The relief of Mount San Calogero consists of a rock outcrop of the Upper Triassic–Lower Lias (stromatolitic limestones with Megalodon of Late Triassic–Early Lias) (Basilone 2018), characterized by Meso-Cenozoic limestones assigned to the “Saccense Unit” (Lentini and Carbone 2014). Its importance, however, is mainly due to a complex system of hypogene karst cavities, which also includes around twenty hydrothermal caves (Guidi and Verde 2001; Badino et al. 2009; Badino 2013; Commissione Grotte Boegan e Associazione Geografica La Venta 2013; Guidi et al. 2014). This geosite is considered a world unique site (ORBD 2025c) due to its speleo-thermal peculiarities (Floriani 1979; Forti 1991) and its significant historical importance since prehistoric Sicily. Human presence is documented here as early as the 6^th century BC in the upper caves, and up to 4000 years ago in the intermediate ones (Verde 2021). The entrances located at the top of the relief, namely the “Stufe di San Calogero,” consisting of two natural steam-filled caves, called the Antro di Dedalo (or Grotta di San Calogero and Grotta degli Animali) are incorporated within an ancient thermal complex built near the Sanctuary of San Calogero, patron saint of Sciacca.

This complex network of cavities and tunnels remains only partly explored due to its extreme conditions, including fluctuating temperatures (15–37 °C) and nearly 100% humidity (Di Piazza et al. 2017). These closed or semi-closed habitats resemble caves, where darkness and nutrient scarcity allow only specialized organisms—such as arthropods and microorganisms—to survive, while others, like bats, use them temporarily (Souza Maysa and Ferreira 2011; Smith et al. 2012). Recent micromycological studies have also identified various fungal taxa, including species from the genera Penicillium, Cladosporium, and Trametes (Di Piazza et al. 2017).

From an ecological perspective, it remains unclear how these internal environmental conditions influence the external characteristics of the site (e.g., the temperature of lithological substrates and soils). Nevertheless, the presence of Periploca angustifolia and Searsia tripartita (La Mantia et al. 2025)—rare shrub species in Sicily and typical of pre-desert formations—clearly highlights their potential climacic role within the area.

Moreover, from a geological perspective, this sector of Sicily has recently been considered by some authors as part of the foreland, similar to the Hyblaean area, whose territories may also have emerged (at least partially) after the deposition of pelagic carbonates, called “Trubi” (Ferranti et al. 2019). According to this hypothesis, the biological colonization of this area could have been even earlier, with a phytogeographical evolution potentially very different and highly significant for Sicily itself (Geraci et al. 2009; Troia et al. 2012; Ilardi et al. 2020).

From a climatic perspective, the mean annual precipitation is 564 mm, distributed over an average of 65 rainy days; precipitation peaks in winter, while a pronounced and prolonged minimum occurs during the spring–summer period (La Mantia et al. 2025). The mean annual temperature is 17.9 °C, with minimum average temperatures in February (7.9 °C) and maximum average temperatures in July (30.6 °C). According to the bioclimatic classification of Rivas-Martínez et al. (2004), the area falls within the lower dry Thermomediterranean belt (La Mantia et al. 2025).

From a biogeographical standpoint, the area is classified within the Mediterranean Region and the Central Mediterranean Subregion (Rivas-Martínez et al. 2004), and further assigned to the Sicilian Province, Eusiculo Sector, Western Subsector, and Drepano–Panormitano District (Brullo et al. 1995).

The land use of the Mount San Calogero area reflects a long history of human activity, from prehistoric therapeutic and religious visits to quarrying, agriculture, and afforestation. Historical toponyms such as Thermae Selinuntinae and Aquae Labodes attest to the region’s thermal heritage (Verde 2021). Quarrying of limestone outcrops (“Cave di pietra”) and extensive agriculture, including dry cultivation of carob and almond trees and sheep grazing, once shaped the landscape. Around the rocky areas, naturalized stands of Opuntia ficus-indica and Agave species mark past human cultivation.

Today, much of the territory forms part of the Sciacca publicly owned Forest within a regional nature reserve managed by the Sicilian Forestry Service. Since 1988, afforestation—mainly with Pinus halepensis, Cupressus sempervirens, and Eucalyptus camaldulensis—has altered the original low maquis dominated by Periploca angustifolia. The area has suffered repeated wildfires, notably in 1997, leading to the loss of large, forested zones and recurrent vegetation degradation (La Mantia et al. 2025). In recent years, additional fires have occurred, highlighting a trend of increasing frequency. According to data from the Regional Forestry Information System, fires in this area have recurred almost annually since 2008, except in 2013 and 2022 (La Mantia et al. 2025). In the same fire-cleared clearings, numerous Periploca angustifolia individuals have regenerated from seeds, evidently dispersed by wind from residual plants located along hedges and at the margins of afforested areas. More developed shrubs are sometimes found entwined with dried branches or even the charred trunks of old conifers consumed by fire. The species tends to create sparse garrigue and low maquis, part of a complex phytocoenotic mosaic, interspersed with the more extensive Hyparrhenia hirta grasslands, as secondary aspects of two distinct native vegetation series: i) Olea europaea on rock outcrops; ii) Searsia tripartita on stony-detritic substrates.

In the surrounding coastal belt, ongoing residential expansion has further transformed the landscape, encroaching toward borders of the protected area. Other land-use types are also notable in the adjacent coastal strip, where in recent decades, intense residential urban expansion has been recorded, approaching some parts of the aforementioned protected areas.

From a floristic and phytosociological viewpoint, the area was little known until now, except in general terms. In fact, the toponym “Sciacca at Monte S. Calogero” (or “Monte Kronio”) is scarcely mentioned in the most important classical floristic works, such as those by Gussone (1827, 1843) and Lojacono (1888–1889, 1891, 1902–1903, 1904–1907, 1906, 1908–1909). As also noted in Giardina et al. (2007), for this locality there are only sporadic bibliographic references, mainly concerning Gussone’s collections from 1827 and 1828. In particular, these include: Searsia tripartita (La Mantia et al. 2025), Astragalus caprinus subsp. huetii, Micromeria microphylla (Gussone 1834), Stipa barbata [Sciacca at S. Calogero (Gussone 1827)].

For other taxa, some specifically listed in the Standard Form of the Natura 2000 site (ORBD 2025c), there are only general bibliographic references (e.g., “Sciacca”), as in the cases of Lonas annua, Malva flava (sub Lavatera agrigentina), and Glandora rosmarinifolia [sub Lithodora rosmarinifolia]. For these taxa, given the absence of specific exsiccata from the area and/or the rarity of their preferred habitats, confirmation of their actual presence within the Natura 2000 site is necessary.

Indeed, among other geobotanical publications for this area, only a few reports can be be mentioned: the sites of the endemic Brassica villosa subsp. tineoi (Ottonello 1978; Raimondo et al. 1991), and forest aspects of Olea europaea stands of habitat 9320, sensu Directive 92/43/EEC (Rivieccio et al. 2021), as well as a more recent study on a small residual maquis patch of Searsia tripartita (La Mantia et al. 2025).

However, comprehensive monographic studies—both on the flora and the vegetation of this protected area—are still lacking. Such investigations would be highly desirable, particularly in light of recent and unexpected discoveries of species with North African affinities, including Searsia tripartita and Periploca angustifolia.

Periploca angustifolia is a shrubby liana belonging to the Apocynaceae family, reaching 2(–3) m in height and adapted to particularly hot and dry climatic conditions (see Suppl. material 1). It has perennial stems with longitudinally striated bark, bearing short, rigid branches, sometimes twisted at the tips, with small terminal shoots. The leaves are lanceolate, coriaceous, either evergreen or summer-deciduous, the upper opposite; the blade is narrowly ovate to oblong-lanceolate, cuneate at the base and obtuse at the apex, glabrous, and subsessile. Flowers are grouped (2–15) in small axillary cymes; they have a corolla about 1 cm in diameter, with glabrous petals, lobes yellowish-green and purplish-brown on the inside. The fruits are narrowly conical follicles (0.6 × 6–10 cm), often paired and divergent. Seeds are dark (7 mm), with white woolly hairs (25–40 mm) that are easily dispersed by wind (anemochory) (Fig. 4a). Although it produces latex, the species is particularly appreciated by livestock, especially in the arid environments of North Africa; in Tunisia it also has medicinal value (Dghim et al. 2018).

The study was conducted through preliminary field surveys, initiated at the beginning of spring 2025, following the incidental discovery of Periploca angustifolia by one the authors, documented by photographs posted on posted on the web (Acta Plantarum 2025). Several excursions were subsequently carried out, aimed at spatially locating the population, monitoring the ecological-stational features of the site, and conducting a preliminary survey of the flora and vegetation of the garrigue and low maquis it forms, as well as the role it plays within the landscape. Relevant literature was also consulted, along with herbarium investigations at the Herbaria of Palermo (PAL) and Catania (CAT), to define an updated distribution range of the species in the regional context, previously known only from some small circum-Sicilian islands (Linosa, Lampedusa, Pantelleria, Marettimo, Levanzo, Favignana), as well as the Maltese Islands, but not from Sicily itself.

For the geolithological characterization of the area, we mainly referred to Basilone (2018) and Lentini and Carbone (2014), as well as to various other publications concerning the geomorphological and hydrothermal aspects of the site (Badino 2013; Verde 2021). Climatic data were derived from the nearby thermopluviometric station of Sciacca (La Mantia et al. 2025), considering the dataset from 1926–1985 analyzed by Duro et al. (1996).

The geobotanical investigation was expanded to include a phytosociological, synecological, and syndynamic characterization of the vegetation landscape, with particular attention to the shrub formations of Periploca angustifolia, primarily established in clearings within afforested areas along the southwestern slope of the hill. Twelve phytosociological relevés were performed in spring, on homogeneous patches of low maquis-garrigue vegetation, although partly discontinuous and of limited extent. The sampled surfaces ranged between 50–100 m², at elevations between 143 and 278 m a.s.l. The relevés were then assembled into a phytosociological table (Table 1). Moreover, it was used for further floristic–phytocoenotic comparison with other analogous communities dominated by Periploca angustifolia and/or related taxa (included in the alliance Periplocion angustifoliae), so far described for the islands of the Sicilian Channel. A synoptic table was subsequently compiled (Table 2), including 22 phytosociological tables and 184 relevés, with the aim of distinguishing groups of characteristic and differential elements within the various syntaxa, as well as the most representative companion species.

Species identification was primarily based on Pignatti et al. (2017–2019), whereas the taxonomic nomenclature followed the Portal to the Flora of Italy (2025), except for Searsia tripartita and Lycium schweinfurthii [according to Plants of the World Online (POWO 2025)]. The phytosociological nomenclature follows the criteria of the International Code of Phytosociological Nomenclature (ICPN: Theurillat et al. 2021), while syntaxonomic classification was based on Mucina et al. (2016) and Brullo et al. (2009, 2020).

The communities reported in the above synoptic table were further classified through cluster analysis, in which frequency classes were converted into numerical values according to van der Maarel scale (1979). Hierarchical cluster analysis was performed using the Bray–Curtis dissimilarity index. An agglomerative hierarchical clustering algorithm with Ward.D2 linkage was applied, minimizing within-group variance. NMDS (Non-metric Multidimensional Scaling) was used to graphically represent floristic relationships among the relevés in a two-dimensional space. Furthermore, a PERMANOVA (Permutational Multivariate Analysis of Variance) was carried out to test whether the studied associations differed significantly in composition, based on a Bray–Curtis distance matrix. Analyses were conducted with R software, version 4.5.1 (R Core 2025), using the “vegan” package (Oksanen et al. 2006) for NMDS, PERMANOVA, and Bray–Curtis distance matrix computation. The “cluster” and “stats” libraries were used for hierarchical clustering, while “ggplot2” and “matplotlib” (via Python) were employed for graphical visualization.

The first records of Periploca angustifolia in the Islands of the Sicilian Channel were provided by Gussone (1827) in the Aegadian Archipelago, where he reported it on the three largest islands (Favignana, Levanzo, and Marettimo). The same author later also indicated its presence on the islands of Pantelleria, Lampedusa, and Linosa (Gussone 1832). In the Maltese Archipelago, it was reported by Caruana Gatto (1890), under the name Periploca laevigata. Further mentions of the species were given by Riccobono (1906), within an account of his collections from Linosa and Lampedusa, where he also simultaneously indicated it in Sicily at “Modica and Girgenti.” However, for these latter localities, no herbarium exsiccata are known, nor subsequent confirmations by other authors have been provided.

Another specimen of the species collected by the same author must also be noted, preserved at the Palermo Herbarium (PAL63537!). It consists of three portions of twigs of the species, on the label of which-handwritten by Riccobono himself-appears the note: “Foresta Capaci – Cultivated. 4/25/1887, Riccobono.” This clearly indicates the origin of the sample, taken from a cultivated plant (in Contrada Carrubbella, where the toponym “Case Riccobono” still exists today).

In addition to this newly documented population on Mount S. Calogero, the current distribution of the species in the Sicilian Channel area—compiled from bibliographic references and unpublished data—includes the following additional localities:

- Marettimo island: Punta Bassana (Gianguzzi et al. 2006); - Favignana island: C.de Boschitto, Montagna Grossa o Grande, Punta Faraglione (Di Martino and Trapani 1967); - Levanzo Island: Faraglione, Pietre Varate, Costa Dogana, Serra Alberello, Cala Fredda (Di Martino and Trapani 1968; Romano et al. 2006); - Pantelleria island: C.da Fram, C.da Camilli, C.da Polacca, Punta Polacca, C.da Sataria (Brullo et al. 1977; Gianguzzi 1999); - Lampedusa (frequent along the coastal belt and in the dry rocky slopes; Bartolo et al. 1988) and Linosa Islands (common in rocky habitat in various localities, as Panzarella, Arena Bianca, Montagna Rossa and Monte Vulcano; Brullo and Siracusa 1996; Malta (Mistra Valley, Salina Bay, Ghar Lapsi, Naxxar, Mnajdra near Qrendi, Victoria Lines, Ghar Lapsi), Gozo (Xlendi Valley, Triq Kalati Punic) and Comino islands (Brullo et al. 2020).

Table 1 reports 12 relevés carried out in the low maquis community with presence/dominance of Periploca angustifolia, localized within the study area, which is here proposed as a new association, described below.

Asparago albi-Periplocetum angustifoliae Gianguzzi, La Mantia et Cambria ass. nov.

Holotypus: Rel. 3, Table 1

Phytosociological data: Table 1 and Table 2 (column 1).

Diagnostic species: Periploca angustifolia (dominant), Asparagus albus, Ampelodesmos mauritanicus, Carlina sicula subsp. sicula, Pistacia terebinthus, Chamaerops humilis.

Short description: Primary or secondary low maquis, 2–3 m tall, more or less dense or open, occurring on xeric, rocky-stony slopes, with high frequency and sometimes dominance of Periploca angustifolia. It is associated with several elements of the alliance Oleo-Ceratonion siliquae and the order Pistacio lentisci–Rhamnetalia alaterni (e.g., Chamaerops humilis, Olea europaea, Pistacia lentiscus, Asparagus albus, Teucrium fruticans, Stachys major, as well as of the class Quercetea ilicis (e.g., Ampelodesmos mauritanicus, Asparagus acutifolius, Pistacia terebinthus, Arisarum vulgare, Allium subhirsutum).

Based on the data reported in Table 1 and in the synoptic table (Table 2), the specific species combination that characterizes the new association, here proposed under the name Asparago albi-Periplocetum angustifoliae, includes the following differential taxa (all more or less frequent within the community, and rare or virtually absent on the small islands of the Strait of Sicily):

Substrates: Rocky calcareous and/or stony-debris slopes, edaphically poor or with shallow soils, occurring at the base of rock walls exposed to the South/South-East.

Bioclimate: Mediterranean oceanic pluviseasonal; lower thermomediterranean thermotype; lower dry ombrotype, tending toward semiarid conditions.

Syntaxonomic notes: The Asparago albi-Periplocetum angustifoliae ass. nov. is framed within the alliance Periplocion angustifoliae (order Pistacio-Rhamnetalia alaterni, class Quercetea ilicis). The distribution area of the alliance comprises the various islands of the Strait of Sicily [where 10 different associations have been described, summarized in the synoptic table (Table 2)], the small southern Aegean islets (Brullo and Guarino 2000), and extends across the Mediterranean fringe of Africa as far as southern Spain (Rivas-Martínez 1975).

Vegetation series: In its most typical form, the association Asparago albi-Periplocetum angustifoliae ass. nov. constitutes a garrigue or low maquis vegetation, acting as a transitional community linked to the following two xerophilous vegetation series: i) the Ruto chalepensis-Oleo sylvestris sigmetum is widespread on rocky carbonate substrates (Gianguzzi and Bazan 2020; Bazan et al. 2021) and its regressive stages are represented by grasslands dominated by Hyparrhenia hirta (Hyparrhenietum hirto-pubescentis) and by small therophytic grasslands with Trachynia distachya (class Stipo-Trachynietea distachyae); ii) the Calicotomo infestae-Searsio tripartitae sigmetum is typical of xeric stony-detrital substrates along eroded slopes. Its secondary aspects are again represented by the same vegetation types described for the previous series. The potential area of these two vegetation series is heavily disturbed by anthropogenic activities (e.g., afforestation, cultivation, more or less urbanized zones). In the upper part of Mount San Calogero, the same series are in catenal contact with the microgeosigmetum of cliffs, marked by sparse chasmophytic aspects referred to the alliance Dianthion rupicolae (class Asplenietea trichomanis) or to the association Capparidetum rupestris (class Parietarietea judaicae); in the lower part, they likely extend almost to the coast (Fig. 5).

Synchorology: The association is endemic to this portion of the southern Sicilian coast near Sciacca, representing a geographical vicariant of association Periploco-Euphorbietum dendroidis, which is found on several small islands of the Strait of Sicily (Brullo and Marcenò 1985; Brullo et al. 2009; Gianguzzi et al. 2016).

The dendrogram resulting from the cluster analysis performed on the associations listed in the synoptic table (Table 2) highlights the presence of two main clusters (Fig. 6). The first includes the associations Asparago acutifolii-Ziziphetum loti, Ephedro fragilis-Pistacietum lentisci, and Asparago horridi-Retametum gussonei, all occurring only in Sicily (where Periploca angustifolia is entirely absent), while the second brings together a more complex and diverse set of communities, subdivided into 10 groups.

Within the first group of this second cluster, two associations dominated by Searsia tripartita can be distinguished: the Calicotomo infestae-Rhoetum tripartitae (also restricted to Sicily and lacking Periploca angustifolia) and the Periploco angustifoliae-Rhoetum tripartitae (exclusive to the island of Linosa).

The remaining associations are distributed across the other 9 groups, as follows:

Overall, it emerges that the most frequent and widely recognized syntaxon across the different islands of the Strait of Sicily—the association Periploco angustifoliae-Euphorbietum dendroidis—shows a certain floristic heterogeneity, splitting into different subclusters depending on geographical provenance. All other communities, on the other hand, tend to separate rather clearly from one another, owing to their markedly different floristic composition.

As for the Asparago albi-Periplocetum angustifoliae ass. nov., it shows a certain autonomy compared to the other communities considered, displaying greater similarity with the aspects of Periploco angustifoliae-Euphorbietum dendroidis subass. typicum found on Favignana Island (Aegadian Archipelago).

The NMDS analysis (Fig. 7) yields similar results, with the relevés of Periploco angustifoliae-Euphorbietum dendroidis broadly grouped into a rather wide and irregular cluster, while the other communities remain fairly distant from each other.

Regarding the Asparago albi-Periplocetum angustifoliae ass. nov., also in this case it shows greater affinity with the relevés of Periploco angustifoliae-Euphorbietum dendroidis from the Aegadian Archipelago, geographically the closest.

Finally, the PERMANOVA analysis highlights significant differences in the floristic composition of the various communities investigated (F = 3.52; p < 0.001), confirming that the dissimilarities among the different associations are greater than their internal variability, and thus justifying the syntaxonomic treatment proposed.