Species and habitat distribution
This study focuses on the native vascular flora of the Azorean archipelago, which includes 149 indigenous species, 59 of them being endemic (Schaefer et al., 2011). The Azores is an oceanic archipelago located close to the mid-Atlantic ridge in the North Atlantic Ocean, composed of volcanic islands of recent origin. It is one of the most remote in the world, being separated by 1376 km from the closest shore in the Iberian Peninsula (Florencio et al., 2021). Its islands are located along a NW - SE axis, with the oldest island, Santa Maria, being located in the southeast, whereas the youngest, Pico, is located in the central region of the Azores (see fig. 1). The climate is temperate and wet with mild summers following the Köppen Climate classification (Borges et al. 2019).
Species occurrences were extracted from the “ATLANTIS initiative in the Azores” GBIF dataset (GBIF.org). It comprises a total of 1,338,102 georeferenced records at a 500 x 500 m cell resolution (Borges et al., 2018), including native and non-native species. Taxonomic and temporal data cleaning procedure is detailed in Leo et al. (2021), resulting in 135,014 suitable records of native vascular plants.
We considered three types of habitats representing the dominant, less disturbed onesin most Azorean islands: native forests, characterized by a dense tree and shrub cover of small stature and currently restricted to high elevations in the islands (detailed description of the current types of native forest can be seen at Elias et al. 2016); naturalized vegetation patches mostly composed by native plants and located outside the native forest; and seminatural pastures, which are mid- and high- elevation pastures that maintain some native plants and are subject to short-term cattle grazing activity and low inputs of fertilizers (fig. 1). The first two categories were already delimited in the shapefiles developed by Picanço et al. (2017), and the latter corresponds to the areas of pastures located above 400 m. Altitudinal data were extracted from a digital elevation model with a 25 m spatial resolution of the islands (EU-DEM v1.1, 2021). Then, we calculated the area and percentage of each habitat type for each 500 x 500 m cell of the occurrences grid; in the case of seminatural pastures this was done by intersecting the altitudinal and pasture shapefiles. These analyses were performed using the QGIS Development Team software (2020). To evaluate if there were any differences in species richness between habitats, we built species accumulation curves for each habitat considering only cells covered with at least 50, 75 or 99% of the focal habitat (Supplementary Material 1). Since the curves at 50 and 75% stabilized at larger richness than the 99%, we used this latter threshold to assign a cell to a specific habitat. We set the threshold at 99% cover instead of 100% to consider a 1% of disagreement due to projection and processing variation of areas. Thus, cells with less than 99% of a specific unique habitat were discarded from the analyses. Due to the absence of cells matching this criterion in Santa Maria and Graciosa, and also the low number of cells identified in Corvo for at least one habitat, we excluded these three islands from this study (fig. 1). The number of cells retrieved from each island and for each habitat within islands is given in Supplementary Material 1.