Originalities of Willow of Salix atrocinerea Brot. in Mediterranean Europe

: Willow communities (genus Salix ) occurring in Mediterranean Europe are presented, showing, through statistical treatment with multivariate cluster analysis, the separation of the di ﬀ erent plant communities and their sintaxonomic a ﬃ liation. Six willow communities have been identiﬁed, whose formations include a set of plants with high heritage value. We highlight plants with legal protection status (Annex IV and II of the Habitats Directive-92 / 43 / EEC), endemic, rare, and endangered species such as Salix salviifolia subsp. australis , Cheirolophus uliginosus , Euphorbia uliginosa and Leuzea longifolia . Therefore, two new willow communities are proposed for the southwest of the Iberian Peninsula. The ﬁrst dominated by Salix atrocinerea , Frangulo baticae-Salicetum atrocinereae ass. nova of ribatagan distribution, under acid substrates, thermomediterranean to lower mesomediterranean, dry to sub-humid. The second, dominated by the endemic Salix salviifolia subsp. australis , Clematis ﬂammulae-Salicetum australis distributed in the Algarve, developing on neutral-basic substrates, exclusively thermomediterranous, dry to sub-humid. In both cases, there are presented on their own ﬂoristic serial, ecology, and substitution steps. A new hygrophytic meadows was also identiﬁed dominated by Molinia caerulea subsp. arundinaceae , Cheirolopho uliginosii-Molinietum arundinaceae ass. new hoc loco , which lives on substrates rich in organic matter, exclusive to the Ribatagano Sector. Through the deepening of knowledge about the composition and dynamics of riparian vegetation, it is possible to adapt management methods to sustain and protect these important edafo-hygrophilic systems in the Mediterranean.


Introduction
Riparian zones are highly heterogeneous and disturbed environments. They are composed of a wide variety of physical habitats in terms of their size of substrate sediment, moisture, and nutrient conditions, inundation duration and frequency, and also susceptibility to drought [1]. The southern

Data Collection
For the confirmation of the new associations we carry out a set of relevés ( Figure 1). In each relevé, all existing plants were recorded and assigned a quantitative index according to the abundancedominance scale, with according by Braun-Blanquet [33]. This scale combines an estimative between the number of individuals of each existing specie and the respective area within the inventory. For each index (in bold), there is a coverage range, namely: + few individuals with very poor coverage (0.1% to 1%); 1 very abundant individuals with low coverage (from 1% to 10%); 2 individuals very abundant or covering at least 1/20 of the surface (from 10% to 25%); 3 any number of individuals covering 1 4 to 1 2 of the surface (from 25% to 50%); 4 any number of individuals covering 1 2 to 3 4 of the surface (from 50% to 75%); 5 any number of individuals covering more than 3 4 of the surface (from 75% to 100%). We compiled an Excel© table with 84 relevés, where 13 were made by the authors and the rest supported by bibliography [2,9,11,12].
Sustainability 2020, 12, x FOR PEER REVIEW 3 of 14 the inventory. For each index (in bold), there is a coverage range, namely: + few individuals with very poor coverage (0.1% to 1%); 1 very abundant individuals with low coverage (from 1% to 10%); 2 individuals very abundant or covering at least 1/20 of the surface (from 10% to 25%); 3 any number of individuals covering to of the surface (from 25% to 50%); 4 any number of individuals covering to of the surface (from 50% to 75%); 5 any number of individuals covering more than of the surface (from 75% to 100%). We compiled an Excel© table with 84 relevés, where 13 were made by the authors and the rest supported by bibliography [2,9,11,12].

Characteristics and differentials
Salix atrocinerea

Description of Willow Communities
I-Carici lusitanicae-Salicetum atrocinereae (clusters 27-38, Figure 1) According to Neto [8], this association is dominated by Salix atrocinerea and occupies swamps, lagoons, and margins of rivers and streams with water flowing over sandy soils subjected to fluctuations in its flow (although they present a high humidity content during the dry season). This association tends to be located in hygrophilous environments in a dry to subhumid ombroclimate in the thermomediterranean belt of the Sadese District.
II-Frangulo baeticae-Salicetum atrocinereae ass. nova hoc. loco (clusters 19-26, Figure 1; holotypus relevé: 08, Table 2) The Frangulo baeticae-Salicetum atrocinereae ass. nova hoc. loco occurs in thermomediterranean and warmer mesomediterranean areas, dry to sub-humid belts of the Ribatejo and Sado Sector, over hydromorphic sandy soils rich in organic matter, with some degree of peat formation, and are found on unusual position, since these willow woodlands are strictly linked to slopes of hills with a supplementary supply of springs. This is because the surface water infiltrates through the permeable pliocene-pleistocene sandy soils and feed the upwelling of subterranean water in many isolated springs, often in positions where the impermeable miocene substrates emerge. Thus, Frangulo baeticae-Salicetum atrocinereae is an silicicolous association developed close to springs, supporting a long period of waterlogging and represents an obvious discontinuities (island) with the surrounding zonal forests of cork oak (Quercus suber): Aro neglecti-Querco suberis sigmetum, which represent the climatophilous vegetation series. In addition to their distinct ecology, this new acidophilous community are physiognomically characterized by vegetation that is markedly different from the willow forests most often associated with the banks of water courses of the valleys from the nearby areas. The new Salix atrocinerea willow forest proposed here is frequently accompanied by Frangula alnus subsp. baetica and other differential species from the Genistion micrantho-anglicae alliance, such as Cheirolophus uliginosus, Leuzea longifolia and Euphorbia uliginosa.
With the destruction of these hygrophyllous formation occurs a new association dominated by Molinia caerulea subsp. arundinaca, accompaneid by Cheirolophus uliginosus: Cheirolopho uliginosii-Molinietum arundinaceae ass. nova hoc loco (holotypus relevé: 07, Table 3). This perennial grassland grows on peaty areas of the Ribatejo and Sado Sector, colonizing sandy soils with a water-table near the surface. We include the phytocoenoses within the Brizo minoris-Holoschoenenion vulgaris sub-alliance (Molinio arundinacea-Holoschoenion vulgaris, Holoschoenetalia vulgaris, Molinio-Arrhenatheretea), which also differs from the other association already described by a pool of rare or endemic species, such as Cheirolophus uliginosus, Leuzea longifolia and Euphorbia uliginosa.  III-Viti sylvestris-Salicetum atrocinereae (clusters 47-53, Figure 1) This is a community that was described as belonging to the gleyed sandy soils of the Doñana area, in Cádiz and Littoral Huelva Sector [9], extending towards the Coastal Lusitania and West Andalusia Province, and reaching the Atlantic Orolusitania Subprovince and western areas of the Lusitania and Extremadura Subprovince. It occurs in dry to sub-humid thermomediterranean belt, and grows on ologotrophic soils, subjected to temporary flooding. According to Rivas-Martínez [9], this community is dominated by Salix atrocinerea and is characterized by the frequency of Vitis vinifera subsp. sylvestris, Thelypteris palustris, among others.
V-Myrto communis-Salicetum atrocinerea (clusters 39-46, Figure 1) For La Madalena archipelago, located between north-eastern Sardinia and southern Corsica (Maddalenino Subscector, Campidanese-Turritano Sector, Sardinian-Corsican province), Biondi and Bagella [12] published the willow forest Myrto communis-Salicetum atrocinerea, dominated by Salix atrocinerea, often accompanied by Rubus ulmifolius, Carex hispida, Myrtus communis and Oenanthe crocata. This association is located in hydromorphic soils on swampy depressions, where the water table is almost permanent or close to the soil surface, within the thermomediterranean dry bioclimatic belt. On more dry soils of riverbeds, occurs a variant of this association, the subass. tamaricetosum africanae, characterized by the presence of Tamarix africana [12]. Following Bacchetta [34], in river and streams with permanent flow, on oligo-miocene deposits located in upper thermomediterranean or lower mesomediterranean belts under upper dry to lower subhumid areas of the western-central Sardinia, the communities enriched with Laurus nobilis are classified as subass. lauretosum nobilis.
VI-Salicetum atrocinereo-australis (clusters 1-13, Figure 1) This association colonizes siliceous soils of the thermomediterranean to mesomediterranean, dry to humid belts of the southwestern part of the Cádiz and Sado Suprovince and Lusitania and Extremadura Subprovince [2,4]. According to Dalila [35] and Quinto-Canas [36], the Salicetum atrocinereo-australis occurs in periodically flooded margins of temporary watercourses, characterized by torrential flows during the wet season and is distinguished by the abundance of Salix salviifolia subsp. australis and the absence or scarcity of nemoral species in the shady understory, due to the substratum instability and strong sediment carriage.
VII-Clematido flammulae-Salicetum australis ass. nova hoc loco (clusters 14-18, Figure 1; holotypus relevé: 04, Table 4) In the southern Portuguese territories included in the biogeographical unit of Algarve District, occurs the association Clematido flammulae-Salicetum australis, which is proposed here as a new willow forest association, exclusive from the limestone substrates of Barrocal algarvio, under thermomediterranean dry to subhumid belts. These riparian forests develop along banks of torrential Algarve's streams, where the hydrographical basin substrata are mostly basic [37]. It is found on oligotrophic soils, with sandy-limey texture, that are periodically flooded, and resisting to prolonged drought periods. The new willow forest is dominated by Salix salviifolia subp. australis, constantly accompanied by Salix neotricha and Clematis flammula. The presence of Fraxinus angustifolia and Nerium oleander, reveals the catenal relationship of this association, with the ash woodlands (Ranunculo ficariiformis-Fraxinetum angustifoliae) and oleander micro-woodlands (Oenantho crocatae-Nerietum oleandri). Out of the trees and shrub observed, the climbing plant species are well represented, with Aristolochia baetica, Lonicera implexa, Rubus ulmifolius, Smilax aspera var. altissima, and Calystegia sepium. In the understory, we can highlight the presence of Brachypodium sylvaticum, Vinca difformis, Equisetum ramosissimum, and Festuca ampla, as well as hygrophilous plants from Molinio caeruleae-Arrhenatheretea elatioris and Magnocarici elatae-Phragmitetea australis such as Oenanthe crocata, Carex hispida, Lythrum salicaria, Cyperus longus subsp. badius, Schoenoplectus lacustris, Typha domingensis, Scirpoides holoschoenus, Mentha suaveolens, Agrostis stolonifera, Dorycnium rectum, among others, associated to environments affected by temporary flooding. The presence of nemoral-thermophile species Aristolochia baetica, Bupleurum fruticosum, and Cheirolophus sempervirens, must be emphasized. Its fringe and first substitution step belongs to the bramble shrublands of Rubus ulmifolius from Lonicero hispanicae-Rubetum ulmifolii. Accordingly, the removal of tree and shrub cover leads to the hygro-nitrophilous reed beds belong to the association Holoschoeno vulgaris-Juncetum acuti. Lastly occurs the perennial grasslands from Narcisso willkommii-Festucetum amplae, dominated by Festuca ampla, growing on neutro basic deep soils, with sandy-limey texture, along torrential streams running through calcareous deposits of the Jurassic and Cretaceous age, always in the Algarve's streams basin [38].

Conclusions
With the information collected from 84 phytosociological relevés, we conducted a comparative and synthetic analysis of riparian woodlands communities dominated by Salix atrocinerea and Salix salviifolia subsp. australis, occurring in Southwestern part of the Iberian Peninsula and Sardinia. This study allowed us to identify the ecological position of Salix atrocinerea next to springs located hills sploes on psamophilic soils. Accordingly, we propose a new association namely Frangulo baeticae-Salicetum atrocinereae, for the acid pliocene-pleistocene sandy soils, in the Ribatejo and Sado Sector. Furthermore, for neutro-basophilous torrentail streams and watercourses of the Algarvese District (Algarve and Monchique biogeographic Sector) we propose the association Clematido flammulae-Salicetum australis.
At sub-serial level we identified a new perennial grassland association dominated by Molinia caerulea subsp. arundinacae, namely Cheirolopho uliginosii-Molinietum arundinaceae, for the thermomediterranean dry to sub-humid areas of the Ribatejo and Sado Sector, and represents a regression stage of Frangulo baeticae-Salicetum atrocinereae.
Such willow forests and perennial grasslands are important for biodiversity conservation, since they constitute a refuge to endemic or protected species, such as Salix salviifolia subsp. australis, Euphorbia uliginosa, Cheirolophus uliginosus, Leuzea longifolia, Agrostis juressi, Euphorbia transtagana, Hyacinthoides transtagana, among others. Therefore, it is important to guarantee the sustainability of these habitats, through conservation measures to avoid the disturbance on riparian woodlands across the Mediterranean, as reported by Fenu et al. [39]. In this sense, it would be necessary to create joint policies for these areas of high biodiversity, through the enhancement of ecosystem services.
Species conservation should give priority to places with a high number of plants threatened by IUCN (International Union for Conservation of Nature) criteria, such as Mediterranean waterways. Set as priorities, these areas should be subject to overgrazing control (mainly cattle grazing), develop programs to eradicate invasive plants (focusing on several species of the genus Acacia and Arundo), and seed harvest for germination and conservation studies in germplasm banks.
Attempt to their high conservational value, the willow woodlands associations should incorporate the habitat 92A0-Salix alba and Populus alba galleries, besides the perennial grasslands of Cheirolopho uliginosii-Molinietum arundinaceae, incorporate the habitat 6410-Molinia meadows on calcareous, peaty or clayey-silt-laden soils (Molinion caeruleae), both from Annex I of Council Directive 92/43/EEC.