The Plant Communities of the Class Isoëto-Nanojuncetea in Sicily

A syntaxonomical revision of the Isoëto-Nanojuncetea class for the Sicilian territory is provided. This syntaxon gathers the ephemeral herbaceous hygrophilous plant communities linked to periodically submerged soils, widely distributed in the European, circum-Mediterranean and Macaronesian territories. Within this class, two orders are recognized, lsoëtetalia, with a prevalently Mediterranean distribution, and Nanocyperetalia chiefly occurring in the central-European and Atlantic territories, with scattered and marginal occurrence in the Mediterranean area. The order Isoëtetalia in Sicily is represented by four alliances, i.e., Isoëtion, Preslion cervinae, Cicendio-Solenopsion laurentiae and Agrostion salmanticae, while within Nanocyperetalia three alliances can be recognized, namely Nanocyperion, Verbenion supinae and Lythrion tribracteati. Overall, 32 plant communities are recognized, 11 of which are described for the first time. Each higher rank syntaxa and related associations are examined from a nomenclatural, floristic, ecological and chorological point of view. In particular, the associations were processed using cluster analysis in order to highlight the correlations between them. Regarding the floristic aspects, a checklist of the species occurring in the phytosociological relevés is provided, as well as a new combination concerning Solenopsis gasparrinii, a critical species of the Sicilian flora, is proposed.


Study Area
The research covers the whole of Sicily, including some neighboring Sicilian islands (Favignana, Isola Grande dello Stagnone, Pantelleria, Lampedusa). This island is situated in the central Mediterranean and represents the southernmost part of the Italian territory ( Figure 1). It is the largest island among those present in the Mediterranean with an area of 25,832.4 km 2 (including the smaller islands) and with a coastline 1637 km long, of which 1152 km regard the main island. It is surrounded to the north by the Tyrrhenian Sea, to the south-west by the Sicilian Channel, to the east by the Ionian Sea, and is separated to the north-east by the Italian peninsula by the Strait of Messina. Furthermore, of great importance for the investigations concerning this study, are the wetlands, that in Sicily are represented by various natural lakes, as well as by artificial basins and perennial watercourses. In particular, the most important lakes are the Biviere of Cesarò, the ponds of Nebrodi mountains, Biviere of Gela, Lentini lake, Preola and Gorghi Tondi, Gurrida, Pergusa, Pantano Gurna, etc. In addition, there are many artificial basins created in the last century, such as Ogliastro, Pozzillo, Arancio, Scanzano, Piana degli Albanesi, Disueri, Ancipa, etc. Among the several rivers that flow on the island are the Salso, Simeto, Belice, Platani, Verdura, Irminio, Dirillo, Alcantara, Anapo, San Leonardo, Cassibile, Fiumefreddo, Ippari, Ciane, etc.
The Sicilian territory shows a very complex geological history with rocky outcrops dated between the Paleozoic and Quaternary ages [94]. In particular, the oldest substrata belonging to Calabride chain units occurring in the Peloritani area (North-East Sicily) are represented by Paleozoic metamorphic rocks. The Nebrodi chain is characterized mainly by siliceous rocks from the late Mesozoic to Oligocene belonging to Numidian Flysch. In the Madonie massif, the substrata are mainly represented by carbonatic, dolomitic and quarzarenitic rocks from the Mesozoic age mainly belonging to Panormide units. The northwestern sector of Sicily and Aegadian Islands are prevalently constituted by Mesozoic carbonatic rocks, as well as the Sicani mountains. The Hyblean territory is characterized by Miocene limestones deeply carved by river valleys (caves), which are covered in the upstate by a layer of Plio-Pleistocenic lava. Most of southern and central Sicily is constituted by Plio-Pleistocenic rocks belonging to the Messinian evaporitic series (clays, sandstones, limestones, marls, gypsums, etc.). Finally, Mt. Etna is an active volcanic cone covered by basalt rocks from the Quaternary age, as well as Pantelleria island.
According to Rivas-Martínez [95] classification, two bioclimates can be identified in the Sicilian territory as reported by Bazan et al. [96], such as Mediterranean pluviseasonal oceanic occurring in almost the whole island, including the Aegadian Islands and Aeolian archipelago and Mediterranean xeric oceanic circumscribed exclusively to the coastal belt near Gela, Pantelleria and Pelagie islands. Besides, based on the investigations carried out by Brullo et al. [97] and Bazan et al. [96], 23-25 bioclimatic units can be recognized on the island. Each bioclimatic unit is closely linked to well defined climatophilous associations, that highlight the close correlations between the vegetation and the climatic conditions of a territory. In particular, it is possible to identify several thermotypes and ombrotypes based on the climatic trend regarding the monthly and annual monitoring of the available thermo-pluviometric stations. Among them, along the coastal belt, the lower Thermomediterranean lower dry type is predominant, while in the hilly areas the upper Thermomediterranean lower dry type is very spread out. Another type, quite frequent in Sicily, is the lower Mediterranean upper-lower dry type linked to sub-mountainous places, which in mountainous places is replaced mainly by upper Mesomediterranean at lower altitudes and lower Supramediterranean at upper ones, with upper dry to lower subhumid ombrotypes.
In the context of the Sicilian territory, the wetlands colonized by the plant communities object of this study occur from the coastal belt to the high mountains of the northern ranges, up to 1500-1600 m of elevation. The habitats that usually host this vegetation are represented by surfaces periodically flooded during the winter-spring period, gradually drying up in the summer, as well as the shores of lakes, swamps, reservoirs, and waterways. On the basis of the geological origin, edaphic characteristics, the length of submersion period and physicochemical properties of the waters, these habitats have been classified by Ernandes & Marchiori [98] and Ernandes et al. [73] into four main types, such as: (a) Cupular pools-Better known as rocky pools, which are usually small catchment depressions on flat outcrops arising from limestone dissolution (Figures 2B,E,F, 3E and 4A-D). The bottom of these ponds is covered by a thin layer of soil, submerged by shallow water, characterized by associations regarding the Isoëtion or Preslion cervinae.
(b) Dolines-They are broad depressed surfaces periodically flooded by rainwater with deep and poorly permeable soils, created by karst phenomena or by subsidence ( Figure 3A). Usually, in these habitats, there are communities of the Preslion crevinae, Agrostion salmanticae and Verbenion supinae.
(c) Waterlogged soils-They correspond to more or less large hollows with impermeable surfaces often localized in the wood clearing and covered by a thick layer of clay-silt soil often abundant in the sandy component (Figures 2A and 4G). These stands are submerged by shallow rainwater for short periods, and the plant communities belong mainly to Cicendio-Solenopsion laurentiae.
(d) Temporary streams-They are tiny and shallow watercourses, already dried up in late spring but with soils that remain moist for a long time (Figures 2C, 3B-F and 4E,F,H). In this category, the shores of lakes and artificial basins that dry up during the summer can be included. These habitats are colonized by sub-nitrophilous vegetation of Verbenion supinae or Lythrion tribracteati.
Based on these arrangements proposed by the aforesaid authors, it is considered more appropriate that the plant communities of this class occurring in the Italian territory can be framed in the following syntaxonomic scheme:

Vegetation Analysis
The Optimclass diagram shows a peak of faithful species at 30 partitions of the dataset. Additionally, compared with the Optimclass diagrams obtained by other clustering methods (e.g., flexible beta, Euclidean distance; UPGMA, Bray-Curtis; UPGMA, Euclidean), the optimal partitions turn out to range from 28 to 34. The Crispness of Classification indicates the clearest separations between two and seven clusters: at two clusters the two orders Isöetalia and Nanocyperetalia result separated (except for Agrostion salmanticae), while the partition at seven groups identifies the seven alliances. According to the Optimclass analysis, the dendrogram was pruned at the level of 32 clusters of relevés. The groups thus identified correspond to the surveyed associations, wholly autonomous from a floristic and ecological point of view and quite well differentiated from each other. Overall, 32 associations, seven alliances and two orders were recognized. On the whole, from the multivariate analysis, the associations identified were well differentiated from each other and were arranged in distinct clusters. Indeed, as shown in Figure 6, two main clusters can be detected, which are separated into ecological groups.
The first to disjoin is cluster A, including the associations referred to as Isoëtetalia, with a winter-spring cycle, linked to periodically flooded surfaces drying up from late spring to early autumn. This cluster splits into two main sub-clusters; of these, the first to separate is that one corresponding to A1, which includes communities subject to shorter periods of flooding, while the second is A2, concerning associations with soils submerged for longer periods, often until late spring, including exclusively those of Preslion cervinae. In the case of sub-cluster A1, the associations of two different alliances are here arranged, such as the Isoëtion (group C) and the Cicendio-Solenopsion laurentiae (group D), which differ mainly in the type of habitat in which they are localized. As regards cluster B, also within it there are two main sub-clusters, both corresponding to communities with a summer-autumn cycle. The most isolated one is indicated with B1, which concerns the associations of Lythrion tribracteati linked to loamy-clayey and nutrient-rich soils, while in B2 there are the associations of Verbenion supinae, typical of more or less nitrified soils and those of the Nanocyperion limited to hyper-humid and nutrient-poor soils. Unfortunately, the vegetation of the Agrostion salmanticae falls within the sub-cluster B2, together with that of the Nanocyperion. This arrangement is totally in contrast with the syntaxonomical framing since from the floristic and ecological point of view it belongs to the Isoëtetalia. This is probably due to the quite floristic poor set of this community, which does not allow clustering analysis to find a correct place in the dendrogram. Concerning the ecology of this vegetation, it is localized in large hollows, mainly represented by dolines, usually used as pastures, with well nitrified soils. Results of the DCA (Detrended Correspondence Analysis) ordination approximately confirm the general pattern highlighted by the cluster analysis. The eigenvalues decrease progressively from the first to the third axis (0.82, 0.49 and 0.36, respectively; higher eigenvalues are related to higher beta diversities). Figure 7 shows the ordination results along axis 1 and 2. On the left side of the DCA graph, the Isoëtetalia alliances (Agrostion pourretii included), are well separated from those ones of Nanocyperetalia that are distributed on the right side. Moreover, Figure 8 displays the relevés distributed based on the orders.

Syntaxonomical Scheme of Isoëto-Nanojuncetea in Sicily
According to the multivariate analysis based on the phytosociological relevés used for this investigation, and considering of the arrangements proposed from the aforesaid authors, the plant communities occurring in the study area can be framed in the following syntaxonomical scheme: Structure and ecology: Ephemeral amphibious vegetation occurring in temporary wetlands with soils periodically flooded by oligotrophic, mesotrophic, eutrophic, or sometimes brackish waters [39,113,114]. Floristically, these plant communities are dominated by hygrophilous therophytes often mixed with small hemicryptophytes and geophytes. These phytocoenoses correspond to a type of ephemeral vegetation, linked to very peculiar habitats characterized by a temporary submersion alternating with marked aridity, which in the absence of environmental alterations do not tend to evolve; therefore, they can be considered as communities representing 'permaseries' of vegetation. The associations of this class usually fall in the habitat of Community interest 3170*, which is considered of priority importance [115]. Structure and ecology: Pioneer ephemeral vegetation with thermophilous or subthermophilous requirements linked mainly to oligotrophic soils submerged up to early spring, sometimes flooded until early summer [39,113]. Usually, it is characterized by hygrophilous microphytes having an early spring blooming.
Geographical distribution: This order shows a Mediterranean and South Atlantic-European distribution.
Note: According to Fernández-González et al. [ Geographical distribution: This alliance has a Mediterranean distribution and in Sicily, it occurs in the several localities of the Islands, as well as Pantelleria Island, Lampedusa Island, Favignana Island and Isola Grande dello Stagnone ( Figure 9).

1.
Isoëtetum  [5] on siliceous substrata where it is localized on small rocky hollows submerged in the winter season. Floristically it is characterized by the dominance of Juncus capitatus and Isoëtes durieui, which grow together with other hygrophilous microphytes. In Sicily, the association occurs in the slightly depressed soils rich in silt and clay on siliceous substrata, where it is characterized by a very poor floristic set.

2.
Pulicario Structure and ecology: The association was surveyed in the rocky ponds of the arenaceous outcrops submerged by rainwater during the winter. At the bottom of these small depressions, a thin layer of silt is deposited, where peculiar amphibious hygrophilous vegetation grows. In this habitat, the occurrence of some hygrophytes, such as Isolepis cernua, Pulicaria vulgaris var. graeca, Damasonium polyspermum, Damasonium bourgaei is quite significant. These species, proposed as characteristic, are usually mixed with several other hygrophytes of Isoëto-Nanojuncetea. The association colonizes small surfaces within the perennial vegetation belonging to Sarcocornio fruticosae-Limonietum ferulacei juncetosum subulati, a hyper-halophilous association of Salicornietea fruticosae [78].
Geographical distribution: Currently, it is known only in the Isola Grande dello Stagnone (Trapani).

3.
Isoëto durieui-Ranunculetum parviflori Brullo Structure and ecology: The small, periodically flooded hollows on basaltic rocks, localized at about 600 m a.s.l., are colonized by hygrophilous vegetation dominated by Isoëtes durieui and Ranunculus parviflorus, which were proposed as characteristic species of a peculiar association named Isoëto-Ranunculetum parviflori [81]. In this vegetation, several species of Isoëto-Nanojuncetea are frequent, such as Ranunculus trilobus, R. muricatus, Juncus bufonius, Lythrum hyssopifolia, Lotus angustissimus, Mentha pulegium, etc. Besides, two sub-associations can be identified within it: isoëtetosum durieui corresponding to the typical aspect and callitrichetosum brutiae, linked to conditions of greater edaphic humidity occurring in the central part of the depressions.
Geographical distribution: It is a very rare association exclusive of Pantelleria Island, where it is localized at Monte Gibele on the bottom of a volcanic crater.

4.
Crassulo vaillantii-Elatinetum gussonei Bartolo Structure and ecology: The association occurs in the rocky ponds circumscribed to the carbonatic outcrops submerged by freshwater during the autumn-winter period. These peculiar habitats are characterized by a thin layer of soil covered by 5-10 cm of water at the time of maximum flooding. Floristically, this vegetation is differentiated by the dominance of Elatine gussonei, endemic to Lampedusa and Maltese islands [124,125], which is related to Elatine macropoda. It usually grows together with Buillardia vaillantii and other hygrophytes of Isoëto-Nanojuncetea. As concerns its bioclimatic requirements, this plant community grows within the upper Infra-Mediterranean belt, with lower semiarid hombrotype [96]. The climatophilous vegetation where it falls is represented by the thermo-xeric maquis belonging to the Periplocion angustifoliae Rivas-Martinez 1975 [88].

5.
Lythro hyssopifoliae-Elatinetum macropodae Brullo, Sciandrello, Tavilla  However, the vegetation is floristically and ecologically well differentiated from these two associations. In particular, Elatinetum macropodae colonizes the basaltic rocky pools and is characterized, apart from Elatine macropoda, by Damasonium polyspermum, Herniaria glabra, Pulicaria vulgaris and Lythrum tribracteatum, which are species fully absent in the Sicilian community, while Junco pygmaei-Elatinetum macropodae is localized along the edge of streams or temporary ponds with muddy-sandy substrates and is differentiated by the occurrences of Juncus pygmaeus and Middendorfia borysthenica growing together with Elatine macropoda. As a whole, both these associations show marked differences in comparison with the vegetation surveyed in Sicily. In fact, the latter is floristically differentiated by the occurrence of Buillardia vaillantii, showing high cover values, which is lacking in the other two aforesaid associations. Therefore, this Sicilian plant community is here proposed as a new association, named Lythro hyssopifoliae-Elatinetum macropodae. It should be noted that this new association is quite similar to Crassulo vaillantii-Elatinetum gussonei from Lampedusa, mainly for the habitat where it grows and also for the occurrence of Buillardia vaillantii, while Elatine macropoda is replaced by Elatine gussonei. Within this association three subassociations can be distinguished, namely: (a) subass. typicum (rel. 10-20) differentiated by the dominance of Elatine macropoda, linked to cupular pools flooded by shallow waters; (b) subass. buillardietosum vaillantii subass. nov. (rel. 1-9, holotypus rel. 8, hoc loco) localized in the stands with quite deep waters; (c) subass. callitrichetosum brutiae subass. nov. (rel. 21-25, holotypus rel. 24, hoc loco) occurring exclusively on stands with very deep waters.
Geographical distribution: This association to the best of current knowledge is exclusive of the Hyblean basaltic plateau in southern Sicily ( Figures 2E and 4B).

6.
Buillardio Structure and ecology: The small pools between limestone outcrops, both of natural and anthropogenic origin, usually named rocky pools, flooded in the autumn-winter period, and host highly specialized amphibious plant communities. In these stands, Bulliarda vaillantii seems to have here its optimum, growing together with various other ephemeral hygrophytes, such as Lythrum hyssopifolia, Juncus bufonius, Poa infirma, Juncus capitatus, Juncus hybridus, Polypogon subspathaceus, etc. The occurrence of a very peculiar species of Elatine, was quite significant; according to current knowledge of this genus, it can be attributed to E. campylosperma, a Mediterranean species with a very scattered distribution [127]. From the phytosociological point of view, this vegetation shows a close relationship with other Sicilian associations dominated by Elatine sp. and Bulliarda vaillantii, such as Lythro hyssopifoliae-Elatinetum macropodae and Crassulo vaillantii-Elatinetum gussonei, occurring in quite similar habitat, but differing among them from the floristic point of view, since characterized by other species of Elatine. Therefore, the plant community at issue is proposed as a new association named Buillardio vaillantii-Elatinetum campylospermae. Previously, this vegetation was wrongly attributed by Pasta et al. [128] to Elatinetum macropodae Br. Bl. 1936.
Geographical distribution: The association is localized in the North-western Sicily, where it occurs in some localities of the Trapani territory, such as Castello della Pietra (Castelvetrano), Isola Lunga dello Stagnone (Marsala) and Favignana island (Aegadian islands) ( Figures 2B, 4A and 5E).

7.
Isoëtetum todaroanae Brullo & Ilardi ass. nova hoc loco (Appendix B, Table A8) Holotypus: rel. 1, hoc loco. Characteristic species: Isoëtes todaroana. Structure and ecology: A rather rare and very peculiar community has been surveyed in small hollows on calcarenitic outcrops, submerged especially during the winter period. In this habitat Isoëtes todaroana, a peculiar species described by Troia & Raimondo [129], is localized. It grows on a thin layer of clay soil together with other hygrophytes of the Isoëto-Nanojuncetea including, in particular: Triglochin laxiflora, Romulea ramiflora, Isolepis cernua, Mentha pulegium, Lythrum hyssopifolia, Juncus bufonius. The temporary wetland where this vegetation currently occurs is a remaining fragment of a larger marsh that has been reclaimed in a cultivated area, and therefore, it takes on a relict meaning. Hence, this phytocoenosis for its floristic and ecological features is proposed as a new association named Isoëtetum todaroanae.
Geographical distribution: Currently, this association seems to have a punctiform distribution localizing in a small area near Mazara del Vallo (Trapani). Structure and ecology: Thermophilous plant communities localized in cupular pools, temporary marshes and dolines with deep stagnant waters or in stands with deep-water runoff flooded for most of the spring. This vegetation is rich in creeping amphibian species mixed with hygrophilous microphytes.

PRESLION CERVINAE
Geographical distribution: This alliance shows a Mediterranean range and in Sicily, it occurs in some localities of the northern and southern parts of the Island ( Figure 10).

8.
Isoëto Structure and ecology: The small rocky pools occurring on the basaltic plateau submerged in the rainy periods with very shallow waters persisting until the early spring, host a hygrophylous vegetation characterized by the dominance of Isoëtes velata and Crassula vaillantii, to which Warnstorfia fluitans, a rare moss recently recorded from Sicily [131], is often associated. It is a silicicolous community localized at 230-360 m a.s.l., that, according to Minissale et al. [132], can be referred to as Isoëto velatae-Crassuletum vaillantii, an association described in southern France by Poiron & Barbero [130]. The Sicilian vegetation shows a floristic set and ecological requirements quite similar to the one surveyed in France. Apart from the subass. typicum (rel. 1-12), in Sicily, is possible to distinguish a subass. ranunculetosum ophioglossifolii Brullo, Minissale, Sciandrello & Tavilla subass. nov. (rel. 13-16, holotypus rel. 13, hoc loco) occurring in the stands with a longer flooding period, floristically differentiated by Ranunculus ophioglossifolius and Warnstorfia fluitans.
Geographical distribution: According to literature data, this association considered circumscribed to France and Italy [51,77,130], occurs also in the Hyblean Plateau (southern Sicily), as already mentioned by Minissale et al. [132].

9.
Ranunculo lateriflori-Antinorietum insularis Brullo [80] as Ranunculo lateriflori-Antinorietum insularis and attributed, even if doubtfully, to Isoëtion. Effectively, due to the high frequency of Antinoria insularis, Ranunculus lateriflorus and Callitriche brutia this association is to be included within Preslion cervinae. Regarding its bioclimatic requirements, the association falls within the meso-Mediterranean subhumid belt. At first, two subassociations were recognized in this plant community, named isoëtetosum and ranunculetosum, differentiated, respectively, by Isoëtes sicula, formerly identified as I. durieui, linked to stands subject to a shorter submersion period, while the other one is dominated by Callitriche brutia, localizing on longer flooded surfaces. According to Brullo & Minissale [39], they must be considered as two distinct associations. The first one coincides with the association at issue, while the second one was named by Brullo & Minissale [39] Ranunculo lateriflori-Callitrichetum brutiae, which will be treated later.
Geographical distribution: This association occurs exclusively on the top of Monte Lauro in the Hyblean Plateau.  Table A11).
Holotypus: rel. 1, Tab. 2, Raimondo [86]. Characteristic species: Myosurus minimus, Spergularia madoniaca, Sagina subulata, Ranunculus marginatus Structure and ecology: The association is localized in mountain stands, at altitudes between 1400 and 1600 m a.s.l., especially on the bottom of large dolines, limitedly to the small depressions where water, resulting from the melting of snow, is stagnant for a long time. The soil is represented by clayey-silty deposits, resulting from the erosion of the surrounding carbonatic rocks, with an acidic pH, usually drying up towards the end of spring. These wet surfaces are colonized by dense vegetation with hygrophilous microphytes, where some of them play a relevant physiognomic role. In particular, Myosurus minimus, Spergularia madoniaca, Antinoria insularis, Ranunculus lateriflorus, are the most frequent, which are associated with several other hygrophytes of the Isoëto-Nanojuncetea. The association, described by Raimondo [86] as Myosuro minimi-Ranunculetum lateriflori, was included in the Isoëtion, while Brullo & Minissale [39] put it in synonymy with Ranunculo lateriflori-Antinorietum insularis. Based on the current knowledge this vegetation is more advisable to be kept as an autonomous association, closely related to the latter. As previously emphasized by Brullo et al. [84], both for their localization in the mountain belt and for the floristic set, mainly due to the occurrence of Ranunculus lateriflorus, Myosuro-Ranunculetum lateriflori and also Ranunculo-Antinorietum insularis can be considered as geographical vi-cariants of other allied associations, such as Sedo nevadensis-Juncetum pygmaeis Quezel 1957 from Atlas range (North Africa), Junco-Isoëtetum velatae Rivas Goday 1955 from Spain, and Veronico-Ranunculetum lateriflori Quezel 1973 from Tauro massif (Turkey).
Geographical distribution: The association is localized in the Madonie massif (North Sicily) ( Figure 3A).

11.
Ranunculetum pratensi-lateriflori Brullo, C. Brullo & Giusso ass. nova hoc loco (Appendix B, Table A12). Structure and ecology: In the mountain range of northern Sicily, at altitudes between 1300 and 1700 m a.s.l., in correspondence with wetlands periodically flooded by shallow waters; ephemeral hygrophilous vegetation with a typical spring cycle. The substrates consist of siliceous rocks, mainly represented by flysch and schists, covered by silty-clayey deposits. The vegetation colonizing these places is characterized by quite specialized hygrophytes, where a relevant physiognomic role is played by Antinoria insularis and Ranunculus lateriflorus, as well as by R. pratensis, Veronica serpyllifolia, Barbarea bracteosa, which allow differentiating a new association, closely related to the Ranunculo-Antinorietum insularis. Previously Brullo & Grillo [84], treated this plant community as a subass. veronicetosum of the last association. Nevertheless, apart from the different floristic sets, it is well diversified also from the ecological point of view, since it is distributed at higher altitudes, well over 1000 m a.s.l., and on different substrates. Therefore, it is proposed as Ranunculetum pratensi-lateriflori, which is spread mainly in the supra-Mediterranean humid belt.

Ranunculo lateriflori-Callitrichetum brutiae
Geographical distribution: The association is currently recorded only from Monte Lauro (Hyblean plateau). Structure and ecology: In the deeper depressions of the basaltic substrata, often submerged until the end of springtime, Anagallido parviflorae-Molinerielletum minutae is replaced by Callitricho brutiae-Crassuletum vaillantii, an association with more hygrophilous requirements. Floristically, this vegetation is characterized by the dominance of Callitriche brutia and Bulliarda vaillantii, which grows together with other hygrophilous species of the Isoëto-Nanojuncetea. It shows some relations with Ranunculo lateriflori-Callitrichetum brutiae, from which it differs in lower hygrophily and absence of Ranunculus lateriflorus, while Coleostephus myconis occurs, which emphasizes the more xericity of the stands.
Holotypus: rel. 1, hoc loco Characteristic species: Pilularia minuta and Myosotis sicula. Structure and ecology: The association was surveyed in wide temporary wetlands flooded during the winter-spring period, localized on clayey-siliceous substrates covered by deep muddy soils. This vegetation is distributed about 200 m a.s.l., within the thermo-Mediterranean bioclimatic belt. It is characterized by the occurrence of Pilularia minuta, a rare and inconspicuous fern, which was recently rediscovered in Sicily by Troia & Lansdown [133]. This hygrophyte usually grows with other species of the Isoëto-Nanojuncetea, such as Myosotis sicula. Ranunculus ophioglossifolius, Isoëtes longissima, Lotus parviflorus, Elatine macropoda, etc. For its ecology and floristic set, it is well differentiated from the Junco pygmaei-Pilularietum minutae, as well as from the Pilulario minutae-Isoetetum longissimae.
Geographical distribution: This association seems localized in Contrada Anguillara near Calatafimi (western Sicily). Structure and ecology: This alliance groups spring communities with acidophilus requirements, markedly more sciaphilous and hygrophilous than those of the Isoëtion, with soils remaining humid for long periods. They are localized on waterlogged soils of large hollows with waterproof surfaces, sometimes represented by a wood clearing, covered by a thick layer of clay-silt soil, usually rich in a sandy component. It can be observed also in rocky ponds with sandy soils. In these stands, the hygrophilous microphytes are submerged by shallow rainwater often until late spring. Mucina et al. [114] synonymized this alliance with Cicendion (Rivas Goday in Rivas Goday & Borja 1961) Br.-Bl. 1967, and included it within the Isoëtetalia. When Rivas Goday & Borja (1961) described this syntaxon, they treated it as Cicendenion (sub Cicendion), considering it as a suballiance of Nanocyperion flavescentis Koch 1926, including within it the Cicendietum filiformis Allorge 1922, which represents, therefore, its nomenclature type. Later, Braun-Blanquet [18] raised this syntaxon to the alliance level, including within it the new association Isoëto velatae-Cicendietum filiformis. In particular, the Cicendietum filiformis, described from northern France is characterized by a floristic set rich in elements of the Nanocyperetalia order, such as Isolepis setacea, Juncus tenuis, Cyperus flavescens, C. fuscus, Lythrum portula, Spergularia rubra, Sagina procumbens, Centaurium pulchellum, Gnaphalium ulginosum, G. luteo-album, while the floristic elements of Isoëtetalia are absent. Besides, it should be noted that from the nomenclatural viewpoint, the Radiolion linoidis Pietsch 1973 represents a synonym of the Cicendion as emphasized by Brullo & Minissale [39]. In fact, the two syntaxa are floristically and ecologically perfectly overlapping, since either way the associations referred to them (Cicendietum filiformis included) have the optimum at last spring to early summer, and occur in the territories with temperate bioclimate. Besides, both are characterized by a peculiar pool of species, such as Centunculus minimus, Radiola linoides, Hypericum humifusum, Montia minor, Chaetonychia cymosa, and many other of the Nanocyperetalia order.

CICENDIO-SOLENOPSION LAURENTIAE
Geographical distribution: The alliance is well represented in the western and central Mediterranean area and in Sicily, it occurs in some localities of the western and southern part of the Island ( Figure 11).  Characteristic species: Archidium phascoides, Isoëtes longissima (= Isoëtes velata) Structure and ecology: Hygrophilous vegetation that prefers small humid depressions, on basaltic substrata covered by a shallow silty soil layer, subject to flooding during the autumn-winter period. From a structural point of view, the vegetation is characterized by a low moss carpet dominated by Archidium phascoides, on which numerous hygrophilous microphytes grow, such as, Solenopsis laurentia subsp. hyblaea, Juncus bufonius, J. pygmaeus, J. capitatus, Anagallis parviflora, Centaurium maritimum, etc. The occurrence of some Isoëtes, such as I. longissima, I. durieui and I. histrix is significant. The association is quite rare localizing at 230-385 m a.s.l., within the thermo-Mediterranean sub-humid bioclimatic belt.

Archidio phascoidis-Isoetetum velatae
Geographical distribution: The association occurs on scattered volcanic rocks in the Hyblean Plateau ( Figures 3E and 4C Characteristic species: Molineriella minuta Structure and ecology: Hygrophilous vegetation linked to flat stands with shallow loamy soils deposited on basaltic substrates, subject to short periods of submersion during the autumn and winter months. It is an ephemeral association with a very early vegetative cycle (late winter-early spring), in which a remarkable floristic set occurs, represented mainly by microphytes of the Isoëto-Nanojuncetea. Among them, Isoëtes durieui, Anagallis parviflora, Lotus conimbricensis, L. angustissimus, Lythrum hyssopifolia, Juncus bufonius, J. capitatus, J. pygmaeus, Polypogon subspathaceus, Mentha pulegium, Poa infirma, etc., can be mentioned. Besides, particularly significant is the occurrence of Molineriella minuta, a species very rare in Sicily, which is treated as a characteristic species of the association. Within this community, two sub-associations were identified: typicum, which occupies poorly wet peripherical places subject to a very short period of submersion, and crassuletosum vaillantii, differentiated by the dominance of Buillardia vaillantii, which is localized in the central part of the depressions with deeper waters [83].
Geographical distribution: This association is reported from Bosco Pisano near Buccheri (Hyblean plateau). This association was surveyed on wide wetlands with siltysandy and slightly brackish soils in stands near the sea. Usually, this vegetation is localized in temporarily flooded surfaces occurring in the large clearings within the maquis and is floristically differentiated by Solenopsis mothiana, a rare endemic microphyte described by Brullo et al. [135]. It grows together with Anagallis parviflora, Radiola linoides, Cicendia filiformis, Centaurium maritimum, Briza minor, Mentha pulegium, Juncus bufonius, Lythrum hyssopifolia, etc. It is very peculiar vegetation floristically and is ecologically well differentiated from the other Sicilian association of the Cicendio-Solenopsion laurentiae.

Kickxio cirrhosae-Solenopsietum gasparrinii
Geographical distribution: This association is exclusive of the Isola Grande dello Stagnone near Marsala, where it is very rare (2A).
Holotypus: rel. 2, hoc loco Characteristic species: Isoëtes sicula Structure and ecology: This association is localized in the wide wetlands that are temporarily flooded, especially in the autumn-winter time, and which tend to dry up since the early spring. It grows on silty-sandy soils which keep the humidity throughout the spring and is differentiated by several small hygrophytes showing a high value of coverage. Floristically, this vegetation is characterized by Isoëtes sicula, which grows with Solenopsis gasparrinii, Ophioglossum lusitanicum, Anagallis parviflora, Anagallis minima, Radiola linoides, Cicendia filiformis, Lotus parviflorus, Romulea ramiflora, Isolepis cernua, Juncus bufonius, etc. Currently, the Solenopsio gasparrinii-Isoëtetum siculae, similarly to most microphytic associations of the Isoëtetalia, has a very narrow distribution due to human pressure, in particular for the reclamation of wetlands and their use in farming land.
Geographical distribution: This association occurs in western Sicily near Calatafimi and Mazara del Vallo (Trapani) ( Figure 5A).
Holotypus: rel. 2, hoc loco. Characteristic species: Isoëtes histrix, Myosotis congesta, Aphanes arvensis. Structure and ecology: The wide wetlands characterized by sandy soils, which are flooded by rainwaters in the autumn-winter period, are colonized by peculiar hygrophilous vegetation dominated by Isoëtes histrix. In this stand, remaining quite wet during the springtime, some rare microphytes, such as Myosotis congesta and Aphanes arvensis, occur too. In particular, Myosotis congesta was recently quoted as a new record from Italy by Azzaro et al. [136] and subsequently typified by Tavilla et al. [137], which provides some information on its morphological, ecological and chorological features. This vegetation, for its floristic and ecological peculiarities, is proposed as a new association, named Myosotido congestae-Isoëtetum histricis, which shows a quite poor floristic set and can be referred to as Cicendio-Solenopsion laurentiae for its ecological requirements and the occurrence of Anagallis parviflora. The association is localized within the clearings of thermophilous Quercus suber woodlands.
Geographical distribution: This association is very rare and was observed only inside the nature reserve "Bosco di Santo Pietro" near Caltagirone (southern Sicily). Characteristic species: Agrostis pourretii, Chamaemelum fuscatum, Trifolium michelianum. Structure and ecology: Spring blooming communities, which are linked to humid depressions with long-lasting waters in winter and spring and with predominantly arenaceous soils. Physiognomically, they are dominated by graminoid therophytes and show their optimum in the late spring.

AGROSTION SALMANTICAE
Geographical distribution: The associations of this syntaxon are frequent in the Iberian-Atlantic and West Mediterranean area (Spain, France, Corse, Sardinia, Sicily, and southern Italy). In Sicily, it is localized in the North-West part of the Island (Figure 11). Structure and ecology: In the wide wetlands characterized by silty-sandy soils deposited on flyschoid clays, dense vegetation dominated by Agrostis pourretii and Trifolium michelianum occurs. In particular, the plant community characterized by these species colonizes small stands localized within clearings of deciduous oak woods, where it is mixed with perennial hygrophilous vegetation linked to surfaces submerged by deeper waters. Previously, Caldarella et al. [138] described a very heterogeneous association including both the perennial vegetation of Phragmito-Magnocaricetea and the annual phytocoenosis with Agrostis pourretii belonging to Isoëto-Nanojuncetea, which was proposed as Trifolio micheliani-Glycerietum spicati. Indeed, this association represents very complex vegetation including two well distinct communities belonging to different phytosociological classes. In fact, these authors carried out their relevés on very large surfaces (70-100 m 2 ) without respecting the criterion of the floristic-ecological homogeneity of the surveyed vegetation. This is evident in the floristic set of the relevés, including species of various phytosociological typologies (Phragmito-Magnocaricetea, Molinio-Arrhenatheretea and Isoëto-Nanojuncetea). In particular, within these wide wetlands, the plant communities belonging to Isoëto-Nanojuncetea are localized on small surfaces (at most 10-20 m 2 ), with soils that are already emerged in early spring compared to the surrounding surfaces still inundated. In fact, the floristic set in these stands consists almost exclusively of hygrophilous therophytes belonging to the Isoëto-Nanojuncetea class. This vegetation is, therefore, proposed as a new association named Trifolio micheliani-Agrostidetum pourretii, which for its floristic and ecological peculiarities must be referred to as Agrostion salmanticae. Previously, associations referring to this alliance were described in the Italian territory from North Sardinia by Biondi & Bagella [75] and Bagella et al. [77] and Apulia by Tomaselli et al. [51], represented, respectively, by Anthoxantho aristati-Agrostidetum salmanticae and Phalarido minoris-Agrostidetum pourretii. Structure and ecology: This syntaxon group's ephemeral vegetation localized in wide wet hollows usually flooded until early summer, with soils being mostly eutrophic or subeutrophic, often hypertrophic, and usually well nitrified since they are used as pastures, or more rarely oligo-mesotrophic. Floristically, it is differentiated by the occurrence of species with summer-autumn blooming, showing a prostrate and creeping habit.

Trifolio micheliani-Agrostidetum pourretii
Geographical distribution: This order is distributed in the Atlantic and central European territories, extending also to the Mediterranean ones with Temperate bioclimate, such as mountain stands or in artificial basins and along the coastal places with slightly brackish soils.
Notes: According to Fernández-González et al. [116], the correct name of the order at issue is Nanocypero-Polygonetalia Koch 1926 for reasons of priority, as it was validly published (Art. 2b, ICPN). Indeed, Koch [3] within this order included two new alliances, such as Nanocyperion flavescentis and Polygono-Chenopodion polyspermi, of which the second is invalid according to Art. 3f (ICPN), while the Nanocyperion flavescentis is a valid alliance. Indeed, Koch [3] did not provide relevés but unambiguously referred it to an association validly published by Braun-Blanquet [139], namely Junco compressus-Parvo-Cyperus-Association. Besides, Cyperus flavescens L. occurs in the typical relevé, which, therefore, validates the Cyperetum flavescentis proposed by Koch [3]. Unfortunately, this name is a superfluous name for the Junco compressi-Parvo-Cyperetum (Art. 29c, ICPN). In conclusion, Fernández-González et al. [116], proposed that, as Nanocypero-Polygonetalia is a disused name, Nanocyperetalia Klika 1935 should be the nomen conservandum, since the latter has always been the name used in literature. Structure and ecology: This alliance groups plant communities rich in annual small caespitose sedges linked to wet, sandy-clayey, or organic soils, which are submerged for long period by oligo-mesotrophic, freshwater. This vegetation, showing its maximum vegetative development in the summer-autumn period, in Sicily is very rare and localized in the sub-mountain belt.

NANOCYPERION FLAVESCENTIS
Geographical distribution: The associations of this alliance are widespread in the Euro-Siberian region and in particular in the Atlantic and central European territories, while they are less frequent in the Mediterranean ones. As concerns Sicily, it is localized in the North-East part of the Island (Figure 12).
Holotypus: rel. 1, Tab. 7, Sciandrello et al. [140] Characteristic species: Plantago intermedia, Cyperus fuscus. Structure and ecology: The association was surveyed in the small ponds localized on metamorphic substrata with acid soils submerged by freshwater for long period, remaining moist also in the summertime. It seems distributed at an altitude of 600-700 m a.s.l., within the meso-Mediterranean sub-humid belt. Physiognomically, this vegetation is dominated by two caespitose sedges, such as Cyperus fuscus and C. flavescens, usually growing with Plantago intermedia, Mentha pulegium, Juncus hybridus, Lythrum hyssopifolia, Juncus bufonius, etc. It can be found in sub-mountains stands characterized by climatophilous deciduous oak woodlands linked to siliceous substrata.
Geographical distribution: This association is circumscribed to a few localities of the Peloritani chain (NE Sicily) (4F).  Structure and ecology: Ephemeral vegetation occurring in wide depressions, represented by lagoons, lakes, artificial basins, riverbanks, etc., which are subjected to long periods of submersion, usually until early summer, and often characterized by well nitrified soils. In these habitats, flooded by eutrophic or hypertrophic water, prostrate-creeping species, often of large size, having a summer-autumnal blooming are frequent.

Coronopo squamati-Sisymbrelletum dentatae
Holotypus: rel. 6, Tab. 8, Minissale & Spampinato [87]. Characteristic species: Sisymbrella dentata, Anthemis cotula Structure and ecology: The association was described by Minissale & Spampinato [87] in a depressed area characterized by basaltic substrata, subject to seasonal flooding, often prolonged until the beginning of the summertime. The rocky surfaces are covered with silty-clayey deposits, flooded by shallow waters, and are more or less dried up during the summer. Physiognomically, this vegetation is well differentiated for the dominance of Sisymbrella dentata, a rare Sicilian endemism, which generally grows together with other hygrophytes, including Anthemis cotula, Hordeum hystrix, Coronopus squamatus, Mentha pulegium, Teucrium campanulatum, Ranunculus sardous, R. trilobus, Eryngium pusillum, etc. Currently, the association is rather degraded, since the areas originally occupied by it are subject to cultural activities, due to their transformation into vineyards.
Geographical distribution: The vegetation is mainly diffused in correspondence with the Gurrida Lake, at the base of the Etna Mount near Randazzo. This association was observed also near Castiglione di Sicilia. Probably, this association in the past was much more frequent in Sicily, since Sisymbrella dentata, a characteristic species, was recorded in many places on the island, where it has today, unfortunately, disappeared [143] ( Figure 3C). Structure and ecology: This association is localized in natural coastal lakes, in correspondence with the peripheral surfaces subject to summer-autumn drying. These stands are characterized by more or less flat and clayey-loamy soils, with a certain concentration of nitrates, still humid under the superficial crust. From the floristic point of view, it appears as an annual pioneer vegetation with a hygro-subnitrophilous character, dominated by therophytes with creeping habits, such as Sporobolus schoenoides, Sporobolus aculeatus, Cyperus fuscus. The differential of the association is Oxybasis chenopodioides, a very rare species in Sicily, linked to brackish lake environments, often near the sea, which tends to cover large surfaces along the dried up edges of the basin. As already highlighted by Brullo & Sciandrello [89], due to its floristic and ecological peculiarities, this association shows a remarkable affinity with Amarantho albi-Chenopodietum botryoidis, described for the territory of Granada (southern Spain) by Martinez Parras et al. [144].

Heleochloo schoenoidis-Chenopodietum botryoidis
Geographical distribution: The association was surveyed at Biviere of Gela, a wide coastal lake in southern Sicily. Structure and ecology: This alliance groups the ephemeral, hygrophilous plant communities occurring in wetlands with a long flooding period, having a summer-autumnal optimum. They are linked to silty-clay soils submerged by eutrophic waters, sometimes weakly brackish.

Coronopo squamati-Corrigioletum litoralis
Geographical distribution: This alliance is distributed in the West Mediterranean territory and in Sicily, it occurs in the southern part of the Island (Figure 12). Structure and ecology: This association is localized on flat surfaces with clayeysandy soils, subject to long periods of submersion. It has its maximum expression in the summer-autumn period, when the soil, which is still quite humid, is no longer submerged. It is a hygrophilous vegetation dominated by therophytes with a prostrate or prostrate-ascending habit, including, in particular, Damasonium bourgaei, Sporobolus aculeatus, Heliotropium supinum, Coronopus squamatus, Euphorbia chamaesyce, Paspalum disticum, etc. The occurrence of Cressa cretica is here even more significant since it highlights the halo-nitrophilous requirements of this association. This association was first described by Rivas-Martínez et al. [145] as Damasonio alismae-Crypsietum aculeatae, whose name was later corrected by Costa et al. [111] since Damasonium bourgaei was misidentified as D. alisma. Among the synonyms of this association the Cresso creticae-Damasonietum bourgaei, described by Sciandrello [90] from southern Sicily, must be mentioned too. Previously this association was included in Helochloion or in the Verbenion supinae.

Damasonio bourgaei-Crypsietum aculeatae
Geographical distribution: From literature data [90] and unpublished relevés, this association in Sicily is localized in the Gela territory. It is also recorded in the Iberian Peninsula [111,145] and Tunisia [146] (Figures 3D, 4E and 5M).
Holotypus: rel. 3, hoc loco. Characteristic species: Ranunculus trilobus Structure and ecology: The association was surveyed on small marshy surfaces within communities dominated by helophytes belonging to the Phragmito-Magnocaricetea. It prefers sandy-loamy soils submerged by slightly salty waters during the winter-spring period while remaining humid during the summer. Floristically this vegetation is characterized by therophytes with erect or prostrate habits, among these can be quoted Ranunculus trilobus, usually associated with Lythrum tribracteatum, which highlights the sub-halophilous character of this vegetation. Other hydrophytes of Isoëto-Nanojuncetea are also frequent, such as Lythrum hyssopifolia, Coronopus squamatus, Damasonium bourgaei, Pulicaria sicula, Hordeum marinum, Schenkia spicata, Mentha pulegium, Polypogon subspathaceus, etc. It is proposed as a new association, named Ranunculo trilobi-Lythretum tribracteati.
Geographical distribution: This association is quite rare and occurs only in some salt marshes near Gela (southern Sicily) ( Figure 5L).  [92], from a locality near the rocky coast, in correspondence with a small temporary pond, drying up during the summer period. It is limited to a depression within a calcareous outcrop covered by a layer of sandy-clayey soil. Floristically, it is characterized by hygrophilous therophytes with a predominant creeping habit, dominated by Pulicaria vulgaris var. graeca and Damasonium bourgaei. Other species linked to wet habitats, even if rather sporadically, occur, such as Heliotropium supinum, Polypogon subspathaceus, Lythrum tribracteatum, Coronopus squamatus, etc. It shows some floristic relationships with Damasonio bourgaei-Crypsietum aculeatae, from which it differs substantially in its ecological requirements since the latter association is localized on wide clayey surfaces and shows a more marked halophilous character.

Pulicario graecae-Damasonietum bourgaei
Geographical distribution: This association was surveyed on a stand of the calcareous coast of the Maddalena peninsula near Syracuse. Plant communities referable to this association, but floristically impoverished, were observed in other coastal places from Sicily.

Dataset
The study was carried out over a period of about 50 years with phytosociological investigations in the field and also using the numerous literature data which have been published in the meantime. The present study is based on 394 phytosociological relevés made following the method of Braun Blanquet [147], of which 215 are from literature data and 179 unpublished data (Appendix B). The relevés were carried out in several Sicilian natural areas: the Simeto river (Catania), Capo Murro di Porco and Archeological ruins of Syracuse, Piana del Signore marshes (Gela), Hyblean Plateau, Monte Lauro, the surroundings of Trapani and Palermo, Madonie, Gurrida lake, Nebrodi, Peloritani, etc. The nomenclature of the surveyed syntaxa follows the 4th edition of the International Code of Phytosociological Nomenclature (ICPN, [117]), while the syntaxonomical arrangement follows Brullo and Minissale [39], Rivas Martinez et al. [109], Brullo et al. [148], Biondi et al. [113], and Mucina et al. [114].

Floristic Nomenclautre
As concerns the floristic nomenclature and life-forms, we have followed Pignatti [149][150][151][152] and the Portal to the Flora of Italy [153]. The checklist of the species occurring in the phytosociological relevés is reported in Appendix A. Chorological types follow Brullo et al. [154].

Data Analysis
In order to verify the syntaxonomical relations among the surveyed plant communities, some relevés (up to five based on availability) were selected and the cover-abundance values (following the scale of Braun-Blanquet [147] were transformed according to the method proposed by van Der Maarel [155]. Thus, a matrix of 143 relevés × 158 species was selected from the original data-set and subjected to multivariate analysis, after removing species with a frequency lower than 1%. Hierarchical clustering on the final matrix was performed by using flexible beta linkage, with the Bray-Curtis coefficient. Beta was set at −0.25 so that flexible beta clustering became a space-conserving method [156]. To determine the optimal number of clusters, we used the "Optimclass 1" method (p < 10-6) [157], applying the function "Crispness of Classification" to each data set partition. For the ordination analyses, we carried out a Detrended Correspondence Analysis (DCA) according to the Hill & Gauch [158] approach. Hierarchical clustering and ordination analysis were run by PCOrd version 6.08. Optimclass and Crispness of Classification were performed by the software JUICE [159].

Conclusions
Our survey allowed us to highlight the great diversity of the plant communities belonging to the Isoëto Nanojuncetea class occurring in Sicily in many different environments, such as small rocky pools, large ponds, or the banks of artificial basins. In Sicily, they are mainly found on carbonatic, volcanic, siliceous, or clayey substrata. Although we do not have a detailed mapping, on the basis of our expert-based we can affirm that most of the detected plant communities fall into protected areas, such as regional parks, nature reserves and Natura 2000 sites, but this is not enough to guarantee their real protection, because their existence depends above all on good land management and often on a delicate balance between grazing and agro-silvopastoral activities [160]. All the plant communities of Isoëto-Nanojuncetea treated here, due to their attribution to Mediterranean temporary ponds, can be referred to as the habitats of priority interest codified as 3170* according to the Annex II of the Habitat Directive (92/43/EEC), whereby they require rigorous protection by the States of the European Union [1]. The criticality of this type of habitat, being temporary ponds, is above all linked to the fact that in many cases they are limited to small surfaces, apart from having a very fragmented distribution. All this tends to make them not very visible and does not give them sufficient importance, therefore, they are usually neglected. Overall, they are quite vulnerable, even to involuntary destruction [161,162], or to changes in land use, which together can contribute to the disappearance or alteration of these relevant micro-habitats [163]. Another problem with the protection and management of these habitats is that in many cases, covering very small surfaces, they escape the cartographic surveys of vegetation on a regional scale, which happened in Sicily [164,165]. Even in the inventories of wetlands on a regional or national scale, Mediterranean temporary ponds are largely under-represented, thus limiting the possibilities of protection and correct management [166]. It is, therefore, understood that there is a need to intensify regional field surveys in order to have a better awareness of the real distribution of this habitat. It is to be hoped that in the future these results will stimulate adequate research and management policies on these Mediterranean temporary ponds and their conservation. Unfortunately, this cannot be separated from effective and coordinated governance at the national level, based on multiple spatial scales ranging from land-use policies to the management of protected areas, agricultural areas, and so on [167].