of spatial detail on the hydrological connectivity between rivers and their adjacent floodplains and that a sound validation of the results is absolutely necessary. Long-term monitoring data for floodplains, especially on hydrological connectivity, are of crucial importance in this respect.

Vegetation-plot resurvey data are a main source of information on terrestrial biodiversity change, with records reaching back more than one century. Although more and more data from re-sampled plots have been published, there is not yet a comprehensive open-access dataset available for analysis. Here, we compiled and harmonised vegetation-plot resurvey data from Germany covering almost 100 years. We show the distribution of the plot data in space, time and across habitat types of the European Nature Information System (EUNIS). In addition, we include metadata on geographic location, plot size and vegetation structure. The data allow temporal biodiversity change to be assessed at the community scale, reaching back further into the past than most comparable data yet available. They also enable tracking changes in the incidence and distribution of individual species across Germany. In summary, the data come at a level of detail that holds promise for broadening our understanding of the mechanisms and drivers behind plant diversity change over the last century. Measurement(s) vegetation-plot resurvey data of vascular plant species Technology Type(s) vegetation-plot records Factor Type(s) Cover of species in plots Sample Characteristic - Organism Vascular plant species Sample Characteristic - Environment Terrestrial habitats Sample Characteristic - Location Germany

Vegetation-plot resurvey data are a main source of information on terrestrial biodiversity change, with records reaching back more than one century. Although more and more data from re-sampled plots have been published, there is not yet a comprehensive open-access dataset available for analysis. Here, we compiled and harmonised vegetation-plot resurvey data from Germany covering almost 100 years. We show the distribution of the plot data in space, time and across habitat types of the European Nature Information System (EUNIS). In addition, we include metadata on geographic location, plot size and vegetation structure. The data allow calculating temporal biodiversity change at the community scale and reach back further into the past than most comparable data yet available. They also enable tracking changes in the incidence and distribution of individual species across Germany. In summary, the data come at a level of detail that holds promise for broadening our understanding of the mechanisms and drivers behind plant diversity change over the last century. Competing Interest Statement The authors have declared no competing interest.