The soils were characterized in accordance with the WRB guidelines (IUSS WG WRB, 2022), leading to their classification based on this system.

The vegetation surveys of the forest nuclei were carried out in the period between April 2018 and June 2023. A total of 38 unpublished phytosociological relevés were carried out, according to the Zurich-Montpellier methodology (Braun-Blanquet 1964). For the syntaxonomic interpretation of the communities, the data were compared with those already available from previous literature (Brullo and Marcenò 1985; Buffa et al. 1986; Gianguzzi et al. 1996; Surano et al. 1996; Brullo et al. 2009; Gianguzzi and Bazan 2020a, b, c). For the elaboration of synoptic tables including similar associations of the Quercetea ilicis class already described for Sicily (Appendix 1: Tables A1, A2), reference was made to Brullo et al. (2009) and other more recent papers (cited in the text). The nomenclature of the syntaxa follows the criteria of the International Code of Phytosociological Nomenclature (ICPN: Theurillat et al. 2021), whereas for the syntaxonomic classification of the communities, we referred to Mucina et al. (2016) and to Guarino and Pasta (2017).

To evaluate the role of “refuge habitat” of these forest ecosystems for the conservation of what’s referred to as “valuable” plant diversity (Gigante et al. 2016; Capotorti et al. 2020; Casavecchia et al. 2021), a census of the ancient trees (with localization and dendrometric survey) was performed.

For the classification of vascular plant species, reference was made to Pignatti et al. (2017–2019), while plant naming follows Bartolucci et al. (2018) and the updates available on the Portal to the Flora of Italy (2023). For bryophytes, their nomenclature follows Hodgetts et al. (2020); the specimens are preserved in the Herbarium Mediterraneum Panormitanum (PAL). The identification of lichens was performed using the the online keys published in “ITALIC, the Information System of the Italian Lichens, version 07” (see Nimis and Martellos 2020) and Smith et al. (2009), while their nomenclature follows Nimis (2024). For fungi, the nomenclature used refers to Ferraro et al. (2022), whereas the dried samples are preserved in the SAF Herbarium of the Department of Agricultural, Food and Forestry Sciences of the University of Palermo.

We studied four soil profiles, the first two in the colluvium between the dunes (1) and on the back of the coastal sandy dune that covers the calcarenites (2), and the third and fourth on the debris of Mt. Pellegrino (4). The parent materials belong to the Quaternary complex of western Sicily, a calcarenitic-sandy-silty complex with mainly lithoid clasts, composed of calcite and siliceous rocks fragments and monocrystalline quartz (sparitic grains and small amounts of bioclasts) (Zimbardo 2016).

The soils are classified respectively as: (1) Solimovic Regosol (Arenic) (Fig. 3); (2) Eutric Arenosol (Chromic); (3) and (4) Skeletic Regosol (Ochric) (IUSS WG WRB 2022). The average textural classification of these soils is 15% clay, 35% silt and 50% sand expressed as a percentage of the fine earth fraction. Average properties of the fine earth are: organic carbon 3.0±1.1%; pH 7.8±0.6; BD soil bulk density 1.030±0.011 kg dm^-3.

We used the European classification of humus forms (Zanella et al. 2011), based on the sequence and morphological characteristics, including evidence of biological activity, of organic and/or organomineral horizons observed and described in the field (Table 3). Our correlations between humus forms and soils are consistent with the cross-harmonization proposed by Vacca et al. (2018) for Mediterranean environments. In line with humus theory (Ponge 2003), the formation of humus primarily stems from nutrient availability and reflects the nutrient management approaches adopted by ecosystems. The presence of mull humus indicates systems rich in nutrients, showing a rapid nutrient cycling strategy. It is assumed that because nutrient and organic carbon dynamics are interconnected and organic carbon turnover occurs swiftly, the substantial carbon reserves found are likely a consequence of high net primary ecosystem productivity. Nevertheless, it is worth noting that establishing a straightforward or essential link between these factors is challenging, even in the presence of fast nutrient cycling (Andreetta et al. 2011).

Four types of forest vegetation were identified in the study area (Fig. 4):

1. Pistacio lentisci-Quercetum ilicis Brullo et Marcenò 1985 subass. viburnetosum tini Gianguzzi, Ilardi et Raimondo 1996;
2. Viburno tini-Phillyreetum latifoliae Gianguzzi et Caldarella ass. nova hoc loco;
3. Ruto chalepensis-Oleetum sylvestris subass. cercidetosum siliquastri Gianguzzi et Bazan 2020;
4. Teucrio flavi-Phillyreetum latifoliae Gianguzzi et Caldarella ass. nova hoc loco.

Pistacio lentisci-Quercetum ilicis Brullo et Marcenò 1985 subass. viburnetosum tini Gianguzzi, Ilardi et Raimondo 1996

Holotypus. Rel. 10, table 5 in Gianguzzi et al. (1996).

Phytosociological data. Table 4, Appendix 1: Table A1 (column 3).

Diagnostic species. Quercus ilex (dom.), Pistacia lentiscus, Viburnum tinus, Arbutus unedo, Clematis cirrhosa and Phillyrea latifolia.

Short description. Dense, closed forest, with a clear dominance of Quercus ilex, of variable height between (5)7–14(20) m, in whose tree layer there are Phillyrea latifolia and sometimes Fraxinus ornus and Celtis australis. Other species include Pistacia lentiscus, Viburnum tinus, Arbutus unedo, and some lianas (Clematis cirrhosa, Rubia peregrina, Smilax aspera, Hedera helix and Asparagus acutifolius), which are represented in the shrub layer. The herbaceous component of the undergrowth is poor (Fig. 5), essentially composed of some native sciaphilous-nitrophilous bulbous plants (Acanthus mollis, Arisarum vulgare, Arum italicum, Smyrnium olusatrum etc.), as well as a rich frequency of seedlings of the dominant tree species (Fig. 6a–e).

Substrate/parent material and soil. Coastal Quaternary calcarenites on deep and evolved soils and with edaphic humidity (due to rising groundwater) are widespread in the internal part of the Palermo Plain. These soils are classified as Solimovic Regosol (Arenic) (IUSS WG WRB 2022).

Bioclimate. Mediterranean pluviseasonal oceanic thermomediterranean thermotype and dry-subhumid ombrotype.

Syntaxonomic notes. The Pistacio lentisci-Quercetum ilicis association has been described as a thermophilous holm oak forest in Sicily (Brullo and Marcenò 1985), linked to xeric alkaline substrates (marly, limestones, calcarenites, etc.). This forest association is rich in species of the order Pistacio-Rhamnetalia alaterni (Brullo and Marcenò 1985; Brullo et al. 2009). The subass. viburnetosum tini has been recognized in the calcarenites of the Palermo Plain (Gianguzzi et al. 1996, 2016a). In addition to the high frequency of Pistacia lentiscus, the following species are identified as differentials of the subassociation: a) Viburnum tinus [Steno-Mediterranean element with relict and fragmentary distribution (Pignatti 1982; Karlson et al. 2005), in Sicily known only in this area and in a few sites in the Sicani Mts. (Gianguzzi et al. 2016a) and Mt. Etna (Siracusa 1998; Brullo et al. 2009), etc.]; b) Arbutus unedo (acidophilous and calcifugal species, also quite rare in the basiphilous holm oak forests of the western sector of Sicily; Gianguzzi and La Mantia 2008); c) Clematis cirrhosa, a very common vine in this community.

Vegetation series. The community constitutes the best structured aspect of the “Climatophilous series north-western Sicilian, thermomediterranean from dry to subhumid, calcarenitic of Quercus ilex (with Pistacia lentiscus and Viburnum tinus)”: Pistacio lentisci-Querco ilicis viburnetosum tini sigmetum (Brullo and Marcenò 1985; Gianguzzi et al. 1996; Brullo et al. 2009). On the more xeric and draining sandy-calcarenitic soils, located near the slopes of Mt. Pellegrino, it is replaced by the Phillyrea latifolia series of the Viburno tini-Phillyreo latifoliae sigmetum (whose more mature forest community is described in the following paragraph). Near watercourses – as in the case of the locality detected in Villa d’Orleans in Palermo, where the Kemonia Stream once flowed – it also finds contact with the hygrophilous series of Salix pedicellata of the Ulmo canescentis-Salico pedicellatae sigmetum (Gianguzzi et al. 2010, 2013, 2014).

Synchorology. The forest stands represent residual aspects linked to the coastal Quaternary calcarenites of the western sector of Sicily, where they were destroyed by man to make room for crops (citrus groves, orchards, and vegetable gardens) and urban areas. A further small nucleus of approximately 1 hectare was also located within Villa d’Orleans, in the urban area of Palermo (rel. 15 of Table 4). It has developed through free evolution over the last 150 years, although the undergrowth has undergone the removal of topsoil, litter and dead wood. There are also long-standing individuals of Quercus ilex here, reaching heights over 20 m.

Eunis code. G2.121(Meso-Mediterranean Quercus ilex forests).

Natura 2000 code. 9340 (Quercus ilex and Quercus rotundifolia forests).

Number of nuclei and total coverage (in ha). 5 units (for 11,86 ha of coverage).

Viburno tini-Phillyreetum latifoliae Gianguzzi et Caldarella ass. nova hoc loco

Holotypus. Rel. 3 in Table 5.

Phytosociological data. Table 5, Appendix 1: Table A2 (column 23).

Diagnostic species. Phillyrea latifolia (dom.), Viburnum tinus, Arbutus unedo, Clematis cirrhosa; Asparagus asparagoides (anthropogenic variant).

Short description. Dense maquis forest of evergreen sclerophyllous scrub, (4)5–6(7) m high, with closed or almost closed canopy structure, clearly dominated by Phillyrea latifolia, which is constantly associated with Arbutus unedo and Viburnum tinus. Pistacia lentiscus also frequently appears, as well as various lianas such as Clematis cirrhosa, Asparagus acutifolius, Smilax aspera, Rubia peregrina and Dioscorea communis. Compared with the Pistacio lentisci-Quercetum ilicis subass. viburnetosum tini, in this community the presence of Quercus ilex is more sporadic and with composed of shrub specimens. The herbaceous component is quite poor and scattered – particularly in the most intact aspects – made up of bulbous and rhizomatous species (Arisarum vulgare, Acanthus mollis, Arum italicum, Allium subhirsutum), as well as hosting a significant presence of woody plant seedlings (Fig. 7a–c).

Substrate/parent material and soil. The community is linked to sandy soils on Quaternary calcarenites, in contexts that are more xeric and draining than those colonized by the holm oak grove of Pistacio lentisci-Quercetum ilicis subass. viburnetosum tini. These soils are classified as (2) Eutric Arenosol (Chromic) (IUSS WG WRB 2022).

Bioclimate. Mediterranean pluviseasonal oceanic thermomediterranean thermotype and dry-subhumid ombrotype.

Syntaxonomic notes. Phillyrea latifolia is distributed in North-Mediterranean and Balkan countries, from southern Portugal to Bulgaria, Greece, and the Aegean islands; it also grows in North Africa (Libya to Morocco) and the Near East, from Anatolia to Syria, Lebanon, Israel, and Jordan (Browicz 1984). In Italy, this species is widespread in almost all regions, but more commonly in the centre-south of the peninsula and islands. It grows more frequently as a sporadic species, rarely playing a dominant role; it participates in aspects of Mediterranean scrub and thermophilous shrublands, but also in the establishment of oak groves and evergreen woods (Brullo et al. 2009). In the central plateau of Morocco (Barbero et al. 1981) for example, is part of thermophilous wild olive forests pertaining to the association Phillyreo latifoliae-Oleetum sylvestris Quézel, Barbero, Benabid, Loisel et Rivas-Martínez ex Gianguzzi et Bazan 2020 (Barbero and Quézel 1976; Barbero et al. 1981; Quézel et al. 1988; Gianguzzi and Bazan 2020b), while in Algeria it is found in formations with Quercus canariensis (Aimé et al. 1986). In the Iberian Peninsula, it takes part in oak forests of various types (Quercus ilex, Q. coccifera, Q. faginea and Q. broteroi) and other mesophilous formations (Costa Tenorio et al. 1997), but also in aspects of Mediterranean scrub.

For example, the Phillyreo latifoliae-Arbutetum unedonis (Velasco 1983) Loidi, Herrera, Olano et Silván 1994 association is described along the northern Cantabrian coast, differentiated with the subass. typicum and viburnetosum tini (Loidi et al. 1994); the first is widespread towards the sea, the second further inland. Another scrub community is also indicated for the Costa Smeralda, in the north-east of France, pertaining to Hedero helicis-Phillyreetum latifoliae (Géhu 2007).

On the island of Menorca (Balearic Islands) the species is present with the endemic variety rodriguezii (P. Monts.) O. Bolòs et Vigo, which dominates a shrubby community described as Aro picti-Phillyreetum rodriguezii (Bolòs et al. 1970). Phillyrea latifolia is also widespread in Sardinia, where it is also present in mesophilous woods with Taxus baccata (Bacchetta et al. 2009; Farris et al. 2012), as well as in Corsica, where it marks aspects of scrub related to Erico arboreae-Arbutetum unedonis subass. phillyreetosum latifoliae (Allier and Lacoste 1980). In Italy, it is indicated as an element of the scrub but also of the undergrowth of formations mixed with Quercus ilex and Q. suber, as in the southern part of Lazio (Blasi et al. 1997); is also present in the Carpinus orientalis Mill. community of the Lonicero etruscae-Carpinetum orientalis phillyreetosum latifoliae (Blasi et al. 2001). In the Apulian region, it is present in the Phillyreo latifoliae-Calicotometum infestae (Di Pietro and Misano 2010), in very intricate shrublands dominated by Calicotome infesta, widespread in the Gravina of Laterza area. Phillyrea latifolia è also widespread in the eastern part of the Mediterranean region, associated with Quercus coccifera, as in the Balkan area (Horvat et al. 1974; Trinajstić 1995) and also in Greece (here described as Querco cocciferae-Phillyreetum mediae; Barbero and Quézel 1976). Other communities characterized by the species are also reported in Turkey, such as Phillyreo latifoliae-Pinetum brutiae (Ozyigit et al. 2015) and Pistacio-Phillyreetum latifoliae (Özen and Kılınç 1995); the latter is probably a geovicariant association.

The Phillyrea latifolia maquis located on the calcarenitic substrates of the Palermo Plain is described as Viburno tini-Phillyreetum latifoliae Gianguzzi et Caldarella ass. nova hoc loco (Table 5, rel. 1–4; Fig. 7a–c), differentiated also an anthropogenic variant wich Asparagus asparagoides (Table 5, rel. 5–12; Fig. 7d, e). It is a climbing species of South African origin, naturalized as far back as historic record in this area (Tropea 1907) and in other locations in Sicily (Giardina et al. 2007). It constitutes a recovery maquis, that is open and more nitrophilous, linked to areas exposed to humid northern winds coming from the Gulf of Mondello.

The synoptic table reported in Appendix 1: Table A2 compares the floristic composition of the two associations dominated by Phillyrea latifolia that are presented in this paper [the one (Viburno tini-Phillyreetum latifoliae ass. nova) and the Teucrio flavi-Phillyreetum latifoliae ass. nova (which will be discussed later)] with the other scrub formations of the Oleo-Ceratonion alliance previously observed for Sicily. These are the first associations dominated by Phillyrea latifolia that are described in the regional territory. It can be noted that among all the Sicilian associations, the Viburno tini-Phillyreetum latifoliae ass. nova records a floristic composition with the lowest average number of species (equal to 12.6 per relevé); this data is also to be related with the old-growth nuclei forest present in this site and the notable environmental integrity of the community.

Vegetation series. The forest formation constitutes the best structured aspect of the “Edapho-xerophilous series north-western Sicilian, dry thermomediterranean, calcarenitic of Phillyrea latifolia (with Viburnum tinus and Arbutus unedo)”: Viburno tini-Phillyreo latifoliae sigmetum. This environmental landscape unit follows the series of Pistacio lentisci-Querco ilicis viburnetosum tini sigmetum, occupying the more xeric and external sandy soils of the Palermo Plain, placed closer to the colluvial substrates. Along the slopes of Mt. Pellegrino, it in turn leaves room for the series of wild olive trees (Ruto chalepensis-Oleo sylvestri cercidetosum siliquastri sigmetum).

Synchorology. The Viburno tini-Phillyreetum latifoliae ass. nova constitutes a community of relict maquis, potentially linked to the coastal calcarenites of the western sector of Sicily; however, it is only known in these small areas of the Palermo Plain, having been almost destroyed by man for the benefit of cultivation and urban development.

Eunis code. F5.51A (Phillyrea thickets). Natura 2000 code – 5330 (Thermo-Mediterranean and pre-desert scrub).

Number of nuclei and total coverage (in ha). 9 nuclei (for 9,32 ha of coverage).

Ruto chalepensis-Oleetum sylvestris subass. cercidetosum siliquastri Gianguzzi et Bazan 2020

Holotypus. Rel. 15, table S1 in Gianguzzi and Bazan 2020a.

Phytosociological data. Table 6.

Syntaxonomic notes. Scientific contributions to the phytosociological characterization of communities of Olea europaea var. sylvestris have recently been produced for Sicily (Gianguzzi and Bazan 2020a) and the Mediterranean Region (Gianguzzi and Bazan 2020b), where these forest formations show notable climacic potential in the more xeric bioclimatic belts. In Sicily, they are often represented with small nuclei and a fragmentary distribution (Gianguzzi et al. 2020; Bazan et al. 2021; Guarino et al. 2021; Rivieccio et al. 2020, 2021; Tavilla et al. 2021), as a result of deforestation and anthropic transformations. The Ruto chalepensis-Oleetum sylvestris association is described for the regional area of Sicily (Gianguzzi and Bazan 2020a), limited to basiphilous substrates (limestones, marl, calcarenites, etc.) and differentiated into various sub-associations. The subassociation cercidetosum siliquastri has been indicated for the xeric detrital slopes of the carbonate reliefs (limestone and dolomite) of north-western Sicily, between Palermo and Trapani (Gianguzzi et al. 2012a).

Diagnostic species. Olea europaea var. sylvestris (dom.), Euphorbia dendroides, Ruta chalepensis, Pistacia terebinthus, Cercis siliquastrum, Cenchrus setaceus.

Short description. Dense scrub-forest, clearly dominated by Olea europaea var. sylvestris, (3–)5–6 m high (potentially even over 12–15 m), linked to detrital-clastic slopes and with a variable bi- or tri-stratified structure. Various thermophilous species of the order Pistacio-Rhamnetalia alaterni are associated with it and of the Oleo-Ceratonion siliquae alliance, such as Euphorbia dendroides, Pistacia terebinthus, Ruta chalepensis, Cercis siliquastrum [tree native to south-eastern Europe and Asia Minor, common in central Italy but rarer in Sicily (Gianguzzi and Bazan 2020a)] and Cenchrus setaceus [invasive and sub-cosmopolitan species of subtropical origin, now widely naturalized on the island, where it shows a highly competitive pioneer character compared to other thermophilous perennial grasses (Gianguzzi et al. 1996; Pasta et al. 2010). The community is found along the western slope of Mt. Pellegrino, at altitudes between approximately 80–100 and 300–350 m a.s.l., in markedly xeric and sunny localities, with exposure to the south and southwest (Fig. 7f).

Substrate/parent material and soil. Forest formation is linked to detrital cones and detrital-clastic materials in sunny and xeric sites, along the slopes of Mt. Pellegrino. These soils are classified as Skeletic Regosol (Ochric) (IUSS WG WRB 2022).

Bioclimate. Mediterranean pluviseasonal oceanic thermomediterranean thermotype, dry-subhumid ombrotype (Gianguzzi et al. 1996, 2015, 2016b; Gianguzzi and La Mantia 2000).

Vegetation series. The forest formation constitutes the best structured aspect of the “Edapho-xerophilous and heliophilous series north-western Sicilian, dry thermomediterranean, colluvial (calcareous-dolomitic) of Olea europaea var. sylvestris (with Euphorbia dendroides and Cercis siliquastrum)”: Ruto chalepensis-Oleo sylvestris cercidetosum siliquastri sigmetum. The secondary aspects mainly consist of shrub vegetation of Euphorbia dendroides [Rhamno alaterni-Euphorbietum dendroidis Géhu et Biondi 1997 subass. euphorbietosum bivonae (Gianguzzi, Ilardi et Raimondo 1996) Gianguzzi, Cuttonaro, Cusimano et Romano 1996], grassland vegetation with Cenchrus setaceus (Penniseto setacei-Hyparrhenietum hirtae Gianguzzi, Ilardi et Raimondo 1996) and therophytic meadows of the class Trachynietea distachiae (e.g. Thero-Sedetum caerulei Brullo 1975). In fact, compared with the other forest formations represented in the survey area, the Ruto chalepensis-Oleetum sylvestris subass. cercidetosum siliquastri are generally more isolated and less extensive. This is because these xeric slopes are mainly reforested with conifers, apart from small plots that have been historically used for dry crops (olive groves, carob groves, almond groves, prickly pear groves, etc.). Crop abandonment and recent fires have partly favoured the Cenchrus setaceus prairie (Penniseto setacei-Hyparrenietum hirtae), in addition to the evolution of the serial mosaic mentioned previously.

Near the rocky walls, the series comes into catenal contact with the microgeosigmetum of the cliffs, dominated by the chasmophytic vegetation of the Scabioso creticae-Centauretum ucriae Brullo et Marcenò 1979, partly altered by the invasiveness of Opuntia ficus-indica. In the shaded part of the debris slopes, in the north/north-west exposures, the sigmetum is replaced by the ombrophilous series of Phillyrea latifolia, belonging to the scrub of Teucrio flavi-Phillyreetum latifoliae ass. nova, described in the following paragraph.

Synchorology. North-western Sicily: Mt. Pellegrino (Palermo) and Mt. Sparacio (Trapani) (Gianguzzi and Bazan 2020a).

Eunis code. G2.41 (Wild Olea europaea woodland). Natura 2000 code – 9320 (Olea and Ceratonia forests).

Number of nuclei and total coverage (in ha). 6 nuclei (for 1,52 ha of coverage).

Teucrio flavi-Phillyreetum latifoliae Gianguzzi et Caldarella ass. nova hoc loco

Holotypus. Rel. 9, Table 7.

Phytosociological data. Table 7, Appendix 1: Table A2 (column 24).

Diagnostic species. Phillyrea latifolia (dom.), Pistacia terebinthus, Teucrium flavum, Euphorbia dendroides and Olea europaea var. sylvestris.

Short description. Dense, intricate, and impenetrable scrub, clearly dominated by Phillyrea latifolia, 3–5 (6) m high, frequently associated with Pistacia terebinthus, Euphorbia dendroides, and Olea europaea var. sylvestris, precisely indicated as diagnostic of the characteristic combination of the community, together with Teucrium flavum, a lithophilous-glareicola fruticose species, also common in the undergrowth (Fig. 8a–c). Compared with the other association with Phillyrea latifolia previously described for calcarenites of the Palermo Plain (Viburno tini-Phillyreetum latifoliae ass. nova), Viburnum tinus and Arbutus unedo are also absent. The herbaceous layer is mainly composed of rhizomatous and bulbous plants, such as Acanthus mollis, Arisarum vulgare and Allium subhirsutum.

Substrate/parent material and soil. The community is linked to detrital-clastic materials along the slopes of Mt. Pellegrino; however, it prefers shady, ventilated, and fresh sites exposed to the north/north-west, subjected to humid currents coming from the Gulf of Mondello. The parent materials belong to the Quaternary complex of Western Sicily, a calcarenitic-sandy-silty complex (Palermo Calcarenites) with mainly lithoid clasts, composed of calcite and siliceous rocks fragments and monocrystalline quartz (sparitic grains and small amounts of bioclasts). These soils are classified as Skeletic Regosol (Ochric) (IUSS WG WRB 2022).

Bioclimate. Mediterranean pluviseasonal oceanic (thermomediterranean thermotype and dry-subhumid ombrotype).

Vegetation series. The forest formation constitutes the best structured aspect of the “Edapho-xerophilous and ombrophilous series north-western Sicilian, thermomediterranean from dry to subhumid, colluvial (calcareous-dolomitic) of Phillyrea latifolia (with Pistacia terebinthus and Teucrium flavum)”: Teucrio flavi-Phillyreo latifoliae sigmetum). In the sunniest xeric parts of the slopes of Mt. Pellegrino landscape unit, it is replaced by the wild olive series (Ruto chalepensis-Oleo sylvestris subass. cercidetosum siliquastri sigmetum), while upwards it connects the microgeotype of the cliffs, mainly characterized by the chasmophytic vegetation of the Scabioso creticae-Centauretum ucriae association.

Synchorology. Known for Mt. Pellegrino, but probably also present in other locations in the Palermo Mts.

Eunis code. F5.51A (Phillyrea thickets).

Natura 2000 code. 5330 (Thermo-Mediterranean and pre-desert scrub).

Number of nuclei and total coverage (in ha). 2 nuclei (for 3,20 ha of coverage).

Old trees are declining in forests around the world (Lindenmayer et al. 2012) because of the consumption of natural resources which, also threatens their survival (Rigoni Stern 1990). They significantly impact the biodiversity of an area and are gaining increasing attention as indicators of sustainable forest management (e.g. Forest Europe 2015). In fact, once silvicultural maturity has been reached, centenarian trees contribute to increasing the ecological value of ecosystems, because of an increase in biomass (variable depending on the age, size, and environmental conditions of the sites), and the development of a diversified range of microhabitats. This is reflected in the species of flora and fauna (at various levels), with an important contribution also made to the conservation of saproxylic species (Siitonen and Ranius 2015).

Our first census of these old or “monumental” trees growing in the study area was carried out on 48 specimen concentrated on a relatively small surface (Fig. 2), whose data are summarized in Appendix 1: Table A3.

Among these surveyed old specimens, the following deserve particular mention: a) the “monumental” Olea europaea var. europaea specimen, with a maximum trunk circumference of 11.10 m (at the stump) and 8.18 m (at breast height) (Fig. 8e); whose estimated age is about 1,000 years-old (Schicchi and Raimondo 2007); b) some Quercus ilex specimens that are 12–15 m tall, with a maximum circumference varying between 4.5 and 5.3 m (at the stump) and of 2–3 m (at the breast height); c) some Phillyrea latifolia specimen, that are over 15 m tall, with maximum circumference of about 3 m (at the stump) and 1 m (at breast height) with these measurements being quite unusual for this species); d) the Pistacia terebinthus specimen that is about 13 m tall, with a maximum circumference over 3 m (at the stump) and about 1 m (main branch at the breast height); e) some Arbutus unedo specimen that are between 6.5 and 8 m tall, with a maximum circumference between 1.95 and 2.95 m (at the stump) and up to 0.72 m (main branch at breast height); f) an over 200 year-old specimen of Pistacia lentiscus, with a maximum circumference of 2.25 m (at the stump).

In addition to some rare species of vascular flora, there are also other species of bryophytes, lichens, and mushrooms, in Sicily that have also exclusively been recognized in this area and/or in just a few other locations, further demonstrating the important conservation role of the site and the forest formations investigated.

Viburnum tinus

Small thermophilic shrub, forming part – together with other species, such as Coriaria myrtifolia L., Arbutus unedo, and Phillyrea latifolia – of what is known as the “paleo-Mediterranean” element, typical of past eras with a subtropical climate (Ladero Álvarez 1976; Domínguez and Martínez 1993). It has a fragmentary and relict distribution throughout the indigenous area (Pignatti 1982; Karlson et al. 2005) and is also quite rare in Sicily. In addition to this area (Brullo and Marcenò 1985; Gianguzzi et al. 1996), it is also reported in the following other locations: a) above the Rotoli Cemetery, near Palermo (Gianguzzi et al. 1996); b) Sicani Mts. [Sibilla coast (Bazan et al. 2007), Santa Maria del Bosco, Cozzo Danesi, Bosco di Santo Adriano, northern slope of Mt. Gristia (Gianguzzi et al. 2016a)]; c) Madonie Mts. (Gianguzzi and Bazan 2017) in Gibilmanna; d) Mt. San Giuliano (Pasta and La Mantia 2021); e) Mt. Conca (Pasta and La Mantia 2021); f) Mt. Etna (Brullo et al. 2009).

Arbutus unedo

Species with a circum-Mediterranean distribution that extends as far as the Atlantic, where it reaches Ireland; it is also present in the Canary Islands. In Sicily, it grows between 50 and 1,050 m a.s.l., with optimum levels between 300–400 and 700–900 m a.s.l. (Sparacio et al. 2022), preferring siliceous or decarbonated substrates. It is sometimes also found as an element of the garrigue (Gianguzzi et al. 2016b), although it is more frequent in maquis scrubland (Costanzo et al. 1998; Gianguzzi 1999; Brullo et al. 2003), evergreen oak forests (Quercus suber and Q. ilex; Brullo et al. 2009), or even pine forests of Pinus pinea (Brullo et al. 2003), P. halepensis (Brullo et al. 2009), and P. pinaster subsp. escarena (Gianguzzi 1999). Arbutus unedo is quite widespread along the Tyrrhenian slopes of the Peloritani (on metamorphites), the Nebrodi (on sandstones, between Caronia and Tusa) and the Madonie Mts. (between Castelbuono, Pollina, Cefalù and Gratteri, also on sandstones), including the volcanic islands of the Aeolian Archipelago (apart from Vulcano and Stromboli). However, in the western sector of Sicily, it becomes very rare, and is generally associated with relict localities: a) Bosco Niscemi, in the Palermo Plain (on calcarenites; Figs 6c, 8f); b) coastal limestone-dolomite reliefs battered by humid marine winds (in sites exposed to the north and sheltered from fires), such as Mt. Gallo (M. S. Margherita; unpublished data), Mt. Cofano (Gianguzzi and La Mantia 2008), and Marettimo Island (Mt. Campana; Brullo et al. 2009; Gianguzzi et al. 2006, 2023); c) arenaceous outcrops, such as near Cozzo Secco (Borgetto, province of Palermo; Gianguzzi et al. 2008), Bosco Gaggera (Calatafimi, province of Trapani; Gianguzzi and Bazan, in Rivieccio et al. 2022), and Bosco Scorace (Buseto Palizzolo, province of Trapani; Scuderi et al. 1994). It reappears, always sporadically, in the plains of south-western Sicily between Marsala and Sciacca, on the internal hills of Agrigento, Caltanissetta, and Enna, and is more common in isolated areas of the Sicani Mts. and the Iblean sector (Sparacio et al. 2022).

Other rare vascular plants

Among the other rarities that stand out in the floristic entourage of the communities, the following should also be mentioned: a) Cistus salviifolius, a typical species of acidophilic/calcifugal submontane hilly garrigues (Gianguzzi et al. 2015), which is currently very rare in the Palermo Plain (only a few sporadic individuals have been detected); b) Limodorum abortivum, an uncommon and localized entity, that had already been identified in the territory, but could no longer be confirmed in recent times (Künkele and Lorenz 1995; Raimondo et al. 1996); c) Acis autumnalis, also typical of garrigues, uncommon (La Rosa et al. 2021).

Bryophytes also play an important role within old-growth forests (Campisi et al. 2020). Investigations carried out in the Bosco Niscemi have led to the discovery of species that are typical of Mediterranean woods, some of which are quite rare. This is the case of Cryphaea heteromalla (Hedw.) D.Mohr – which has been identified in Sicily so far in very few other residual localities – in addition to Hypnum cupressiforme Hedw. and Leptodon smithii (Hedw.) Weber & D.Mohr, which have also never previously been found at such low altitudes and in contexts so close to the urban environment.

Cryphaea heteromalla (Hedw.) D.Mohr

This is a suboceanic-Mediterranean epiphytic species of the Cryphaeaceae Schimp., sensitive to air pollution (Dierßen 2001), and indicated in regression for some European areas (Sérgio et al. 1994). In Sicily, it is quite rare; so far it has only been recognized in only three other localities, all of which are at high altitudes. Two of them are located on isolated reliefs in the north-western sector, specifically Mt. Bonifato (Alcamo) and Mt. San Giuliano (Erice), both on artificially planted conifer populations; the third concerns the island of Pantelleria (Raimondo and Dia 1980; Raimondo et al. 1981), on natural woodland formations (Lo Giudice 1991; Gianguzzi 1999). A small population had also been reported for the Palermo Mts. (Dia et al. 2000), however this can no longer be confirmed, following recent devastating fires. Therefore, its discovery in the Bosco Niscemi, on the trunks of Phillyrea latifolia, constitutes an interesting confirmation of this species for northern Sicily (Fig. 8d).

References on the lichenological component relating to the investigation area are lacking, apart from a study by Giovenco et al. (1996) on epiphytic lichens as biomonitors of air pollution in the urban area of Palermo. Our recent sampling within the forest nuclei of the Bosco Niscemi also confirmed the important role of the cryptogamic component. Some of the lichens present with greater frequency and coverage values include Parmotrema perlatum (Huds.) M. Choisy, Punctelia subrudecta (Nyl.) Krog., and Ramalina canariensis J. Steiner., which are generally quite common in the Mediterranean area. The presence of species of conservation interest was also detected (according to Nimis 2024), including Bacidia rosella (Pers.) De Not., as well as the epiphytic Gyalecta derivata (Nyl.) H.Olivier, Ramalina roesleri (Schaer.) Nil. and Waynea stoechadiana (Abbassi Maaf & Cl.Roux) Cl.Roux & P.Clerc.

Bacidia rosella (Pers.) De Not.

It is a nemoral species, rare globally, with wide distribution in Europe and northern Africa, in areas with a mild temperate to Mediterranean-Atlantic climate. It is indicated as rapidly declining in most of its distribution area, and indeed reported in “Red Lists” for many countries, such as Austria, Denmark, Germany, Norway, Sweden, and Switzerland. In Italy, it has been found in open and humid woods, even in riparian ecosystems, localized on deciduous trees (especially of the Acer and Fraxinus genera), but also evergreens, such as Quercus ilex. It is listed as “possibly extinct” in much of northern Italy as well as “locally abundant” in suitable habitats in southern Italy. It is included in the “Red List of Italian epiphytic lichens” as “Near threatened” (Nascimbene et al. 2013).

Gyalecta derivata (Nyl.) H. Olivier

This species is widespread in Europe and northern Africa. In Italy, it is mainly widespread along the Tyrrhenian side of the Peninsula, where it is considered “very rare” (in the humid Mediterranean belt) to “extremely rare” (Nimis 2024). In Sicily, has only been identified on Marettimo Island (Nimis et al. 1994); therefore, the analysed locality represents the first report for the regional area. Similar to the previous species, it is also included in the “Red List of Italian epiphytic lichens” as “Near threatened” (Nascimbene et al. 2013).

Mycocenological studies have demonstrated the significance of the relationships between old forest formations and fungi, with the latter providing the forest with the “pabulum” of organic substance (Barluzzi et al. 1991); in fact, important correlations, trophic and otherwise, can be hypothesized between the woody component of the same communities (trees, shrubs and lianas) and the saprotrophic, in particular lignicolous and leaf litter components. Macrofungal diversity in forest ecosystems is strongly influenced by different management practices (Tomao et al. 2020). Unmanaged forests are distinguished by the richness of wood-inhabiting fungi and indicator species. In contrast, ectomycorrhizal species are more diverse in managed stands, whereas terrestrial saprotrophs are highly diversified in both managed and unmanaged mixed forests (Dvořák et al. 2017). Specific investigations carried out on the mycological flora in the Bosco Niscemi (Venturella et al. 2001; Ferraro et al. 2022) led to the discovery of 295 taxa in the site (30 Ascomycetes and 265 Basidiomycetes), in addition to Eichleriella leucophaea, a very rare lignicolous entity has recently been found (Saitta 2015), which will be discussed below. The most representative families are Tricholomataceae (61 taxa), Agaricaceae (24 taxa), Russulaceae (18 taxa), and Cortinariaceae (16 taxa). The numerically richest genera are Amanita Pers. and Agaricus L. (13 taxa), Russula Pers. (11 taxa), and Tricholoma (Fr.) Staude (11 taxa). The occurrence of infrequent species such as Trichoglossum hirsutum (Pers.) Boud. (Geoglossaceae) and Hydnocystis piligera Tul. & C. Tul. (Pyronemataceae) and the rare Typhula fistulosa (Holmsk.) Olariaga (Typhulaceae) is noteworthy. Significant ecological adaptability of species usually found in Sicily at altitudes above 800 m, such as Rubroboletus satanas (Lenz) Kuan Zhao & Zhu L. Yang (Boletaceae), Hygrophorus russula (Schaeff. ex Fr.) Kauffman (Hygrophoraceae), Lactarius mairei Malençon (Russulaceae), Phellodon niger (Fr.) P.Karst. (Thelephoraceae), Aureoboletus gentles (Quél.) Pouzar (Boletaceae), and Tricholoma acerbum (Bull. ex Pers.) Quél. (Tricholamataceae) has been detected. Another infrequent species is Mollisia cinerea (Batsch) P.Karst. (Mollisiaceae), confirming its lowest presence in the most disturbed clear-cut forests. Among other taxa, Xylaria polymorpha (Pers.) Grev. (Xylariaceae) and Hymenoscyphus serotinus (Pers.) W.Phillips (Helotiaceae) are old-growth forest indicators (Schmid and Helfer 1999; Parmasto 2001).

Eichleriella leucophaea Bres.

This is a species with a wide distribution, albeit very rare and fragmentary, with its only known locality so far for Sicily and Italy in the study area (Saitta 2015). In Europe, it is also reported in Spain (Dueñas 1997, 2002; Hernández-Crespo 2006; Prieto-García et al. 2010), France (Bourdot and Galzin 1928), Bulgaria (Pilát 1937), Poland (Bresadola 1903), Germany (Aron et al. 2005) and Norway (Saitta 2015), collected on dead wood of various forest species such as Q. ilex (Dueñas 2002).