Aim: Bogs and transition mires in Central Europe have undergone tremendous changes in the last decades, declining in spatial extent and favorable conservation status. However, species extinctions have been documented only rarely because of a lack of reliable floristic data. Here, we assessed species losses of bog complexes and analyzed their potential drivers.

Location: Black Forest, Germany.

Methods: We made use of the unique situation that the majority of bogs in the Black Forest (124 sites) had been systematically surveyed from 1972 to 1980 and resurveyed the flora of the same sites between 2017 and 2020. In addition, we included further data from the preceding decades.

Results: Out of 88 species for which we compiled site occupancy data, two species went extinct in the whole study area and 37 decreased from 1972 to 2020, losing on average 33% of their initial frequency. In contrast, 46 species displayed a positive trend. While decreasing species were characteristic of raised bogs, moorland ponds and base-rich mires, increasing species were typical of poor mires, fens and wet meadows. Species losses were higher at low elevation, pointing to increasing temperature increase and decreasing precipitation as main drivers of extinction, while habitat area, distance to the nearest site and land use played no significant role. The mean altitude at which extinctions of populations occurred increased with time. Assuming a continuation of the observed negative trends for declining bog species, our models predict the extinction of further ten species by 2045.

Main conclusion: Our study highlights the value of systematic and comprehensive past biodiversity surveys to assess biodiversity change. We demonstrated that low-mountain habitats have already suffered biodiversity loss as a result of climate change. There are only limited management options against rising temperatures and decreasing precipitation.

Study site. The study area comprised the southern part of the Black Forest, an area of about 500 km² (Figure 1). While the study area only covers about one third of all bog sites in the Black Forest, the vast majority of rare and regionally typical bog species occurred here. Within the study region, elevation ranges from approximately 300 m above sea level in the Rhine valley in the south to almost 1,500 m above sea level at the summit of the Feldberg in the northwest. This elevational gradient offers the opportunity to relate possible changes to climatic drivers, in particular to precipitation and temperature.

The majority of the bog complexes is located in forested valleys and on slopes and not used for forestry, except for some marginal areas. Some other bog complexes are found on sloping valley floors or plateaus that are used for low-intensity agriculture. Almost all bog complexes have been affected by drainage ditches, which in most cases were created more than 50 years ago and are no longer maintained today. To our knowledge, in the past few decades, new drainage ditches have only been established in exceptional cases, affecting only small areas (von Sengbusch, 2015). As we were not able to collect water level data at all sites and drainage conditions did not change in almost all sites, we did not include them in the subsequent analyses.

Data compilation. In total, species inventories of typical vascular plant species of bog complexes were compiled for 124 bogs with presence/absence. In particular, 106 bogs were included that had been surveyed by Barbara Dierßen and Klaus Dierßen four to five decades ago in the study area (B. Dierßen & Dierßen, 1984). Locations of all sites of bog complexes (as listed in Appendix Table S1 in Supporting Information) were digitized as polygons.

In addition to the bog sites surveyed by Dierßen & Dierßen themselves, B. Dierßen & K. Dierßen (1984) included a species list from one site from literature and eight further sites were included that had been recorded at the same time by Schuhwerk (1988). These records were made between 1972 and 1980 (Appendix Table S2 in Supporting Information). Further nine bog complexes were included that were only surveyed in the 1980s and 1990s. There are further bog sites inside the study region, which however, are either very small or mostly very species-poor, and thus, not considered in our study. From 2017 to 2020, the first author resurveyed 123 sites, most of them twice. To complete these census species lists, we added occurrence data from habitat mapping campaigns carried out in these four years. For the period between 1980 and 2017, we collected occurrence data from habitat mapping campaigns, expert reports for nature conservation value of some of the sites, the species protection programme of the federal state of Baden-Württemberg and unpublished records based on the expertises of local biologists (Appendix Table S2 in Supporting Information).

Out of the species lists of all surveys, we selected 109 vascular plant species. On the one hand, the selected species can be considered typical of raised and quaking bogs, as well as mires and, in particular, transition mires. On the other hand, species were included that are common in peat woodlands, wet meadows and fens. Species from this second group are usually absent in undisturbed and intact bogs and mires, but are widespread in degraded stands. These species were included to detect effects of drainage and eutrophication (see species` habitat preferences in Appendix Table S3 in Supporting Information). We then checked for species with insufficient number of observations. From the initial pool of 109 target species, we removed those without repeated records as well as those that were not recorded by the majority of the authors or whose identification by some recorders was questionable. For example, we excluded Scutellaria minor because the species has never been recorded by B. Dierßen & K. Dierßen (1984) although its presence had been documented in bog complexes at low elevation since at least 1961 (Sebald, O. et al., 1996). Species taxonomy follows Rothmaler (Jäger, 2017). We aggregated Dactylorhiza traunsteineri and D. majalis as D. majalis agg. because these species were not distinguished in all past surveys. Finally, 88 species remained, with a total of 5076 occupancy change observations across all sites (for the data see Appendix Table S5 in Supporting Information). Across all sites, the mean number of occupancy change observations per species (that is whether a species was newly recorded at a site or no longer occurred) was 57.7 (min 1, max 118). Overall, the final database consisted of 814 species lists and a total of 72,268 presence/absence (p/a observations, with on average 583 species records per site and 6.5 (min 1, max 16) repeated records per species per site. The lists vary in completeness. An exception is the most recent species inventory for the period 2017 to 2020, which can be considered nearly complete. The assessment of extinction events at the scale of hectares generally involves the difficulty that absences of every species have to be examined across the total area of every bog complex. Thus, even with the extensive data on which our study is based, past extinction events always pose a risk that species may have been overlooked (Beck et al., 2018). Thus, it is difficult to assess when exactly species that were absent in our last systematic survey from 2017 to 2020 went extinct. For example, Carex frigida went extinct at site 160 (Quellgebiet Sägenbach) between 1973, when the species was recorded by B. Dierßen & Dierßen (1984) and 2018. Within this period, the extinction could have happened any time because we had no further survey data.

Site_ID: number used in the map in Figure 1 and in Appendix Table S1; Species: Taxon name; Year_first_obs: Year when the species was first observed at this site; Year_last_obs: Year when the species was last observed at this site; Occ_change: indicates whether a species went extinct (-1), remained occurring (0) or newly occurred (+1) at this site; Year_subsequent_survey: Year  of the subsequent survey in which the species was no longer detected at the site; Ext_event: refers to declining species and shows the extinction event (-1) that corresponds to the year of the subsequent survey.