A new pioneer association of detrital substrata of the hilly and low-mountain belts in Central Apennines (Italy)

Plant communities dominated by Petrosedum ser. Rupestria in Italy have always been referred to the Sedo albi-Scleranthetea biennis Br.-Bl. 1955 class. This group of species was also often found more or less abundant in the garrigue and pasture vegetation. Some unusual plant communities dominated by Petrosedum rupestre (L.) P.V.Heath with a conspicuous presence of typical species of scree vegetation were found and sampled in the Abruzzo region, on more or less mobile debris. In order to classify such communities, syntaxonomic, structural, chorological and ecological analyses have been performed. All the Petrosedum ser. Rupestria dominated communities reported in Italy have been compared with the sampled vegetation. A new perennial pioneer association, linked to more or less mobile detrital substrata of the hilly and lower-mountane belts of the calcareous Central Apennines has been here described with the name Linario purpureae-Petrosedetum rupestris. Based on the syntaxonomic analysis, this association has been included in the Linarion purpureae alliance (Scrophulario bicoloris-Helichrysetalia italici order). The Linario-Petrosedetum has been divided into two subassociations: a typical one named petrosedetosum rupestris and a thermophilous one of the lower altitudes named teucrietosum flavi. The study revealed the originality of such communities and their appropriate classification in the Thlaspietea rotundifolii class. This is the first record in Italy of a Petrosedum ser. Rupestria-rich community framed in the scree vegetation class.

During phytosociological investigations on the vegetation that colonizes debris substrates in the Abruzzo region, some unusual communities dominated by Petrosedum rupestre and Cephalaria leucantha were found and sampled. Although Petrosedum rupestre is considered a diagnostic species of the Sedo-Scleranthetea class (Mucina et al. 2016;Biondi et al. 2014), a certain debris mobility together with the presence of several species considered as diagnostic of the scree vegetation of the Thlaspietea rotundifolii class, casted doubt upon an easy classification of the observed plant community. In order to properly un-derstand and describe it, several kinds of analyses were performed together with a comparison with all other Sedum ser. Rupestria-dominated communities described in Italy.

Materials and methods
The vegetation survey has been carried out in several places of the Abruzzo region inside or just outside the "Gran Sasso and Laga Mountains" National Park, "Abruzzo, Lazio and Molise" National Park and "Sirente-Velino" Regional Park (Fig. 1). A complete list of the localities of sampling is provided in the Appendix I. The climate of the study areas was identified using published data of temperatures and rainfalls (Ufficio Idrografico e Mareografico di Pescara, 1971Pescara, -2000 spatialized for the whole Abruzzo region (unpublished). According to Rivas-Martínez et al. (2011), the thermotypes were found to be ranging between Upper Mesotemperate and Lower Supratemperate while ombrotypes were found to be ranging between Upper Subhumid and Lower Humid.
To better understand the ecology of the surveyed vegetation for each group of relevès, the Ellenberg indicator values (Pignatti, 2005) were calculated in three different ways: normal (based on the presence/absence of species in the relevès group), frequency-based (based on the occurrences of the species within the group) and weighted by cover of each species.
We used the species indicated by Mucina et al. (2016) as diagnostic of the vegetation classes to develop syntaxonomical spectra of the surveyed vegetation, that were also calculated normal, frequency-based and weighted.
A synoptic table including all the relevès available in the Italian phytosociological literature, having taxa belonging to Petrosedum ser. Rupestria as dominant species, was built in order to highlight possible floristic differences among different geographical areas.
Finally, we compared the results of all these analyses with the diagnoses of the alliances, orders and classes in Biondi et al. (2014) and Mucina et al. (2016), in order to identify the most suitable syntaxonomic frame for the surveyed vegetation, in terms of structural, bioclimatic, phytogeographical, ecological and floristic features.
For the syntaxonomic references at the ranks of alliance, order and class, we referred to Biondi et al. (2014). Plant species nomenclature follows Bartolucci et al. (2018).  ) occur only in the second group of relevès (rel. 9-13), representing a good set of differential taxa compared to the first group (rel. 1-8). The more abundant presence of chamaephytes, that are typical of garrigue vegetation, in the second group is to be correlated with the lower altitude at which they spread.
The synoptic table reported in Table 2 was built including all the Petrosedum ser. Rupestria-dominated vegetation types described in Italy (Poldini 1989;Di Pietro et al. 2006;Pignatti and Pignatti 2016). It shows the autonomy and the peculiar floristic composition of the studied vegetation, that results clearly characterized by a significant presence of diagnostic species of the class Thlaspietea rotundifolii. These species are almost completely absent in the communities surveyed on Prenestini Mts (Di Pietro et al. 2006), on Dolomites (Pignatti and Pignatti 2016) and in Friuli (Poldini 1989  The syntaxonomical spectra of the Abruzzo communities, calculated in order to establish the relative weight of the various groups of species (see Appendix II), show the prevalence of the species of the Festuco-Brometea class in the normal spectrum, while the species of the Thlaspietea rotundifolii class are prevalent in the frequency-based spectrum. Those of the Sedo-Scleranthetea and Cisto-Micromerietea classes prevail in the spectrum weighted on cover values.
The life form spectra show the prevalence of hemicryptophytes both in the normal and in the frequency-based spectra, while chamaephytes prevail in the weighted spectrum ( Figure 2).
The chorological spectra reveal the prevalence of the Mediterranean species in all the cases, with a significant presence of the S-European species mostly in the spectrum weighted on cover values ( Figure 3).
The application of the Ellenberg indicator values (Pignatti 2005) to the two groups of relevès of Table 1 (see Appendix III) shows a high light intensity (L) and moderately high values of temperature (T), both of them with maximum values in the second group of relevés; moderately high values of continentality (C), very low values of humidity (U), high values of soil pH (R), and very poor nutrient conditions (N).

Discussion
As shown in the synoptic table (Table 2), the coenological autonomy and floristic identity of the surveyed vegetation are evident. A high number of Thlaspietea rotundifolii diagnostic species allow the Abruzzo communities to be distinguished from similar communities described for other Italian areas.     The syntaxonomical spectra were analyzed in order to weigh the different role of taxa with reference to the various vegetation classes (see Appendix II). They show that the Festuco-Brometea is represented by a high number of species, showing low frequency values and even lower coverage values. The species of the Thlaspietea rotundifolii class are clearly prevalent in the frequency-based spectrum, while those of the Sedo-Scleranthetea and Cisto-Micromerietea classes prevail in the spectrum weighted on cover values. Obviously, the dominance of Petrosedum rupestre drives the role of Sedo-Scleranthetea prevailing in the weighted life form spectrum (Fig. 2C and 2F), while the high values of Cisto-Micromerietea of the second group of relevés (9-13 of Tab. 1) is due to the greater contribution of the chamaephytes in this group. The high values of the diagnostic species of Thlaspietea rotundifolii are linked to the mobility of the debris on which the surveyed vegetation preferentially grows. For these reasons, we include the surveyed vegetation in this last class. Subordinate syntaxa may be represented by Linario-Festucion dimorphae Avena &Bruno 1975 andThlaspietalia stylosi Avena &Bruno 1975 which are widely spread in Central Apennines (cf. Feoli Chiapella and Feoli 1977;Feoli Chiapella 1983;Biondi et al. 2000;Di Pietro et al. 2001, 2008etc.). Other alliances and/or orders reported for the Apennines such as  (Biondi et al. 2014). The communities of Linario-Festucion dimorphae which are normally found in the montane and subalpine belts of Central Apennines are characterized by the prevalence of Apennine Endemic, South-European Orophyte and Eurasian elements (e.g. Conti and Manzi 1992;Petriccione 1993;Biondi et al. 1999;Di Pietro et al. 2004). On the contrary, the analysis of the chorological spectra of the studied vegetation, as noticed, reveals a clear prevalence of the Mediterranean element, with relevant percentages of Steno-Mediterranean species, mostly in the normal and frequency-based spectra and in the second group of relevés (Figure 3). Such occurrence makes this collocation difficult.
A more consistent option is the order Scrophulario bicoloris-Helichrysetalia italici described by Brullo et al. (1998) for Sicily, recently used to classify some communities found in Central Italy. In fact, Di Pietro et al. (2017) provisionally referred to this order the association Helichryso italici-Dittrichietum viscosae Trinajstić ex Di Pietro, Germani and Fortini 2017, found on Cornicolani Mts. in southern Lazio, in the alliance Linarion purpureae Brullo 1984. Moreover, the vegetation of Artemisio variabilis-Helichrysetum italici Brullo and Spampinato 1990, reported for the terraces of the Trigno river (central Italy), was also referred to Scrophulario-Helichrysetalia, in the alliance Artemision variabilis Biondi, Ballelli, Allegrezza, Taffetani & Francalancia 1994(Pirone et al. 2009). The Linarion purpureae alliance includes the pioneer communities of carbonatic or pyroclastic volcanic screes which mainly develop in mountain areas of Southern Italy and Sicily (Brullo 1984), while Artemision variabilis includes the chamaephytic nitrophilous communities growing on river terraces and other gravelly incoherent substrata, in the Thermomediterranean and Mesomediterranean thermotypes of Southern Italy (Biondi et al. 2014).
The studied vegetation shows similar ecological features to Linarion purpureae, together with the presence of the diagnostic species Linaria purpurea and Rumex scutatus subsp. scutatus, and others similarly linked to mobile debris with a Mediterranean distribution (Scrophularia canina, Galeopsis angustifolia susbp. angustifolia, Lactuca viminea subsp. chondrilliflora).
On the basis of these considerations, we describe here a new association, with two subassociations, classified in the Linarion purpureae alliance and Scrophulario bicoloris-Helichrysetalia italici order. They are hereafter characterized. Mucina et al. (2016), include this order in the Balkanic Drypidetea spinosae, whereas Biondi et al. (2014) include it in Thlaspietea rotundifolii. In this paper we have preferred to follow the syntaxonomical interpretation provided by Biondi et al. (2014), since the species that are indicated as diagnostic of the Drypidetea spinosae class by Mucina et al. (2016) are almost absent in Central and Southern Italy, while the species of the Thlaspietea rotundifolii class are well represented both in our relevés and in those from Southern Italy.
Physiognomy and structure. Discontinuous vegetation with hemicryptophytes and chamaephites, with dominance of Petrosedum rupestre, Cephalaria leucantha, Linaria purpurea and Scrophularia canina. Diagnostic species. Petrosedum rupestre and Cephalaria leucantha. The first is a succulent chamaephyte with a Central-W-European distribution, linked to stony, rocky and debris habitats; the second is a semi-rosette hemicryptophyte with a S-European distribution, which lives mostly in garrigues, stony pastures and other debris environments (Pirone and Tammaro 1997;Allegrezza et al. 1997;Biondi et al. 2005). The choice of these species has been made on the basis of their dominance and frequency, and because they (more than others) reveal a peculiar vegetation in the frame of the Thlaspietea rotundifolii class. Syntaxonomy. For the reasons explained in the discussion, the new association Linario purpureae-Petrosedetum rupestris ass. nova is framed in the Linarion purpureae alliance (order Scrophulario bicoloris-Helichrysetalia italici, class Thlaspietea rotundifolii). Syndynamics. The association is framed in the vegetation series of both mixed thermophilous and semi-mesophilous oak forests of the order Quercetalia pubescenti-petraeae Klika 1933 and mesophilous forests of the order Fagetalia sylvaticae Pawłowski in Pawłowski, Sokołowski andWallisch 1928 (Pirone et al. 2010). The subassociation petrosedetosum rupestris is mainly linked to the series of semi-mesophilous and mesophilous woods. Synecology. The association develops on carbonatic incoherent sediments (gravel, debris, etc..), at altitudes between 400 and 1300 m a.s.l. The bioclimatic context ranges from Upper Mesotemperate to Lower Supratemperate, with ombrotypes from Upper Subhumid to Lower Humid. The typical subassociation is linked to an altitudinal range between 700 and 1,300 m a.s.l. with a Supratemperate thermotype. Synchorology. The association is known at the moment only for the hilly and lower mountain belts of the Gran Sasso, Sirente-Velino and Marsica Mts. groups and their surroundings; its potential distribution might include also other areas of the Carbonatic Apennines.
Linario purpureae-Petrosedetum rupestris ass. nova, teucrietosum flavi subass. nova (Table 1, rels. n. 9-14; holotypus: rel. n. 15) Physiognomy and structure. Comparable to the typical subassociation, with a more abundant presence of chamaephytes. Differential species. Teucrium flavum subsp. flavum and Euphorbia spinosa. These species have been chosen since they well highlight the more thermophilous character of this subassociation. Syndynamics. The subassociation is framed in the vegetation series of mixed thermophilous and semi-mesophilic oak woods of the order Quercetalia pubescenti-petraeae Klika 1933 (Pirone et al. 2010). Synecology. The subassociation settles on carbonatic incoherent sediments (gravel, escarpments with clastic debris, etc..), at an altitudinal range between 400 and 700 m a.s.l. The bioclimatic context can be referred to Upper Mesotemperate thermotype and Upper Subhumid ombrotype. It describes the most thermophilous aspects of the association, characterized by a greater presence of species of the Cisto-Micromerietea class. Synchorology. The subassociation is known at the moment only for the Gran Sasso and Sirente-Velino mountain groups and their surroundings; its potential distribution might include also other areas of the Carbonatic Apennines.

Conclusions
The performed syntaxonomic, structural, chorological and ecological analyses clearly showed the autonomy and the peculiarity of the studied vegetation that is formalized in the new association Linario purpureae-Petrosedetum rupestris ass. nova. The two subassociations petrosedetosum rupestris, typicum, and teucrietosum flavi, have been identified inside the association, the latter with reference to the most thermophilous aspects linked to the lower altitudes and differentiated by chamaephytes that are typical of the garrigue vegetation.
On the basis of the floristic, ecological and chorological features of the new association, we think that its best classification is in the Linarion purpureae alliance (order Scrophulario bicoloris-Helichrysetalia italici). At the moment we agree with the interpretation by Biondi et al. (2014) who put the order in the Thlaspietea rotundifolii class even if Mucina et al. (2016) includes it in the Drypidetea spinosae. A complete revision of the scree plant communities spread in the whole Center and Southern Italy and the Balkans should be performed in order to better understand the relationships between the two classes, but this goes beyond the aim of this work. This is the third report from Central Italy of communities referable to the Scrophulario-Helichrysetalia italici order after the ones by Pirone et al. (2009) andDi Pietro et al. (2017). However, Di Pietro et al. (2017) cast doubt on the possibility that typical classes of high-altitude screes and talus slopes such as Drypidetea and Thlaspietea may could represent strictly Mediterranean vegetation units like the ones surveyed on Cornicolani Mts. As to the Linarion purpureae alliance, this may be considered its first record from Central Italy, since the inclusion of the Helichryso italici-Dittrichietum viscosae in this alliance was provisional (Di Pietro et al. 2017).
Our results confirm the presence of the Linarion purpureae alliance and the Scrophulario bicoloris-Helichrysetalia italici order in Central Italy.