Presence-absence of plant habitat specialists in 15 patches of dry calcareous grassland

Abstract Background Dry grasslands on calcareous bedrock in warm climates around the Oslo Fjord are naturally fragmented biodiversity hotspots. This habitat geographically coincides with the most densely populated area of Norway. Many habitat specialists, along with the habitat itself, are red-listed because of land-use change, forest encroachment, and invasive species that cause habitat loss and greater isolation of remaining patches. To ensure effective conservation, data on species presences and absences are necessary to quantify states, changes, and extinction risks in specific populations and habitat patches. New information We present presence-absence data of 49 vascular plant species in 15 patches of dry calcareous grassland habitat, surveyed in 2009, 2019, and in 2020. The species are considered to be habitat specialists and, thus, unlikely to occur between the patches.


Introduction
Norway's most densely populated area coincides geographically with a national biodiversity hotspot. Warm climates and calcareous bedrock have created dry grasslands in places with shallow soil and historical land-use practices. These grasslands are naturally fragmented, but are now Red-listed along with several habitat specialists that depend on them. Land-use change, forest encroachment, and invasive species cause habitat loss and greater isolation of remaining patches (Evju et al. 2018).
We present presence-absence data for 49 vascular plant species in 15 mapped patches of dry calcareous grassland habitat surveyed in three growing seasons. The data combine surveys from 2009, 2019, and 2020. The 2019 and 2020 surveys are repeated surveys of a subset of data described in previous publications (Wollan et al. 2011, Bakkestuen et al. 2014, Evju et al. 2015. As the species are considered to be habitat specialists, they are unlikely to occur between the patches (Evju et al. 2015). Although substantial habitat loss has occurred in the past ~ 80 years , dry calcareous grasslands occur sparsely and fragmented by nature. Therefore, it is plausible that the populations of habitat specialists, until recently, were in approximate equilibrium with their environment.
Dry calcareous grasslands are characterised by drought-tolerant grasses, forbs, shrubs and occasionally trees. Though originally defined as lacking tree cover, the borders are transient between naturally treeless and potentially forested semi-natural habitats kept open by grazing. The surveyed habitat patches also include semi-natural vegetation where dry pine forest has potential to grow. Interannual variability plays an important role, as occasional intense droughts keep competitive vegetation away from the driest patches. For example, the summer of 2018 was exceptionally dry and hot, and visibly impacted the vegetation the following years. Many drought-intolerant plants and young trees died, and some of the habitat specialists came back in full vigour the next season.
Some patches of dry calcareous grasslands, like those presented here, are protected inside nature reserves. A combination of appropriate theory and good data can provide insights into extinction risk scenarios and guide choices between alternative conservation strategies. Ecological theory, such as island biogeography and metapopulation and metacommunity theory, can provide hypotheses and general predictions. Good data, including absence records, are necessary to test these hypotheses and provide insight into specific populations and habitat patches. The present dataset was collected to study hypotheses of turnover and persistence in the metapopulations of habitat specialists and to quantify states and changes in a habitat of considerable conservation interest. As all the patches are on the same island, they form a closed system where species may persist, become extinct, or be colonised from neighbouring patches or patches on different islands. The dataset builds on a study design set up to monitor this hotspot habitat (Bakkestuen et al. 2014), following their habitat patch delineation and definitions of habitat specialist species. Sampling description: Fieldwork in 2009 was carried out by KB, OES, and AKW, all experienced botanists. Potential locations of dry calcareous grassland had been identified by aerial photos and distribution modelling. The patch locations were then visited by the fieldwork team and either discarded or confirmed and delimited. Polygon coordinate points were recorded using a handheld GPS with an estimated accuracy of 1-10 m. All three in the team looked for species in the same patch, one being in charge of writing down occurrences. When no new species were found, all three tried to find one more species each before declaring the list complete. Species lists per patch were noted in a notebook. Fieldwork in the 2019 and 2020 surveys was carried out by EL alone. Patches were found using a map with semi-transparent polygons over a recent aerial photo. Species presences were recorded in a spreadsheet on a smartphone. Patches were inventoried by carefully walking back and forth across the polygon to cover the entire area. EL took pictures of each patch. EL also took pictures of uncertain species with the iNaturalist (iNaturalist 2008) app to aid identification after fieldwork as it is illegal to collect plants inside the protected area.

General description
Quality control: We took several steps to quality-check the data. Each occurrence was evaluated manually to ensure the highest possible quality of data: after data collection was complete in 2019 and 2020, each observation was checked against the data from the previous year(s), occurrences in GBIF, and pictures taken during fieldwork. Some uncertainty was resolved using pictures stored in iNaturalist, where pictures were stored for individual plants along with their coordinates and other metadata. Some of these pictures were sent to OS, SLO and other experts to confirm uncertain species identifications. Additional trips to Gressholmen by EL along with SLO were carried out in September 2019, and alone in June 2020, to check uncertain presences and suspected false absences.
Step description: For 2009 and 2019, there are more data available than described here. These data include all vascular plant species, not just the specialists, and some additional descriptive variables for each patch.
In spite of quality control, some uncertainty remains. Delineating polygons inevitably involves uncertainties, and GPS polygon boundaries were not physically delimited in the field. In addition, changes in patch size may have occurred between 2009 and 2019, due to regrowth or other factors. To a lesser degree, this is also an issue between the 2019 and 2020 surveys. False absences may occur in the dataset, which is common in field surveys (Morrison 2016), either from incomplete survey effort or because the species was not detectable at the time of fieldwork. As flowering phenology varies, early-flowering species (e.g. Veronica arvensis, Draba verna) are especially susceptible to being omitted, even though special care was taken to look for them. Misidentification could also occur, though the availability of previous data and use of trained botanists with expert knowledge of the local flora should minimise this risk.

Geographic coverage
Description: Calcareous grasslands occur naturally fragmented on shallow soils by the Oslo Fjord, mainly on exposed and/or grazed Cambrian-Silurian marine sedimentary rock. The climate is relatively warm, with an annual temperature of 5.7°C (Aune 1993) and annual precipitation of 763 mm (Førland 1993). The area is amongst the most species-rich in Norway, as the combination of calcareous bedrock and warm summers is rare and hosts species that are mostly found further south in Europe.
The species were recorded in 15 patches of dry calcareous grassland on Gressholmen Island, situated in the inner Oslo fjord (Fig. 1). Habitat patches were identified through a combination of distribution modelling at 5 x 5 m resolution, interpretation of aerial photos, and in the field (Wollan et al. 2011). The mean patch size was 1078 m , range 264 m to 2382 m . Total habitat area, as the sum of all patch areas, was 48516 m .
Humans have shaped and impacted the habitats on Gressholmen, probably for as long as the Oslo fjord has been settled. Today, the Island is artificially connected to two other Islands, Heggholmen and Rambergøya. Most of this Island trio is protected as nature reserves, but has previously housed a shooting range, an airport, a soap factory with connected houses, and a paint factory. It has likely been grazed by domestic animals while the Island was inhabited. From the 1970s until its eradication in 2007, a large rabbit   year The year the occurrence was recorded. scientificName The scientific name of the recorded taxon, in Latin and including author name, mapped to the GBIF taxonomic backbone from the closest match in the Norwegian Species Nomenclature Database. kingdom The full scientific name of the kingdom in which the taxon is classified, here Plantae. taxonRank The taxonomic rank of the most specific name in the scientificName, for exampe, species or variety. Most of the occurrences were recorded on species level.
vernacularName Common name in Norwegian. Provides a link back to the original data collection sheet and to the Norwegian Species Nomenclature Database.