Ptilostemon greuteri is a very rare woody species belonging to the Asteraceae family, described by Raimondo & Domina (2006) for a single locality in north-western Sicily (Mount Inici, municipality of Castellammare del Golfo, Trapani province) just some 15 years ago. This sympodial nanophanerophyte can exceed a height of 3 m and is crown bears 8-10 branches, white-tomentose at the apex, branch off from its striated monocauline stems, forming a dense, globular crown. In contrast to the Mediterranean woody sclerophylls, characterised by small, leathery and strongly cutinous leaves, P. greuteri has persistent, soft, long lanceolate, lauriphyllic leaves. Numerous and densely imbricated, these leaves are pinnate, lanceolate and subsessile, flat or slightly convex upwards, their apex obtuse to acute; they grow on the mid-terminal portion of the branches - where they persist for a long time even after drying - and give the plant a very distinctive habitus (Fig. 1). The upper side of the leaves is shiny green, while the lower side is covered with a dense, fluffy-white tomentum. The lower leaves measure 25-35 ×1.3-4 cm, and their size gradually decreases towards the top of the branches, which host lax corymbs bearing 2-8 ovoid flower heads.

The size of the leaves of P. greuteri represents a remarkable anomaly in the framework of native woody species from Sicily and the Mediterranean in general: in fact, their large surface area appears a trait that does not fit in well with the long-lasting summer thermo-hydric stress typical of the climatic conditions of the entire basin. In view of its morpho-anatomical peculiarities, P. greuteri is considered a palaeoendemic (Brullo & Brullo 2021) whose extreme rarity is presumably a consequence of the glacial and post-glacial climatic changes occurred during the Quaternary. At present, the species grows wild only in two sites (Pasta et al. submitted), both of which are of high environmental and conservation value. The phylogenetic position of P. greuteri within the genus is still debated; the species was in fact referred to the section Ptilostemon by Raimondo & Domina (2006), while subsequent molecular investigations (Vilatersana et al. 2010) suggest it should better be included in the section Pterocaulos.

The survival of this species is threatened by the frequent recurrence of summer fires, the last of which occurred on 2 July 2017 and affected a vast area of Mt. Inici, completely wiping out the adult population of the locus classicus (Raimondo et al. 2021; Pasta et al. submitted). At the same time, however, the fire caused the partial destruction of the conifer reforestation, eventually favouring the dynamism of the local vegetation where P. greuteri occurs, giving rise to a community that is extremely peculiar from a physiognomic point of view in the phytocoenotic panorama of Sicily and probably of the entire Mediterranean Basin.

Drawing on the above observations and considerations, the present work aims at improving the available knowledge on the synecology of the species and its syndynamic role in the plant communities of the area where it grows. Indeed, P. greuteri had hitherto been considered a species linked to the local thermophilic chasmophytic communities (Raimondo & Domina 2006; Raimondo et al. 2021), referred to the association Scabioso creticae-Centauretum ucriae Brullo & Marcenò 1979 and included in the class Asplenietea trichomanis (Br.-Bl. in Meier & Br.-Bl. 1934) Oberdorfer 1977 (Brullo & Brullo 2021). However, the habit of this woody species suggests ascribing the assemblages where this species occurs to the maquis communities belonging to the class Quercetea ilicis Br.-Bl. in Br.-Bl., Roussine & Nègre 1952. The surveys carried out made it possible to identify and distinguish from both a floristic and phytocoenotic point of view, two different plant communities in the habitats colonised by P. greuteri, i. e. the ledges and fractures on vertical cliffs, and the screes made of unstable and incoherent clastic detritus at the base of the carbonatic relief.

For the geolithological and geomorphological characteristics of the two locations of P. greuteri, the work of Catalano et al. (2011) was consulted. As for local climatic patterns, reference was made to the dataset (years 1926-1985) analysed by Duro et al. (1996), focusing on the closest recording points, i. e. the thermometric station of Calatafimi (350 m a.s.l.), located just 10 km east of the study area (Tab. 1), and the rainfall station of Castellammare del Golfo (15 m a.s.l.), located at the foot of Mount Inici (Tab. 2).

The bioclimatic characterisation of the territory according to the indices of Rivas-Martínez (2004), was performed by consulting Drago et al. (2005) and some scientific papers concerning the whole region of Sicily (Bazan et al. 2015; Gianguzzi et al. 2016b) and some neighbouring areas of the province of Trapani (Gianguzzi & La Mantia 2009). Repeated field surveys focused on local plant communities were carried out between 2018 and 2022 and allowed to make several phytosociological relevés, performed following the Zurich-Montpellier School methodology, i.e. adopting the abundance-dominance indices proposed by Braun-Blanquet (1964), updated according to the suggestions by Géhu & Rivas-Martínez (1981) and Biondi (2011). The treatment of the syntaxa mentioned in the text follows the criteria of the International Code of Phytosociological Nomenclature (ICPN: Theurillat et al. 2021).

For the classification of the vascular plants found during field surveys, reference was made to the Pignatti et al. (2017–2019), while the nomenclatural treatment of the vascular plant taxa follows Bartolucci et al. (2018). To produce a synoptic overview of phytosociological data concerning the Sicilian syntaxa framed into the alliance Oleo-Ceratonion, reference was made to Brullo et al. (2009) and several recent works, cited in the text.

Fig. 2 provides an updated distribution map of P. greuteri, showing the position of the second subpopulation, recently discovered c. 2.2 Km south of the locus classicus (Pasta et al. submitted).

The study area concerns the northern slopes of Mt. Inici (1,064 m a.s.l.), which falls within the municipal territory of Castellammare del Golfo (Trapani province), where the two stands of P. greuteri are located. Both sites share many ecological patterns (Pasta et al. submitted): they are located at intermediate altitudes between Cozzo Monaco (771 m a.s.l.), Pizzo Brando (635 m a.s.l.), Pizzo Stagnone (783 m a.s.l.) and Pizzo del Dottore (620 m a.s.l.); morevoer, they both fall within two Natura 2000 sites, the Special Protection Areas ITA010029 “Monte Cofano, Capo San Vito e Monte Sparagio” and ITA010015 “Complesso dei Monti di Castellammare del Golfo”. From a geolithological point of view, both sites are characterised by Trias-Lias carbonate platform deposits, ascribed to the Trapanese Domain and mainly referred to the “Monte Bonifato-Calatubo” Unit, which is covered by sediments belonging to the “Monte Inici” Unit. The substrates are made of dolomitic limestones with megalodonts, stromatolitic and lopheritic dolomites, calcilutites rich in algae and foraminifera (Catalano et al. 2011). The slopes of Mt. Inici are partially covered with coarse and loose clastic debris resulting from the continuous collapse of rocks from the nearby vertical cliffs. The slopes are rather steep and furrowed by several incisions, where water flows during the most abundant and violent rainfall events, concentrated between autumn and spring. Both subpopulations occur in cool and shady habitats within two of these gullies, characterised by a thick cover of incoherent and unstable debris. While awaiting the interpretation of the meso- and microclimatic data collected at both P. greuteri stands (Marcenò et al. submitted), local mesoclimatic patterns were inferred from the thermometric data recorded at the Calatafimi station (Tab. 1), while reference was made to the station of Castellammare del Golfo for rainfall values (Tab. 2). More in detail, as far as rainfall is concerned, the annual average value is 642 mm, distributed over a total of 82 days; the mean daily temperatures are around 16.7 °C, with monthly averages ranging from 9 °C (January) to 26.7 °C (August) and average annual excursions of 9.2 °C; the absolute maximum and minimum temperatures recorded are 41.7 °C in August and -2.0 °C in December, respectively. The proximity of the sea strongly influences local climate, which is characterised by a particularly mild winter season: in fact, the average maximum temperatures of the coldest months are 11.9 °C in January and 12.3 °C in February.

According to the classification proposed by Rivas-Martínez et al. (2004a), the study area is subject to an oceanic pluviseasonal Mediterranean bioclimate. Based on the available thermo-pluviometric data, the area falls within the upper thermo-Mediterranean belt with a lower sub-humid ombrotype.

From a biogeographical point of view, the area belongs to the Mediterranean Biogeographical Region, Western Mediterranean Subregion, Italo-Tyrrhenian Province, Sicilian Sector (Rivas-Martínez et al. 2004b), Western Subsector, Drepano-Panormitan District (Brullo et al. 1995).

Tab. 3 includes 12 phytosociological relevés carried out in both localities Pedrazzi and Il Finestrone, in the territory of Castellammare del Golfo. These relevés provide an outline of the characteristics of the new association described here. Malvo olbiae-Ptilostemonetum greuteri is a low and usually dense maquis formation, physiognomically characterised by the striking dominance of P. greuteri, a laurophyllic phanerophyte that gives a very peculiar habitus to the whole community, with several cespitose and lianose phanerophytes/nano-phanerophytes co-occurring in the shrub layer, and with an herb-dominated understorey rich in hemicryptophytes and geophytes.

Based on field observations and the semi-quantitative analysis of the survey data, the authors propose to include this community in the class Quercetea ilicis (order Pistacio-Rhamnetalia alaterni, alliance Oleo-Ceratonion). This choice is based on the high number of plants (12-15 taxa per relevé: see Tab. 3, rels 1-8) considered as characteristics of the afore-mentioned syntaxa. These taxa clearly prevail over the species of forest edges and mantle communities ascribed to the class Crataego-Prunetea (mostly summer green deciduous phanerophytes) and over the rupicolous species referred to the class Asplenietea trichomanes (where the vegetation hosting P. greuteri were previously included). In fact, the species characteristic to the latter two classes plays a secondary role from the physiognomic point of view, and they sporadically occur in the relevés just as transgressive species that usually take part to other communities present in the neighbouring areas.

Despite being dominated by an extremely narrow-ranged palaeoendemic plant, the Malvo olbiae-Ptilostemonetum greuteri shows a surprising high vitality and ecological plasticity. The primary aspects - referred to the subass. euphorbietosum bivonae - characterise fragments of shade-tolerant and relict low maquis (rels 9–12), located on almost inaccessible rocky ledges and small cracked surfaces on steep carbonatic cliffs. Here, P. greuteri and Euphorbia bivonae, often co-dominant, are associated with many other phanerophytes, like Chamaerops humilis, Pistacia terebinthus, Teucrium flavum, Prasium majus, etc., and with the perennial tussock grass Ampelodesmos mauritanicus (common in many primary communities of the class Quercetea ilicis; see Gianguzzi et al. 2015, 2016). Other frequently occurring species are proposed among the characteristics of the association, like Centranthus ruber, Malva olbia - seldom recorded within the Oleo-Ceratonion communities and considered by several authors as a characteristic of the Pruno-Rubion alliance (class Crataego-Prunetea) - and several lianas referred as characteristics of both the class Crataego-Prunetea and Quercetea ilicis, like Smilax aspera, Rubia peregrina, and Lonicera implexa.

A secondary aspect of the investigated community - referred to subass. typicum – mostly occurs on scree, and is currently spreading, especially in locality Pedrazzi. This plant community plays an important role in local vegetation dynamics, representing the stage that precedes the development of the holm oak woodland of the association Rhamno alaterni-Quercetum ilicis. Even if its dynamic role could induce to interpret this community as a mantle assemblage and to frame it into the class Crataego-Prunetea, the authors believe that it is more correct to treat it as a secondary scrub formation belonging to Oleo-Ceratonion, just like other synecologically similar associations, such as Rhamno alaterni-Euphorbietum dendroidis. Indeed, this latter coenosis may be considered a vicariant of the Malvo olbiae-Ptilostemonetum greuteri on the screes, steep slopes and ledges of the carbonatic mountains between Palermo and Trapani (northwestern Sicily), albeit under more xeric climatic conditions. In these contexts, infact, Rhamno alaterni-Euphorbietum dendroidis plays a secondary role as well, tending towards the holm oak woodland referred to the Rhamno alaterni-Querco ilicis sigmetum series (Brullo & Marcenò 1985; Brullo et al. 2009; Gianguzzi et al. 2015, 2016).

Overwhelmed by the dominance of P. greuteri, the species of the class Quercetea ilicis found in the relevés of the Malvo olbiae-Ptilostemonetum greuteri typicum generally perform low cover but are nonetheless very frequent (usually 12-15 taxa and half of the total species number per relevé: see Tab. 3, rels 1–8).

Further interesting remarks arise from the analysis of Tab. 4 (and Appendix II), which provides a synoptic overview on the communities framed into the Oleo sylvestris Ceratonion siliquae alliance known so far for Sicily. In fact, this table underlines the strong connection of some of the “critical” species mentioned above, like Rubia peregrina, Smilax aspera and Ampelodesmos mauritanicus, with the communities belonging to the class Quercetea ilicis. The table also highlights that the relevés of Malvo albiae-Ptilostemonetum greuteri presented in this paper count as many as 22 characteristics of the class Quercetea ilicis. Altogether, these data provide further support to the syntaxonomic interpretation given by the authors and to the classification proposed in the following scheme.

Sicily and its satellite islands represent one of the main hotspots of plant biodiversity individuated in the Mediterranean Basin (Médail & Quézel 1997), showing and exceptionally high rate of endemism, as highlighted by several authors either from the floristic (Brullo et al. 1995; De Castro et al. 2008, 2015a, 2015b; Gianguzzi & La Mantia 2004; Guarino & Pasta 2017, 2018; Brullo & Brullo 2021), and the phytocoenotic point of view (Brullo et al. 2009; Gianguzzi et al. 2011, 2012, 2013, 2014a, 2014b, 2015, 2017; Guarino & Pasta, 2017). The coastal stretch of the western sector of Sicily (including the Egadi Archipelago) is home to few and often single stands of many palaeoendemic, disjunct and/or relict species which are regionally or even nationally rare, like Oncostema hughii (Tineo) Speta, Thymus richardii Pers. subsp. nitidus (Guss.) Jalas, Erodium maritimum (L.) L’Hér., Daphne sericea Vahl and Thymelaea tartonraira (L.) All. (Marettimo Island), Pseudoscabiosa limonifolia (Vahl) Devesa (Marettimo, Monte Corvo and Mt. Gallo), Brassica macrocarpa Guss. (Marettimo and Favignana islands), Erica sicula Guss. subsp. sicula (Mt. Cofano), Simethis mattiazzi (Vand.) Sacc. (Marettimo and Mt. Cofano), Hieracium cophanense Lojac. (Mt. Cofano and Monte Passo del Lupo), H. lucidum Guss. and Genista gasparrinii (Guss.) C. Presl (Mt. Gallo). Most of these taxa are considered climatic relicts, a small group of plants that were able to survive the Quaternary glacial events (Garfì et al. 2021). P. greuteri belongs to this group; it provides a very peculiar and somehow ‘archaic’ physiognomy to the woody communities described in this paper, where it occurs as a dominant/co-dominant species, incredibly set aside in few stands of high ecological and conservation value, which have only recently been discovered.

The field surveys carried out have shown that P. greuteri is strictly localised on rock ledges and incoherent north-facing screes located within two narrow stream incisions, on carbonatic substrates, at altitudes between 300-500 m a.s.l., in the thermo-Mediterranean bioclimatic zone, with a sub-humid ombrotype. Due to their physiognomy and the dominance of this enigmatic broadleaved lauriphyll, these communities are markedly different from other Mediterranean forest communities, particularly due to the dense lauriphyll foliage and the silvery colour of the young leaves and branch tips. In fact, this species suffers considerably from summer drought (July and August), given the characteristics of the leaf blade that tolerates little direct sunlight. Although scarce, from a purely physiognomic point of view, such scrubland aspects are very rare in the panorama of phytocoenoses related to the order Pistacio-Rhamnetalia alaterni and to the whole class Quercetea ilicis, and it is therefore proposed here to refer them to a new lithophilous, thermophilous and shade-tolerant coenosis Malvo olbiae-Ptilostemonetum greuteri ass. nova, framed in the Oleo-Ceratonion alliance. The only woody communities that show a certain physiognomic-structural analogy with those dominated by P. greuteri are those characterised by the dominance or co-dominance of Bupleurum fruticosum, i. e. Hippocrepido emeri-Bupleuretum fruticosi, described by Brullo et al. (1993) for the hot and humid slopes of some canyons of the Iblei Mountains (south-eastern Sicily) and Spartio juncei-Bupleuretum fruticosi occurring on the N-facing hills of the Madonie and Nebrodi Mts. (Raimondo & Ilardi 2009).

Considering the multiple disturbances affecting the species - also due to increasing anthropogenic pressure and widespread urbanisation of the nearby territory - P. greuteri results to be seriously threatened with extinction in the wild. Inspite of the recent demographic increase of the maquis community where it belongs, P. greuteri remains an intrisically vulnerable species due to its peculiar ecological requirements, which allow this species to survive only on the cool and shady slopes of the two aforementioned gullies located on the eastern slope of Mt. Inici, either on coarse and mobile screes (subass. typicum) or on the outcropping rocks of some ledges (subass. euphorbietosum bivonae).

QUERCETEA ILICIS Br.-Bl. in Br.-Bl., Roussine & Nègre 1952

PISTACIO LENTISCI-RHAMNETALIA ALATERNI Rivas-Mart. 1975

Oleo sylvestris-Ceratonion siliquae Br.-Bl. ex Guinochet & Drouineau 1944

Malvo olbiae-Ptilostemonetum greuteri ass. nova

typicum subass. nova

euphorbietosum bivonae subass. nova