The genus Sempervivum L. s. str. (excl. Jovibarba Opiz) is subendemic to the European continent and was most recently recognised as consisting of 40 to 60 species (Gowler & Tebbitt, 1995; ‘t Hart et al., 2003; Thiede & Eggli, 2007). The type of this genus is S. tectorum L., established by Parnell (1993: 87), with a lectotype designated from the Clifford herbarium (BM). Unfortunately, the proposed lectotype represents S. ×fauconnetii Reut.; therefore, the conservation of the name with a conserved type was proposed (Gallo & Jarvis, 2016: 1431) and recommended (Applequist, 2019: 849). The conservation of the name has facilitated the clarification of the taxonomic application of the binomial S. tectorum, which, in some cases, had been used too frequently to identify different taxa (e.g. S. alpinum Griseb. & Schenk, S. riccii Iberite & Anzal. and, perhaps, S. boutignyanum Billot & Gren. in F. Schultz) while in other cases obvious synonymies were not implemented, as in the case of S. arvernense Lecoq & Lamotte and S. andreanum Wale.

The S. tectorum group represents a morphologically variable taxon, which is distributed over a very wide range, including particularly central-western Europe where it is almost always spontaneous, while in eastern Europe it is frequently cultivated and more or less naturalized (‘t Hart et al., 2003). Research carried out to date has already made it possible to understand, at least in part, the taxonomic complexity of this group but much work remains to be done especially in the westernmost part of its range. This study contributes to a better understanding of the populations of Sempervivum in southern France and presents the results of a field-based study of a population of Sempervivum discovered near Champcella, in the Durance valley (Dép. Hautes-Alpes, Region Provence-Alpes-Côte d’Azur, France) that was, morphologically and ecologically attributable, at first sight, to S. tectorum. A more in-depth study has instead revealed that this population is characterized by peculiar morphological features and a precise geographical distribution beyond that of the S. tectorum. This material is described as a new species, named S. druentianum L. Gallo.

Several of the morphological traits defining S. druentianum are unique and differentiate this new taxon from what is currently interpreted as S. tectorum. Especially the glandular pubescence on both the surfaces of the rosette leaves has been observed to be consistently present in the case of S. druentianum. The trait is reminiscent of the leaf vestiture found in S. riccii, a species endemic of central Italy, from which S. druentianum can be distinguished by the glandular nature of carpels on abaxial surface vs (as in S. riccii) (Table 1).

The hybrid between S. arachnoideum L and S. druentianum, which is present at the type locality of S. druentianum, is here described as S. ×brigantiacum L. Gallo. An identification key is presented for morphologically similar taxa in southern of France.

Comparative morphological, ecological and biological (protandry and protogyny) studies that compare S. druentianum with related taxa present in southern France (S. alpinum, S. calcareum Jord. and S. tectorum) were carried out in the field and in the private collection of the author for three years. In addition to the specialised literature on the genus Sempervivum as well as French floristic bibliography pertaining to the study area, the more than 200 names that have been included in the synonymys of S. tectorum s. l. (L. Gallo, ined.) were considered to rule out the possibility that S. druentianum had already been described earlier.

Neither the literature survey not the study of the more than 200 synonyms attributable to S. tectorum s. l. indicated any data associated with ×S. druentianum. The population of Sempervivum of Champcella is therefore not yet known and is here described.

Sempervivum druentianum L. Gallo spec. nov. (Fig. 1)

Type: France, Provence-Alpes-Côte d’Azur, Dép. Hautes-Alpes, between Le Chambon and Chanteloube, neighborhood of Champcella, before the place named “La Bourgea”, on the side of the D38, 986 m a.s.l., rocks covered with xerophilous vegetation, 44.711056 N, 06.587453 E, 22-30.VI.2019, L. Gallo & M. Gai (holotype: MRSN-12989) (Fig. 2).

Diagnosis: Sempervivum druentianum is very similar to S. tectorum but differs in its leaves being glandular on both surfaces (vs. glabrous in S. tectorum), and oblanceolate to sometimes rhombeus (cf. Stearn 1987: 320, 493) (vs. oblanceolate to linear in S. tectorum) and the carpels more or less glandular on the back (vs. glabrous in S. tectorum). Other peculiar traits are inconsistently present, including that the filaments are sometimes glabrous at the base and not hairy as in S. tectorum and S. alpinum, and the inflorescence branches of S. druentianum are not scorpioid [these are morphological characters that are present in natural populations and cannot be overlooked. We must be honest and say that there are objective difficulties in determining them].

Description: rosette (20)30-60 mm diameter. Leaf erect ± oblanceolate-pyriform (pear-shaped), light green with white base, apex brown-violet apex, 25-35 × 10-14 mm, glandular on both the faces and on the margins. Inflorescence (8)15-25 cm height, branched but not scorpioid, often reddish, with rare bracts; the cauline leaves are similar to rosette leaves, green often reddish, glandular, with long cilia and apex violet. Flowers round in bud, sessile or with very short pedicel (<1 mm), 20 mm of diameter. Petals 13-14, patent, linear, 6-9 × 1.0-1.5 mm, white with central green strip, base pink, sometimes pink-spotted adaxially. Filaments violet, glabrous or glandular. Anthers oval, not mucronate, bronze or violet. Carpels green, glandular throughout. Stylus erect, green, sometimes spotted of violet toward the apex.

Area and habitat: to date, only a single population of S. druentianum is known in the Département Hautes-Alpes (Region Provence-Alpes-Côte d’Azur), on the hills near the Durance River at Champcella, between Le Chambon and Chanteloube (St. Crépin) (Fig. 3) at about 1000 m a.s.l., on huge rounded limestone rocks. This locality was strongly affected by the periods of glaciations/deglaciations in the Alps, with a permanence of ice up to 20,000 BP (Jorda et al., 2000; Buoncristiani & Campy, 2004; Rosique, 2004). In the last glacial maximum (LGM), on the French side, the alpine glacier occupied an elevation between 1200 and 400 m a.s.l. with widespread diffusion in the Durance valley and therefore also in the Champcella area, which was covered by the glacier (Buoncristiani & Campy, 2004). Climatically, the area is characterised by strong thermal fluctuations with many cold days and many very hot days. Precipitation is highest in autumn and lowest in summer, a Mediterranean type rainfall pattern, with consequent atmospheric aridity in the vegetative period, which is increased by the very high insolation rate (Bardin, 1967; Guiter, 1975). Péguy (1947) exhaustively compared environmental parameters between different localities of the Durance Valley. In the neighbourhood of Guillestre at 980 m a.s.l., which is close to Champcella (type locality of S. druentianum), annual rainfall of 690 to 763 mm was recorded. Widmann (1950) reported rainfall at the well-known location of Juniperus thurifera L. at St. Crépin, which is opposite that of S. druentianum, on the left of the valley, of 744 mm. Rain is mainly concentrated between September and October while the winter appears rather dry (see also Braun-Blanquet, 1922); moderate rainfall from April to June is of little use for the vegetation due to the wind and the strong insolation (Widmann, 1950). The area comprising Gap and Sisteron is located on the boundary between the Mediterranean region that is characterised, at least potentially, by Quercus pubescens Willd. and an alpine forest with prevailing Pinus sylvestris L. (Braun-Blanquet, 1922; Breistroffer, 1946; Widmann, 1950; Ozenda, 1966; Richard & Tonnel, 1987; Garraud, 1998) on dry, calcareous soils. The type locality of S. druentianum is located near 1000 m, but it is possible to hypothesize that the range of its elevation may be between 750 and 1500 m in the area near the Mediterranean zone, where Sedetum brigantiacum Br.-Bl. is a pioneer on rocks overlain with little calcareous soil (Ozenda, 1966). These plants along with several other representatives of the Crassulaceae are exposed to drought conditions and frost (Bardin, 1967). Sempervivum druentianum has very limited natural geographical distribution range and its remarkable morphological similarity to S. tectorum, together with taxonomic confusion in the genus Sempervivum as a whole, have delayed its recognition as a new species.

Phenology: Sempervivum druentianum predominantly flowers in July. The first flowers opened on 24 June (during the 2019 season) and the last flower opened on the 3 August (during the 2014 season).

Etymology: from Druentia, old classic name of the Durance River where, in its surroundings, S. druentianum has been found. This name was cited by some classic authors and sometimes reported as Durentia, Drancia, Drance, etc. (Alessio, 1949).

Remarks: despite repeated field expeditions in the vicinity of Champcella, no other populations of S. druentianum have been found. However, traces of its former presence, in the form of hybrids with other species, such as S. arachnoideum, have been identified in two locations not far from Champcella: at the Plateau d'Herbonne (Châteauroux-les-Alpes) between 900 and 930 m a.s.l. and at the Lac de Siguret (Embrun) between 1060 and 1100 m. No other species belonging to the S. tectorum group were found at the type locality of S. druentianum, although near La Roche-de-Rame, a population of S. alpinum was located by the author (GL-9007).

Different distributions of the S. tectorum group were reported in Chas (1994: 203) and Atlas en ligne de la flore du département des Hautes-Alpes (SAPN-GF, 2022), where S. alpinum, S. tectorum and perhaps hybrids between them, all been brought together under the same name (S. tectorum), which have not yet been separated. Another species of the S. tectorum group found in this area is S. calcareum, which reaches the northern limit of its distribution range at Lac de Serre-Ponçon (Embrun) (see SAPN-GF, 2022), about 30 km north from the type locality of S. druentianum.