Contribution to the Knowledge of Rocky Plant Communities of the Southwest Iberian Peninsula

The rocky habitats of southern Portugal are ecosystems with extreme xericity conditions, associated with special abiotic strains. In these unstable ecological conditions, a considerable diversity of plant communities occurs. The objective of this study, carried out in the Algarve and Monchique, and the Mariánica Range biogeographical sectors, is to compare chasmo-chomophytic communities of the southwestern Iberian Peninsula, using a phytosociological approach (Braun–Blanquet methodology) and numerical analysis (hierarchical cluster analysis). From these results, two new communities were identified, Sanguisorbo rupicolae-Dianthetum crassipedis and Antirrhinetum onubensis, as a result of floristic and biogeographical differences from other associations already described within the alliances Rumici indurati-Dianthion lusitani and Calendulo lusitanicae-Antirrhinion linkiani, both included in the Phagnalo saxatilis-Rumicetea indurate class.


Introduction
Habitats are very important natural or semi-natural places that need to be continuously studied to preserve them and their inhabitants, especially the plant endemic species that characterize them [1,2]. In fact, different habitats in the world are threatened by different types of pressures and threats [3][4][5][6][7][8][9][10]. Among the wide spectrum of habitats, we focus on plant communities that develop on rocky habitats, which are often associated with crests, cliffs, lithosols, rocky soils or rock outcrops. In these edaphoxerophilous biotopes that appear to be evidence of extreme xericity conditions, namely great edaphic drought as a result of the soil's reduced capacity for water retention [5,11]. In fact, this environment offers the ideal conditions for a highly specialized type of vegetation: the chasmo-chomophytic communities, which encompass rupicolous vegetation that colonizes earthy broad crevices, rocky soils and lithosols (chomophytes) or narrow rocky fissures (chasmophytes).
The perennial chasmo-chomophytic communities of the southwestern Iberian Peninsula are included in the Phagnalo saxatilis-Rumicetea indurati vegetation class [12,13]. The Phagnalo saxatilis-Rumicetalia indurati is the only order of this class and is further divided into six alliances in the Iberian Peninsula [13]. Because of the substrata nature given to the descriptions of the chasmo-chomophytic communities within the Phagnalo saxatilis-Rumicetalia indurati order, such as schist or greywackes, limestone and dolomite, Costa et al. [12] recognized the presence of two alliances in southern Portugal: Rumici indurati-Dianthion lusitani, which is ascribed to the communities developing on fissures of siliceous rocks of the West Iberian Mediterranean and Oroiberian territories, whereas the communities growing in the Dividing Portuguese Sector and Arrábida range (central and western Portugal), typically associated with limestone and dolomitic substrates, are classified within the Calendulo lusitanicae-Antirrhinion linkiani class.

Study Area
Located in the southwest of the Iberian Peninsula, the study area covers two distinct units: the first, in the southern part of the Algarve and Monchique biogeographical sector, encompasses the sub-littoral lower altitude reliefs of Algarve limestone-Barrocal algarvio (maximum 480 m high), where there is frequent presence of basic rocky outcrops. The second includes the schist, greywacke and quartzite cliffs of the low-altitude range (with an altitude of less than 400 m) of the lower Guadiana valley, in the southwestern part of the Mariánica Range Sector. According to the most recent study of Peninsula Iberica bioclimatic characterization by Rivas-Martínez et al. [14], the study area is classified as mediterranean pluviseasonal oceanic, dry to subhumid thermomediterranean bioclimate.

Data Collection
Field research was carried out from 2011 to 2020. Phytosociological relevés were collected according to the Zurich-Montpellier phytosociological method [12,13,[15][16][17], where we found two new distinct associations in the Algarve and Monchique, and Mariánica Range biogeographical sectors (Figure 1), based on the comparison of the phytosociological relevés, performed in Table 1. Following Biondi [18], each relevé is a floristically and ecologically homogeneous plant community that represents the plant association on the ground. Within this definition, for each relevé, all plants that are found in an area whose floristic, structural and ecological conditions are homogeneous, were identified and assigned a quantitative value or index for their coverage, using the conventional abundance-dominance scale of Braun-Blanquet.

Data Analysis
For statistical data processing of the samples, we first generated a data matrix that included 64 relevés and 135 species from our field sampling (Tables 1 and 2; association 1; Figure 2, clusters 1-8; Table 3, association 2; Figure 2, clusters 9-16) and relevés taken from the literature [20][21][22][23]25,35,36]. The matrix was subjected to the unweighted pair-group method using arithmetic averages (UPGMA), with Bray-Curtis distance, to produce the dissimilarity measure, using the software Primer 6 [38,39]. The transformation of Braun-Blanquet's abundance-dominance values follows Van der Maarel [40]. This transformation is required as a solution for converting the nonnumerical values into numerical scale and in this form used as input data for numerical analysis, with the following equivalence: r = 1, + = 2, 1 = 3, 2 = 4, 3 = 5, 4 = 6 and 5 = 7. Group A has a high dissimilarity in relation to the other associations and includes relevés dominated by Dianthus lusitanus. The Digitali thapsi-Dianthetum lusitani is an association co-dominated by Dianthus lusitanus and Digitalis thapsi which occur in the mesomediterranean to supramediterranean bioclimatic stages of the Lusitania and Extremadura, Carpetana and León, and Oroiberian Subprovinces, on rocky fissures of schist, quartzite and granite [21]. The Silene acutifoliae-Dianthetum lusitani occur mostly on quartzitic outcrops in the mesomediterranean belt of the São Mamede mountains (São Mamede Sierran District, Oretana Range and Tajo Sector), well characterized by the presence of Silene acutifolia [22]. The Sileno montistellensis-Dianthetum lusitani is found in submediterranean areas, with the supramediterranean to oromediterranean themotype, on granitic fissures of the Estrela mountain biogeographical territory, and is characterized by the presence of Silene x montistellensis (hybrid between Silene acutifolia and Silene foetida) [36].

Results and Discussion
The cluster analysis also shows a group of relevés clearly separated from the rest, which are included in the cluster group B, divided into two subgroups. The relevé cluster subgroup B1 corresponds to the association Antirrhinetum onubensis, which is proposed here as a new association, largely confined to the limestones of Algarve District (Algarve and Monchique Sector). The subgroup B2 comprises samples ascribed to both silicicolous and calcicolous associations included in the alliances Rumici indurati-Dianthion lusitani (such as Phagnalo saxatilis-Rumicetum indurati and Sanguisorbo rupicolae-Dianthetum crassipedis) and Calendulo lusitanicae-Antirrhinion linkiani (such as Phagnalo saxatilis-Dianthetum barbati and Sileno longiciliae-Antirrhinetum linkiani), respectively. The floristic similarities between these four associations are a result of the high presence of Phagnalon saxatile and Sanguisorba rupicola in its characteristic species set, both with indifferent soil preferences, and also, in the scope of companion species, the presence of Dactylis hispanica subsp. lusitanica, Sedum forsterianum, Hyparrhenia sinaica, Helichrysum stoecha, Sedum sediforme, Melica minuta, Polypodium cambricum and Sedum album. The Phagnalo saxatilis-Rumicetum indurati is dominated by Phagnalon saxatile and Rumex induratus, and is widely distributed throughout the thermomediterranean to supramediterranean areas of the West Iberian Mediterranean, and Coastal Lusitania and West Andalusia Provinces. The Sanguisorbo rupicolae-Dianthetum crassipedis, a new association physiognomically characterized by Dianthus crassipes, occurs in the thermomediterranean to mesomediterranean dry areas of the East Mariánica District (Mariánica Range Sector). The Phagnalo saxatilis-Dianthetum barbati, a calcicolous community characterized by the co-dominance of Dianthus barbatus and Phagnalon saxatile, is found in the northern part of the Divisório Portuguese Sector [24]. The Sileno longiciliae-Antirrhinetum linkiani, which has been described by Ladero et al. [25] in the limestones of the Divisório Portuguese Sector and Arrabida Sierran District (Ribatejo and Sado Sector), is characterized by the dominance of species from the Calendulo lusitanicae-Antirrhinion linkiani alliance, such as Antirrhinum linkianum, Silene longicilia, Calendula suffruticosa subsp. lusitanica, Biscutella valentina, Arabis sadina, Avenula lodunensis subsp. occidentalis and Saxifraga cintrana.
As evidenced in Table 1, in the main characteristics and companion species group, the associations of subgroup B2 encompass differential and territorial species, which support the ecological concept of divisions proposed by Ladero et al. [25] for the Phagnalo saxatilis-Rumicetalia indurati order in the West Iberian Mediterranean territories: the alliance Rumici indurati-Dianthion lusitani for the siliceous rocks and Calendulo lusitanicae-Antirrhinion linkiani for the outcrops of limestone and dolomitic rocks.
The relevés of the new association Sanguisorbo rupicolae-Dianthetum crassipedis (holotypus Table 2, relevé 8) appear to be clearly defined in group A (clusters 1-8; Figure 2). It is a perennial chasmo-chomophytic community, which develops on acid rocky fissures of schist or greywackes and quartzitic outcrops of the Guadiana basin, in the southeastern part of Portugal (East Mariánica District, Mariánica Range Sector). The Sanguisorbo rupicolae-Dianthetum crassipedis is an association characterized by Dianthus crassipes and Sanguisorba rupicola [41] (Figure 3). As shown in Table 2, the floristic composition also contains other chasmo-chomophytic species from the Phagnalo saxatilis-Rumicetea indurati class, such as Phagnalon saxatile and Rumex induratus. The rupicolous character is emphasized by the presence, in the companion species group, of chasmophytic elements from the Asplenietea trichomanis class, such as Cheilanthes maderensis, Cheilanthes guanchica and Cosentinia vellea. Regarding the xerophilous position, this new association develops on the most exposed sector of the cliffs or crests and is distributed in holm oak woodland domains of the Myrto communis-Quercetum rotundifoliae or, in the driest siliceous areas of the lower part of the Guadiana valley, on potential areas of the sclerophyllous shrubs of Juniperus turbinata, from the Phlomido purpureae-Juniperetum turbinatae. We place this new Dianthus crassipes community, at association rank, within the Rumici indurati-Dianthion lusitani alliance, which comprises the heliophilous and xerophilous chasmo-chomophytic communities growing on acid siliceous rocks of the West Iberian Mediterranean Province [12].
We place the Antirrhinetum onubensis at the association rank, in the Calendulo lusitanicae-Antirrhinion linkiani alliance, which encompasses the rupicolous vegetation of calcareous crevices of central and western Portugal (in the Divisório Portuguese Sector and Arrabida Sierran District) [10,26]. Nevertheless, according to our results, we propose to modify the alliance diagnosis, extending over the biogeographical territories of the southern Portugal (Algarve District, Algarve and Monchique Sector). Moreover, this community should be classified as a priority habitat from Habitats Directive 92/43/EEC, under the Natura