The studied maquis is located within the “Monte San Calogero (Monte Kronio)” Nature Reserve, according to the Regional Decree of the Territory and the Environment no. 366/44 (26 July 2000) and managed by the Department of Rural and Territorial Development. The reserve spans an area of 52.25 ha. Additionally, a Natura 2000 site (SAC ITA040009) is located within the area, which spans 112.538 ha; it is located northeast of the town of Sciacca (province of Agrigento), on the southern coast of Sicily (Fig. 4). Biogeographically, it falls within the Central Mediterranean Subregion of the Mediterranean Region, following the classification proposed by Rivas-Martínez et al. (2004). On a local scale, according to Brullo et al. (1995), it is part of the Sicilian Province, Eusiculo Sector, Western Subsector, Drepano-Panormitano District.

The study area is characterized by limestone outcrops belonging to the Meso-Cenozoic carbonate succession known as the “Saccense Unit”, dominated by calcareous rocky outcrops, namely stromatolitic limestones with Megalodon, dating back to the Upper Triassic-Lower Lias (Lentini and Carbone 2014; Basilone 2018). It culminates at Mount San Calogero (397 m a.s.l.), a rocky hill characterized by a complex karst system rich in caves known as the “Stufe di San Calogero”, “Grotta del Lebbroso”, “Grotta di Mastro”, “Grotta del Gallo” and “Grotta di Cucchiara” (Gullì 2013). This is influenced by secondary volcanic activity. The average temperature of the thermal vapors is around 38 °C (sometimes rising higher) which impedes a complete exploration of the site. The site also features well-known thermal baths, used historically for therapeutic purposes, built next to the Sanctuary of San Calogero, the patron saint of Sciacca (Badino and Torelli 2014).

Based on data from Sciacca’s thermo-pluviometric monitoring station (Duro et al. 1996), the local average annual rainfall is 564 mm, distributed over 65 rainy days (Table 1).

The average annual temperature is 17.9 °C, with a minimum of 7.9 °C in February and a maximum of 30.6 °C in July. According to Rivas-Martínez et al. (2004) bioclimatic classification, the area falls within the lower dry thermomediterranean bioclimatic belt (Bazan et al. 2015; Gianguzzi et al. 2016).

This study was carried out between the spring of 2023 and the summer of 2024. Besides field vegetation surveys, all the relevant literature and the available herbarium specimens of Searsia tripartita in Sicily [PAL, CAT, and Gussone Herbarium (NAP)] were checked, to compile data on the past and present regional distribution of the species.

Vegetation was surveyed adopting the Zurich-Montpellier methodology. The observed plant taxa were identified using Pignatti et al. (2017–2019), following the nomenclature from the Portal to the Flora of Italy (2024), aside from S. tripartita, for which we referred to Plants of the World Online database (POWO 2025). Repeated field surveys were carried out between 2021 and 2024 that enabled the collection of several phytosociological relevés, adopting the abundance-dominance indices proposed by Braun-Blanquet (1964). The phytosociological relevés were carried out both in spring (during the optimal vegetative period of the investigated community) and in late summer (with most of the species in vegetative stasis); this was done to better perceive variations in the floristic composition and the degree of coverage of the species present in the maquis vegetation.

For the syntaxonomic interpretation of the surveyed plant community, our field data was compared to that reported in existing literature (Brullo and Marcenò 1985; Brullo et al. 2009). Syntaxon names cited in the text agree with the International Code of Phytosociological Nomenclature (ICPN: Theurillat et al. 2021), whereas their syntaxonomic treatment agrees with Mucina et al. (2016). To produce a synoptic overview of phytosociological data concerning the Sicilian syntaxa with S. tripartita framed into the alliance Periplocion angustifoliae, reference was made to Brullo et al. (2009) and several recent works, cited in the text.

A diachronic analysis was conducted on vegetation dynamics in the areas occupied by maquis within the study site, using historical images from the Regional Orthophoto Repository or available on Google Earth (over 27 years, from 1998 to 2024).

Phylogenetic analyses of the Rhus complex, using DNA and gene spacers (Miller et al. 2001; Yi et al. 2004), have shown that the genus Searsia is clearly monophyletic and strikingly distinct from Rhus s.s. (Moffett 2007). Below, we provide a taxonomic prospect of all the epithets related to this species.

Searsia tripartita (Ucria) Moffett in Bothalia 37: 173 (2007)

≡ Rhamnus tripartitus Ucria in Nuov. Racc. Opusc. Aut. Sicil. vi (Pl. Linn. Op. Addend. et Secund. Linn. Syst.): 249 (1793)

≡ Rhus tripartita (Ucria) Grande in Bull. Orto Bot. Regia Univ. Napoli 5: 62 (1918)

= Rhus oxyacantha Schousb. ex Cav., Anal. Cienc. Nat. 3: 36 (1801)

= Rhus oxyacanthoides Dum.-Cours., Bot. Cult. 3: 359 (1802)

= Rhus crataegiformis Pers., Syn. 1: 326 (1805)

= Rhus dioica Brouss. ex Willd., Enum. Pl.: 325 (1809)

= Rhus ziziphina Tineo, Pl. rar. Sicil. Pugill. 1: 8 (1817) (nom. illeg.)

Sub Rhus dioicum [s.d., s.l., Gussone (NAP); Sciacca a S. Calogero; s.d., s.l., Gussone (NAP), two labels, the first reporting “Rhus dioicum Will. p. 328; – oxyacanthioides Pers.; – zizyphina Tin. Martio, Aprili. In collibus aridis calcareis”, while in the second we read “Rhus oxyacanthoides Dum. Sciacca a S. Calogero” (Fig. 3d); s.d., s.l., Todaro (PAL); in collibus sterilibus calcareis. Palermo a Monte Pellegrino / rupi calcaree colline marittime Villa Belmonte, gennaio, Todaro (PAL); ex insula Linosa/ in collibus aridis, aprile 1893, Sommier (PAL); rupi calcaree colline marittime Villa Belmonte, marzo, Lojacono-Pojero (PAL); Linosa, 9 giugno 1840, Ajuti (PAL)]; sub Rhus zizyphina [Linosa, 9 giugno 1848, s.c. (PAL); Belmonte, 26 febbraio, s.c. (PAL); S. Croce (near Palermo, authors’ note), s.d., s.c. (PAL); in collibus Palermo alle falde di M. Pellegrino, s.d., s.c. (PAL); Linosa, giugno, (Gussone) (PAL); S. Croce, s.d., s.c. (PAL); s.l., s.d., Gussone (PAL)]; sub Rhus tripartita [Linosa M. Vulcano, s.d., G. Scafidi (PAL); Scoglitti, s.d., s.c. (PAL); Scoglitti: neighborhoods, 36°52'N, 14°27'E, 16/06/1990, Raimondo et al. (PAL); Sampieri (Pozzallo), 07/10/968, S. Brullo, (CAT035332); Sampieri (RG), 30/04/2010, D’Agostino, Minissale e Sciandrello (CAT009735); Sampieri c.da Trippatore, 30/04/2010, Sciandrello (CAT); Cava d’Aliga, 30/04/2010, Sciandrello (CAT)].

Apart from the locality that is the focus of the present study, in Sicily’s regional area Searsia tripartita currently grows spontaneously in the southern and eastern sectors of the Hyblaean area and in Linosa Island. It was not possible to confirm the historical reports for the western coasts of Sicily, regarding San Vito Lo Capo (for which no herbarium specimens are present) and Santa Flavia (near Palermo).

S. tripartita usually dominates the shrubland where it grows (Brullo et al. 2009). It sometimes co-occurs with the congeneric Searsia pentaphylla (Jacq.) FA. Barkley [= Rhus pentaphylla (Jacq.) Desf.)], another woody species that also has a North African distribution (Pignatti et al. 2017–2019). However, it should be noted that the distinguishing features between these two shrubs are quite subtle and primarily concerning the leaves (in S. tripartita, the leaves are divided into 3 obovate-digitate segments, while in S. pentaphylla, they are divided into 3–5 digitate segments that are linear-spatulate, entire, or with 3–5 teeth at the apex) and the size of the plant (S. tripartita does not exceed 3 meters in height, whereas S. pentaphylla can reach up to 5 meters). Both species are very rare and may co-occur in extremely arid coastal habitats. Indeed, as emphasized by Lojacono-Pojero (1891, translated by authors), “both species grow intermixed in several places in Sicily... They are highly polymorphic in their foliage, and whatever Gussone may say […], they cause a serious aberration in the specific value attributed to them. Moreover, habitus is not a reliable clue for distinction; apart from the size of the leaves, I believe there is no character that is not shared by both species. In my opinion, the mixed growth has led to hybridizations that have produced intermediate forms”.

Regarding the earliest records concerning both species, some misleading bibliographic inconsistencies need to be clarified. According to Tineo (1817) S. pentaphylla (under Rhus thezera) grows “prope Xaccam” [= near Sciacca], whilst S. tripartita (under Rhus ziziphinus) only occurs “in montosis S. Flaviae” [Santa Flavia, district of Palermo] and in “S. Vitu” [= San Vito Lo Capo, district of Trapani]. Since much of Tineo’s collection was destroyed during the uprisings of 1820 (Borzì 1897), no specimen was available to confirm the identity of the plant in Sciacca and to ascertain that he personally observed it. It is possible that Tineo received this information from Gussone, whose herbarium (NAP) still contains two specimens bearing the toponym “Sciacca”. The first bears the handwritten note “Rhus dioicum. Sciacca a S. Calogero”, while the second has two herbarium labels, one reading “Rhus dioicum Will. p. 328; – oxyacanthioides Pers.; – zizyphina Tin. Martio, Aprili. In collibus aridis calcareis”, while the other reads “Rhus oxyacanthoides Dum. Sciacca a S. Calogero”. Although the labels of both specimens have no date, Gussone writes in his field excursion manuscript notes (Gussone unpublished) that he was there twice (May 12, 1817, and May 2, 1818). Thus, Gussone may have communicated the discovery of this nucleus before the publication of the first volume of Pugillus by Tineo (August 1817).

Later, Gussone (1827) reports S. pentaphylla (under Rhus thezera Pers.) for “Misilmeri, Bagheria, Sciacca, S. Croce, Comiso” and S. tripartita (and Rhus “dioicum” Willd.) for “S. Vito (Tin.), S. Croce, Sciacca, Scicli”. Gussone (1843) also adds Linosa for S. pentaphylla (under Rhus “pentaphyllum” Desf.). Based on our investigations, only individuals of Searsia tripartita were found within the study area.

After the discovery of the nucleus of maquis with S. tripartita on Mt. S. Calogero, the surroundings were systematically explored to assess the size and the conservation status of the population, to better identify the ecological requirements and the threats affecting the species and to survey the plant community where it grows. S. tripartita is located in a restricted area on the southern slope of a rocky hill, between 290 and 330 m a.s.l. (WGS84 coordinates: 41.54038°N, 33.3369°E). Specifically, the population forms a rather dense low maquis patch covering approximately 3,060 m² and consisting of around 70 mature individuals; however, as denoted by the findings reported in Table 2, natural regeneration of the species was not observed during our survey period.

This is another relict station of the species (Biondi et al. 2009; Mucina et al. 2016), as also demonstrated by its historic presence on the site, as already documented by Gussone (1827). Although the vegetation occupies a small area, the habitat is highly unique and conductive to its survival, considering the continuous and intense anthropic disturbance the vegetation still constantly faces today (fires, reforestation, grazing).

A diachronic analysis allowed us to trace the recent evolution of this maquis nucleus by comparing the images from the Regional Orthophoto Repository (Geoportale Regione Siciliana 2024) or accessible on Google Earth (over 27 years, from 1998 to 2024). Photogrammetric interpretation (Fig. 5a–d) showed that during this time lapse this nucleus has remained almost unchanged in terms of the number of individuals, overall distribution pattern and occupied surface area. The steep slope hosting this nucleus appears quite xeric, as it is south-facing and presumably influenced by thermal vapors associated with the system of karstic-subterranean cavities characterizing the entire area.

The S. tripartita plants grow in a very steep area, characterized by numerous rocky outcrops intermittently covered by scree made of small incoherent clasts resulting from erosion processes affecting the slopes close to the top of Mt. S. Calogero. At the mountain summit, the thermal bath buildings stand alongside the Sanctuary of San Calogero, patron of the city of Sciacca, testifying that the surveyed site has been used by humans since ancient times.

Until the 1990s, the southern slope of Mt. S. Calogero was occupied by extensive almond orchards, occasionally interspersed with carob trees, alongside pastures and shrublands near the cliffs, within which the S. tripartita maquis was likely present. As documented by local testimonies, starting in 1990 this area underwent large-scale plantations of Pinus halepensis and Eucalyptus camaldulensis subsp. camaldulensis, before being incorporated into the newly established nature reserve and managed by the Regional Sicilian Forest Service. However, subsequently, the whole area and the entire hillside was struck by some significant wildfire events. A first major fire occurred in 1997, with slight damage in the lower part of the reforested area, where only the understory was affected, experiencing a light surface fire. The fire intensified further uphill, where the flames grew significantly taller, completely destroying the forest plantation and probably reaching the maquis with S. tripartita as well.

The problem of fires in this territory has become even more acute in recent years. In fact, according to the records reported in the Forestry Information System of the Region of Sicily, in the area of Mt. S. Calogero fires have occurred annually since 2008 to date, except for the years 2013 and 2022.

Consultation of the historical imagery available in the Regional Orthophoto Repository from 1998 proved invaluable for understanding the evolution of the landscape where the nucleus of S. tripartita maquis is located (Fig. 5a). The year after the first fire event, the green crowns of S. tripartita are easily recognizable and show evident signs of vegetative recovery after burning. Further diachronic sequences up to the present can also be tracked through other available satellite images from June 2013 (Fig. 5b), August 2014, May 2015, June 2016, October 2017 (Fig. 5c), and March 2024 (Fig. 5d). An image analysis revealed the gradual recovery of local maquis until June 2016, while the subsequent image from October 2017 (Fig. 5c) once again shows clear signs of extensive damage to the vegetation caused by a second summer fire event. Nonetheless, in recent years, the vegetation has been able to recover rapidly, recolonizing the habitat’s surface and showing remarkable resilience. This further demonstrates its unique ecology, which emphasizes the remarkable subclimactic potential of the maquis vegetation.

Focusing on the same color images shown in Fig. 5b – as well as on the others – the different colorations of the vegetation stand out, reflecting the various seasonal periods in which the photos were taken, highlighting the interesting phenomenon of the species estivating behavior. Indeed, during the winter-spring period – which corresponds to the vegetative peak of the community – the vegetation canopy typically appears spotted with the light green characteristic of S. tripartita. This contrasts sharply with the images from the summer-autumn months, where the brown color of the trunks and leafless branches of the dominant species is also prominent.

In Table 2, we present six relevés carried out within the investigated shrubland dominated by S. tripartita. The first four were carried out during spring, when the community was in full vegetation, while the last two were carried out at the end of July, when S. tripartita, Euphorbia dendroides, Asparagus albus, and other summer-deciduous woody plants were completely leafless, and the aboveground organs of almost all the perennial herbs were dry. The community is referred to the association Calicotomo infestae-Rhoetum tripartitae, described by Bartolo et al. (1982) for eastern Sicily.

Calicotomo infestae-Rhoetum tripartitae Bartolo, Brullo et Marcenò 1982

Holotypus: in Bartolo et al. (1982): rel. 6 of table 30.

Phytosociological data: Tables 2, 3 (columns 1 and 2).

Diagnostic species: Searsia tripartita (dom.), Cytisus infestus subsp. infestus.

Short description: Dense, intricate, and impenetrable shrubland, with a clear dominance of S. tripartita, reaching heights of 1.5–3 m. Other recorded species in the woody layer, which are all sporadic, include Euphorbia dendroides, Cytisus infestus subsp. infestus, Olea europaea, Teucrium fruticans and Chamaerops humilis. The herbaceous layer is also somewhat sparse, mainly consisting of rhizomatous and bulbous species such as Acanthus mollis, Arisarum vulgare, Allium subhirsutum, found beneath the very dense canopy. S. pentaphylla does not occur in the similar plant communities previously described for the coastal limestone substrates of southeastern Sicily (Bartolo et al. 1982; Brullo et al. 2009).

Substrate: Rocky coastal S-facing slopes on shallow and nutrient-poor and stony soils, on calcareous substrates.

Bioclimate: Mediterranean pluvioseasonal-oceanic, lower thermomediterranean thermotype, lower dry ombrotype.

Syntaxonomic notes: The association Calicotomo infestae-Rhoetum tripartitae is referred to the alliance Periplocion angustifoliae (order Pistacio-Rhamnetalia alaterni, class Quercetea ilicis); according to Mucina et al. (2016), this alliance includes “thermomediterranean semiarid deciduous relict low matorral of the coastal regions” in the south-western Mediterranean (Rivas-Martínez 1975). It is characterized by summer deciduous species, specifically S. tripartita, S. pentaphylla, Periploca angustifolia, and Ziziphus lotus subsp. lotus [along with other North African species such as Gymnosporia senegalensis subsp. europaea and Calicotome intermedia, which however are not present in the area of Sea of Sicily]. The alliance distribution range includes the coasts of the western part of the Euro-Mediterranean Region (Spain, Sicily) and North Africa (Rivas-Martínez 1975) as well as the islands south of Crete (Brullo and Guarino 2000). In the Tyrrhenian area, it is present in some small circum-Sicilian islands of the Strait of Sicily (Egadi and Pelagie archipelagos, Pantelleria Island and Maltesi Island) and along the coasts of south-eastern Sicily (Brullo et al. 1977; Brullo and Marcenò 1985; Brullo et al. 2009; Gianguzzi et al. 2016). Similar association to the Calicotomo infestae-Rhoetum tripartitae is the Periploco angustifoliae-Rhoetum tripartitae, also dominated by S. tripartita, located on the Island of Linosa, at Montagna Rossa (Brullo et al. 2009). In addition, in the regional area of Sicily other rare associations of the alliance Periplocion angustifoliae are described, in which S. tripartita is absent. They are as follows: a) Periploco angustifoliae-Euphorbietum dendroidis [Islands of Levanzo, Favignana, Marettimo (Brullo and Marcenò 1983; Brullo et al. 2009), Linosa (Brullo and Siracusa 1996), Lampedusa (Bartolo et al. 1990), Pantelleria (Brullo et al. 1977; Gianguzzi 1999) and Malta (Brullo and Marcenò 1985)]; b) Periploco angustifoliae-Juniperetum turbinatae [Islands of Lampedusa (Bartolo et al. 1990), Linosa (La Mantia 2003) and Pantelleria (Gianguzzi 1999); it has also been recorded on Khrisi, a small island south of Crete (Brullo and Guarino 2000) and Tunisia (Brullo et al. 2009)]; c) Lycio intricati-Pistacietum lentisci [South Sicily, between Palma di Montechiaro and Scoglitti (Brullo et al. 2000, 2009; Turrisi et al. 2002)]; d) Asparago stipularis-Retametum gussonei [South Sicily, between Gela and Punta Braccetto (Brullo et al. 2000, 2001, 2009)]; e) Asparago acutifolii-Ziziphetum loti: this association [described for the coastal strip of the north-western sector of Sicily and previously classified into the Oleo-Ceratonion siliquae (Gianguzzi et al. 1996)] should also be referred to the Periplocion angustifoliae where it also constitutes a thermoxerophilous scrub dominated by Ziziphus lotus, a summer deciduous species characteristic of this alliance.

Vegetation series: Southern Sicilian, dry thermomediterranean climatophilous series of the African sumac: Calicotomo infestae-Searsio tripartitae sigmetum. Secondary aspects are represented by the xerophilic perennial grassland dominated by Hyparrhenia hirta (Hyparrhenietum hirto-pubescentis A. et O. Bolòs et Br.-Bl. in A. and O. Bolòs 1950) and annual swards within the class Stipo-Trachynietea distachyae S. Brullo in S. Brullo et al. 2001. On carbonate rock outcrops, it is replaced by the wild olive series (Ruto chalepensis-Oleo sylvestris sigmetum; Gianguzzi and Bazan 2020), while on rocky walls it is dynamically connected with cliff microgeosigmetum, mainly characterized by the chasmophytic Scabioso creticae-Centauretum ucriae association (Figs 3e, f, 6).

Synchorology: The association has been initially described for the coastal stretch between the localities of Sampieri and Cava D’Aliga (Bartolo et al. 1982; Brullo et al. 1998), considered endemic to the coastal belt of southern Sicily. In fact, the presence of S. tripartita has also been recorded in other coastal locations (Giardina et al. 2007; Brullo et al. 2009), albeit as small nuclei or isolated individuals, indicating a broader potential for the formation of maquis.

Annex I Habitat: 5220* Arborescent matorral with Ziziphus and other xerophilous species to the alliance Periplocion angustifoliae (Biondi et al. 2009; Angelini et al. 2016).

EUNIS Classification system: S.54 (formerly: F5.5) Thermomediterranean arid scrub (Chytrý et al. 2020).

The synoptic table (Table 3) compares the floristic composition of the shrubland of Mt. S. Calogero with the two other similar communities dominated by S. tripartita previously described for the coasts of southeastern Sicily (Calicotomo infestae-Rhoetum tripartitae, to which this community is referred) and for the island of Linosa (Periploco angustifoliae-Rhoetum tripartitae), respectively.

From a phytosociological point of view, the surveyed vegetation should be referred to Calicotomo infestae-Rhoetum tripartitae, an association considered endemic to the coastal belt of southern Sicily. Besides its specific ecology, it is distinguished by the presence of Cytisus infestus subsp. infestus, albeit it is not very common. The first four relevés, performed in spring, represent the optimal facies of the coenosis, while the other two are related to the summer period, with the plant community in full estivating phase. As an adaptation to extreme summer aridity, several shrubby species appear leafless (S. tripartita, Cytisus infestus subsp. infestus, Euphorbia dendroides, Asparagus albus, Teucrium fruticans), as do many herbaceous plants, which also exhibit completely dry aerial parts. Sclerophylls that even retain their green foliage even in the summer are limited to Chamaerops humilis and Olea europaea; herbaceous annual plants disappear or are sometimes recognizable by their aerial parts, entirely desiccated, namely hemicryptophytes and geophytes.

The Periploco angustifoliae-Rhoetum tripartitae association (Brullo et al. 2009), which also occurs on the island of Linosa, especially on Montagna Rossa, is indicated as vicariant of the Calicotomo infestae-Rhoetum tripartitae. Also, while this shrubland community is dominated by S. tripartita, instead it colonizes incoherent volcanic substrates and is differentiated by the presence of other summer-deciduous plants, such as Periploca angustifolia and Lycium intricatum.

From the phytosociological viewpoint, the above-mentioned plant communities are classified within the alliance Periplocion angustifoliae characterized by few summer-deciduous species which dominate the most drought-tolerant shrubland communities throughout Europe. Due to their rarity, according to the Habitats Directive, they were assigned to a priority habitat (code: 5220; name: “Predesertic scrubs”). This vegetation is at its vegetative peak during the rainy (autumn-winter) season, while a total vegetative stasis occurs between the end of spring and the end of summer.

Given both the rarity of the species at the European scale and the small size of the nucleus, extending over just 3,000 square meters, this plant community should be listed as an Italian threatened ecosystem (Capotorti et al. 2020; Casavecchia et al. 2021). This habitat is affected by at least three threat factors identified by IUCN-CMP (2019), namely “1.1 - Habitat loss/degradation, agriculture”, due to the potential decrease in available habitat located within areas undergoing reforestation, “10.5 - Human disturbance, fire” and “12.1 - Other threats”, including habitat fragmentation, the small size of the subpopulation and its distance from the nearest nuclei, located approximately 200 km afar in the coastal sector of the Hyblaean district.

QUERCETEA ILICIS Br.-Bl. ex A. et O. Bolòs 1950

PISTACIO LENTISCI-RHAMNETALIA ALATERNI Rivas-Martínez 1975

Periplocion angustifoliae Rivas-Martínez 1975

Calicotomo infestae-Rhoetum tripartitae Bartolo, Brullo et Marcenò 1982

Periploco angustifoliae-Rhoetum tripartitae Brullo, Gianguzzi, La Mantia et Siracusa 2009

Periploco angustifoliae-Euphorbietum dendroidis Brullo, Di Martino et Marcenò 1977

Periploco angustifoliae-Juniperetum turbinatae Bartolo, Brullo, Minissale et Spampinato 1990

Lycio intricati-Pistacietum lentisci (Brullo, Guarino et Ronsisvalle 2000) Brullo 2020 in Cambria et Giusso del Galdo 2020

Asparago stipularis-Retametum gussonei Brullo, Guarino et Ronsisvalle ex Brullo, Giusso, Siracusa et Spampinato 2001

Asparago acutifolii-Ziziphetum loti Gianguzzi, Ilardi et Raimondo 1996