2.1 Study area

2.2 Textual archives

2.3 Land survey and individual censuses

The prospected area is situated on the north- and west-facing slopes of Mont Cenis (Fig. 1) at elevations between 1900 and 2500 m a.s.l., in the subalpine and the lower-alpine belts. The surveyed communes are Lanslebourg, Lanslevillard and the western part of Bessans, between 06°59′N–45°18′E and 06°51′N–45°16′E. The systematic exploration was achieved during one week by a team of six persons with the aim to record all individuals, if possible.

The survey revealed 23 pines showing Pinus mugo characteristics, i.e. pines prostrated with perfectly symmetrical cones (Table 1). Uncertainties of identification could occur for a few pines that lacked cones, and the small height could result from confusion with young Pinus uncinata. However, it was noticeable that, in the explored area, no mountain (erected) pine was observed, although Pinus uncinata forests exist lower in the same valley between 10 and 30 km apart, especially on dry and calcareous soil.

The discovered dwarf pines are distributed between 2180 and 2400 m a.s.l. (Table 1). Most individuals are in pastured grassland, rich in ericoides species such as Arctostaphylos uva-ursi, Vaccinium uliginosum, V. myrtillus, V. vitis-idaea, Rhododendron ferrugineum, Empetrum hermaphroditum, but also Juniperus nana and Cotoneaster integerrimus. Near a large Pinus mugo individual in the commune of Lanslevillard, we observed carpets of Arctostaphylos alpinus, Alpine bearberry, a species rare enough to be mentioned here. A few dwarf pine are situated at the upper treeline of subalpine forests, near 2250–2300 m a.s.l., mainly composed of cembra pine, larch, and very few spruce.

Dwarf pines are distributed on two different sectors of the investigated area, first in the eastern part into the Crozats pasture localised northward from the peak called Signal du Grand Mont Cenis (06°57′N–45°17′E), Lanslevillard (5 ind.) and, second in the Femaz pasture on the west-facing slopes of the peak called Ouillon des Arcellins (06°54′N–45°15′E), localised at the Mont Cenis Pass, Lanslebourg (18 ind.). No altitudinal pattern is observed, both in crown height and circumference. Individuals covering the largest area are not at the highest, nor at the lowest altitude (Fig. 2a, b). Four of the five individuals from the eastern sector, Crozats pasture (nos 1–4), display a large crown (max. circumference: 31 m, Fig. 2c), with dense needles and carrying cones (Fig. 3). The last individual (no. 5) is smaller with needles partly necrosed (Fig. 3). Individuals from the Femaz pasture show small crowns (circumference <7 m; max. height 1.5 m). The crown upper part of these pines is partially necrosed (Fig. 3, ind. nos 6–27).

The analysis of the archives of the forest administration shows that no tree plantation was performed on the communes of Lanslebourg and Lanslevillard (Table 2). From all archives consulted, from Sollières-Sardières to Bessans, none indicates the plantation of dwarf pine, whatever the forest status, i.e. communal or national. The sole reported plantations were in 1975 in the valley bottom at Bessans, nearby the main road, then in 1982 and 1987 with cembro pine and spruce in the communal forest, composed of larch on the north-facing slopes of the valley, in the aim to “diversified tree resources”. Some plantations were made in national land of other communes located lower in the valley, i.e. Termignon and Sollières-Sardières. However, the species used are not indicated.

The lack of pine plantation, and any other tree species on the communes of Lanslebourg and Lanslevillard suggest than the discovered population of Pinus mugo is spontaneous and natural. The sole recorded plantations do not concern the surveyed area. The forest species used by foresters were larches, spruces, and cembra pines and no dwarf or mountain pine.

The biogeographical situation of Pinus mugo within the Mont Cenis area is remarkable. The dwarf pine is a centro-European species with dense and wide cover. In the western Alps, the Mont Cenis population is the first recorded in the northern French Alps. The sole known stands are in the south, in the Durance valley (Queyras massif, Étroite valley), in Italy in the Susa and Orco valleys and, far south in the valleys of the Maritime and Ligurian Alps (Bono et al. [19]; Montacchini [5]; Montacchini and Caramiello [20]; Ozenda [6]; Chas [7]; Fig. 1). Northward, the western populations are localised in the SW Wallis county, Switzerland (WSL [21]) and in the Aosta and Ossola valleys, NW Italy (Montacchini [5]). Although in terms of distances, dwarf pines of Mont Cenis appear rather isolated, an ecological and biogeographical corridor allows the connecting of all stands within the inner Alps, from the Queyras massif to the Mont Cenis through several passes, e.g. those of Mont Genèvre, of Thures and of the Étroite valley between France and Italy. All these passes are under 2200 m a.s.l., and are within the natural altitudinal range of Pinus mugo. Although these stands along the Italian–French boundary are generally extremely isolated, they seem distributed along a virtual axis that would illustrate a wider and better connected distribution within the inner western Alps in the past, from the Savoy to the Durance valleys in France, through the Susa valley in Italy (Fig. 1). The southern and northern stands of dwarf pine within the Maritime and Ligurian Alps and the western Wallis appear totally unconnected from the inner Alps stands due to too elevated altitudes of massifs and passes. A possible connection appears at lower altitudes. However, this is not possible for dwarf pine during an interglacial environment (e.g. the current Holocene), because of the competition with erected trees of the mountain and plain belts that is unfavourable to Pinus mugo. The lower altitude connexion is possible only during a glacial geological period, either by the Po plain in Italy, or by hills of the Mediterranean coast in France (Fig. 1).

Palaeo-biogeographic data suggest an older broader extension of the dwarf pine phenotype of Pinus mugo in the inner alpine valleys (Montacchini [5]). Indeed, fossil imprints of Pinus mugo cones were discovered in one travertine of the Susa valley, dating from the Lateglacial-Holocene transition (Ali et al. [22]). However, these data must be considered with caution, due to the identification method involved, based on the qualitative morphology of imprints. A more systematic investigation of travertine archives is stressed, involving a quantitative analysis of morphological structure of cone scale imprints. Hence, our investigations would better depict the past distribution of this dwarf pine, and would analyse the ecological forcings and processes that fragmented its western geographic area. Among the fragmentation causes, the intensification of cultural land-uses in subalpine territories appears a likely explanation, knowing that Pinus mugo stands are very dynamic in eastern areas under a severe land-use abandonment (Dullinger et al. [23]). However, biotic factors such as interspecific competition seem realistic, particularly during the early Holocene expansion of Pinus cembra, the dominant and naturally frequent subalpine species (Ali et al. [16]). Pollen approaches are not efficient to reconstruct the history of Pinus mugo that is mixed with those other diploxylon pines of the surroundings of SW Italian–French Alps (Pinus sylvestris, P. uncinata, P. nigra, P. halepensis, P. pinaster, P. pinea), except with the sole haploxylon species Pinus cembra. Only the fossil cones appear able to decipher the postglacial history of Pinus mugo.

The Mont Cenis population of dwarf pine is not homogeneous, nor concentrated into a single pasture, although the effectives are very low (N=23). It is remarkable to report that there is no altitudinal pattern. For instance, we could expect smaller and eventually prostrated individuals at higher altitudes. However, according to the situation of individuals, their physiognomy are different. Obviously, individuals from Lanslevillard (eastern pasture) display a better fitness than those of Lanslebourg (western pasture) that are almost all necrosed (Fig. 3) and of smaller size, both in crown height and circumference (Fig. 2c). Individuals from Lanslebourg are all situated in an area dedicated to winter sport and particularly to skiing. It is possible that ski activities could explain this observed pathological status on dwarf pines, i.e. the necroses of top crown needles (Fig. 3). The repeated passages of skis, eventually of devices to manage ski-trails, contribute to densify and reduce the snow layer thickness, exposing artificially needles to weathering, particularly to strong winds containing ice particles. Indeed, the Mont Cenis pass is known for its strong, glacial wind. The current status of Pinus mugo individuals seems critical, and their presence could be rapidly altered if a conservation program were not started to protect them. Furthermore, dwarf pine Pinus mugo is on the French red list of protected species (“annexe 1, arrêté du 20 janvier 1982 relatif aux espèces végétales protégées sur l'ensemble du territoire national français”). This law indicates that species cannot be destroyed, cut, mutilated and broken-off, except in the framework of rural exploitation of lands that are traditionally cultivated, which is not the case of the dwarf pine of Mont Cenis, that are situated in former subalpine pastures that are currently used for winter sports.

Dwarf pines of the Mont Cenis compose a spontaneous and natural population of Pinus mugo Turra. This population appears fragile in light of effective numbers. It is currently isolated, but would result from populations that probably covered wider territories during the early Holocene, from the Durance valleys to the Maurienne valley, through the Susa valley. The dwarf pine population of the Mont Cenis appears altered, except the individuals that do not grow in the ski station area. A conservation program is urgent to preserve this population, the sole known in the northern French Alps.