The territory of study covers the Pyrenean Mountains with the limits already explained in the introductory article of this same publication. Their total area is about 50,000 km² and the area of the alpine belt is of 1,973 km² (Figure 1), of which 949 km² are located in France, 881 km² in Spain and 143 km² in Andorra. Ancestral exploitation of the forests of Pinus uncinata Ramond ex DC, makes it difficult to discern the alpine belt lower delimitation (Camarero & Gutiérrez, 2004). Moreover, a latitudinal gradient of such boundary –which increases from the edges to the centre of the mountain range according to climate continentality and topography– together with the current recovery of woodland, also complicates the establishment of a single timberline altitude value. In most descriptions of the Pyrenean vegetation this altitudinal threshold tends to be placed between 2100 and 2450 m of altitude (Rivas-Martínez et al., 1991; Ninot et al, 2007). In order to simplify, we have adopted the average value of 2300 m.

The climate of the Pyrenean alpine belt is still little known because of its high variability and the scarcity of weather stations providing long-time series of records. Concerning rainfall, the longest data collection is that at the Midi de Bigorre station, located in Hautes Pyrénées (French Central Pyrenees) at 2860 m. a. s. l., with data of over more than a century. In this location average yearly precipitation between 1960 and 1990 was 1,700 mm (Rivas Martínez et al., 1991), an amount that increases substantially westwards due to Atlantic influence, and which significantly decreases towards the central zone, particularly on the southern slopes of the range, with a marked continentality and Mediterranean influence (Creus, 1982; Izard, 1985). Yearly average temperatures in the alpine belt range between -1 and 3°C, with an average decrease of 0.6°C per 100m height (Rivas-Martfnez et al., 1991). Regression models of the existing data mark the 0°C isotherm at 2726 m and, what is more determinant for the flora and vegetation, a mean durations of the vegetative period of 85 days at 2300 m, with a decrease of 11 days per 100 m of altitude and considerable fluctuations imposed by topography (Del Barrio et al., 1990).

As alpine plant we have chosen those found above 2300 m. a. s. 1. in at least 10% of their existing citations. Using this criterion, taxa whose presence is considered accidental have been discarded. In many cases these correspond to nitrophile plants found occasionally and adventitiously and related to the movements of herbivores or frequented mountain paths. Besides, a poor taxonomic and chorological knowledge of quite a few taxa included in apomictic genera and others with numerous microtaxa has led us to take into account just those clearly identifiable. The genera treated in this synthetic fashion have been: Alchemilla, Taraxacum, Hieracium, Pilosella, Armeria and Festuca. The selection described is based upon the data compiled in several floristic Atlas: the ones from the Pyrenees (http://www.florapyrenaea.org), Aragon (http://floragon.ipe.csic.es/index.php), Catalonia (http://biodiver.bio.ub.es/biocat/) and France (http://www.tela-botanica.org/), along with other floras and regional catalogues (Aseginolaza et al., 1985; Aizpuru et al, 1999; Bolós & Vigo, 1984-2001; Villar et al, 1997-2001 and Lorda, 2013) and also upon the “expert-criterion” revision made by the authors.

Values of abundance and spatial distribution have been obtained from the revised maps of the Floristic Atlas of the Pyrenees and also from the number of 10x10km UTM grids. Altitudinal range has been obtained considering the lowest and highest customary altitudes of each plant. Lifeforms have been classified according to Raunkiaer (1934). The vegetation belt has been assessed following (Ninot et al., 2007). The chorology has been synthesized from www.florapyrenaea.org. In order to assess the abundance/rarity we have considered the average abundance assigned in www.florapyrenaea.org to each plant in each of the six geographic sectors of the territory (see Gómez et al, in this issue for sector description). The following classes have been considered: “very rare”, “rare”, “scarce”, “frequent”, “common” and “very common”, which have been allotted to each plant according to the number of references without considering population density. Soil affinity and chorology have been extracted from the aforementioned botanical works but simplifying the categories (Table 1) in order to standardize such information.

Habitat characterization in the alpine belt, up to three for each taxon, has been defined using the information available in herbarium citations, field notes and phytosociological filiation extracted from the bibliography and databases of the Herbarium JACA. If more than one habitat was assigned to a taxon, we chose as “priority habitat” the one in which the plant lives most frequently. The 13 habitats considered are: pastures (calcareous and siliceous), screes (calcareous and siliceous), rocky areas (cliffs calcareous and siliceous), snow beds, wetlands (springs, muddy swamp soils and mires), shrublands, megaphorbs and ruderal communities. Habitat naturalness has been evaluated considering their independence from human influence (Loidi, 1994) with four ordinal values (Table 1).

To test if the distribution of Pyrenean alpine flora according to different biotic and abiotic factors was significantly different from the entire flora, we performed Chisquare tests. Departing from the percentages of the entire flora we calculated the expected absolute values relative to N=645 –total alpine plant found in this study– to perform the test together with the observed values for Pyrenean alpine plants. Table 1 explains the criteria tested and the classes used. In addition to that, Spearman correlation coefficient was used to determine the relationships between the number of species per altitudinal intervals and their areas. To analyze the differences of chorology between sectors, the Kruskal-Wallis test was used.

Alpine plants account for 645 taxa (630 species, of which 11 have two alpine subspecies and two, Silene vulgaris (Moench) Garcke and Ranunculus parnassifolius L., three). These represent 15% of the total native vascular flora of the Pyrenees and are integrated into 231 genera (30% of the total) and 68 families (47% of the total).

All the alpine taxa have been found in the French Pyrenees, except Alyssum alpestre L., Arenaria ligericina Lecoq & Lamotte, Brassica repanda (Will.) DC. subsp. cadevallii (Font Quer) Heywood, Carex lachenalii Schkuhr, Erigeron cabelloi A. Pujadas, R. García-Salmones y E. López, Oxytropis lapponica (Wahlenb.) J. Gay, Plantago monosperma Pourret subsp. discolor (Gand.) M. Laínz, Senecio boissieri DC, Taraxacum aquilonare Hand.-Mazz., Teucrium pyrenaicum L. subsp. guarensis P. Montserrat, Tofieldia pusilla Pers., Veronica aragonensis Stroh and Woodsia pulchella Bertol. On the southern slope (Andorra included) the only alpine taxa not found were Dracocephalum ruyschianum L., Festuca prudhommei Kerguélen & Plonka, Hierochloe odorata (L.) P. Beauv., Saxífraga adscendens L. and Trisetum distichophyllum (Vill.) P. Beauv.

With regard to taxonomie representation, Figure 2 shows the most abundant families in the alpine flora. Among the 15 families with the largest representation of taxa, 12 coincide with the whole of the Pyrenean flora. In the alpine belt CYPERACEAE, SCROPHULARIACEAE, RANUNCULACEAE, CAMPANULACEAE, SAXIFRAGACEAE and PRIMULACEAE show a proportion of double or even higher that of the proportion in the whole area of the Pyrenees. In contrast, COMPOSITAE, LEGUMINOSAE and APIACEAE are clearly under-represented, with frequencies of half or less of that in the whole of the Pyrenean flora. As for the taxonomic major groups, alpine flora differs significantly from the total Pyrenean flora being dicotyledonous less represented and monocotyledonous and ferns more represented (summarized in table 5).

The regions with the highest number of alpine plants (Table 2) are Catalonia and Aragon which, after MidiPyrénées, have the largest areas of alpine belt. At the other end of the scale are Andorra, with just 7% of the alpine territory but 77% of the alpine flora, and the Basque Country, without alpine territory itself, but with more than 62% of the alpine taxa. Figure 3 show the distribution map of the alpine and boreoalpine plants in the Pyrenees with the highest concentration in the central and East sectors.

Table 3 shows the distribution of alpine taxa along the altitudinal gradient. The highest representation occurs between 2300 and 2500 m and almost a quarter of them surpass the 3000 m. A high positive and significant correlation exists between the number of taxa and the area of the different administrative regions (Rho = 0.729; p = 0.011) as well as a significant positive correlation is also found between the altitudinal stretch and the number of taxa per area (Rho = 0.614; P <0.001).

On the other hand, it is worth stressing the high presence of alpine plants recorded in the lower altitudinal strips: about 90% of alpine taxa are represented between 2000 and 2300 m and half of the alpine flora is found below 1500 m whereas 10% is beneath 500 m. At the other extreme, only 23 plants live exclusively above 2300 m, that is to say, they can be considered “strict alpines”. In addition, Trisetum spicatum (L.) K. Richt, subsp. ovatipaniculatum Hultén ex Jonsell and Draba fladnizensis Wulfen, live exclusively above 2600 and 2700 m respectively, while Phyteuma globulariifolium Sternb. & Hoppe subsp. pedemontanum (R. Schulz) Becherer, Saxífraga adscendens L., S. retusa Gouan, Senecio boissieri DC. and Erigeron cabelloi A. Pujadas, R. García-Salmones y E. López (this one recently described and therefore still with an imprecise distribution area) are to be found only above 2500 m.

Considering the different vegetation belts (Figure 4), the highest representation of alpine flora is observed both in the subalpine (almost 95%) and alpine (88%) belts, but more than 25% and 10% respectively descend into the basal and montane belts.

With regard to altitude span, it is observed that the average amplitude of the alpine flora is 1369 m versus 1035 m for the whole of the flora. About 75% of the alpine flora occurs within an altitudinal range larger than 1000 m, whereas in the whole of the flora just 54% show the same value (Figure 5). Boreoalpine plants and alpine orophytes, which are dominant in the highest zones, show an average altitudinal interval of 1150 m. Altogether, alpine plants occupy significantly larger altitudinal intervals than the whole of the flora (Table 5).

Figure 6 shows the distribution of the different chorological groups of the alpine flora. Boreoalpine plants and alpine orophytes represent more than 52%. One fifth of the alpine flora are Eurosiberian taxa. Among the alpine plants there are 72 Pyrenean endemisms that account for 37% of the total endemisms of the Pyrenees (considering endemic subspecies and microspecies). If only strict endemisms are considered at species level, the alpine belt harbours 48 of a total 88 endemic Pyrenean species. Hardly 4% (45 species) of the Mediterranean Pyrenean flora and 11% of the “wide distribution” Pyrenean plants are represented in the alpine belt. Altogether, the alpine flora has a distribution of chorological groups significantly different from the general flora (Table 5).

Considering their total distribution area, 237 alpine taxa have border populations in the Pyrenees: 16 correspond to the northern limit of Mediterranean plants and the rest to southern limits (52 of the 91 Boreoalpine taxa, 148 of the 246 Alpine orophytes and 21 of the 129 Eurosiberian taxa).

Figure 7 shows the degrees of abundance of alpine flora considering the breadth of its distribution. Proportion of very rare plants is lower in the alpine flora than in the whole of the flora. At the other extreme, frequent, common and very common plants show similar frequencies.

When looking at the distribution range of alpine plants, only 13 species are found in a single sector of the six aforementioned for the Pyrenees, 38 in two, 45 in three, 95 in four, 82 in five and 377 in all the six sectors. Considering the number of 10 x 10-kilometre grids, Table 4 shows the frequency of the alpine flora and of the whole Pyrenean flora measured by their presence in 10x10 km² grids. Little more than 5% of alpine plants are represented in less than 10 grids, compared to 39% of the whole flora. Nearly three quarters of them are found in more than 50 grids, while only 34% of the flora as a whole shows such a wide distribution. Altogether, the alpine flora shows a distribution in abundance classes significantly different from that of the whole flora (Table 5).

Alpine flora of the Pyrenees is dominated by hemicryptophytes (55.1% compared to 43.9 of the whole flora) and chamaephytes (24.3% vs 14.2%). Geophytes (12.1% vs 12.2%) and hydrophytes (2.6% vs 2.4%) show frequencies similar to those of the whole flora. Fanerophytes (2.6% vs 6%) and, above all, terophytes (2.8% vs. 20.4%) show a markedly smaller representation, with only three species {Euphrasia alpina Lam., Mucizonia sedoides (DC.) D.A. Webb and Sedum atratum L.) reaching 3000 m. Altogether, distribution of life forms within the alpine flora is significantly different from that of the whole flora (Table 5).

At higher altitudes, there is an increase of chamaephytes that reach 28% above 2800 m and 33% above 3000 m, at the expense of hemicryptophytes that descend, respectively, to 60% and 56% above these altitudes, and geophytes, that decrease from 7% to 5%.

Edaphic affinity of the alpine plants with respect to the flora as a whole shows a similar percentage of calciphilous plants (about 35%), a notable higher presence of acidophilus plants (39.5% vs 21.4%) and a lower representation of the indifferent (25% compared to 43.5%). These differences are significantly different from that of the whole flora (Table 5).

49% of alpine flora can be found in three or more habitats, 35% in two of them and 16% in a single habitat. Habitat distribution (Figure 8), considering only the preferred environment occupied by each species, shows the predominance of alpine flora in pastures and rocky habitats (rocks and screes) compared to the whole of the Pyrenean flora; in addition, a similar representation is observed in wetlands (springs, streams and peaty soils) but in shrubland and nitrophile communities it is much smaller.

However, if all the habitats of alpine plants (up to 3) are considered, their distribution shows a smaller difference with respect to that of the total flora, with 30% of the alpine flora in pastures, 23% in ridges and rocky slopes (screes), 19% in rocky areas, 12% in wetlands, 6% in snowbeds, 5% in shrublands, 3% in megaforb communities and 2% in livestock resting areas. Finally, naturalness of the Pyrenean alpine flora shows that 85% of the taxa are characteristic of high or very high natural environments, 12% of middle naturalness habitats and 3% of the low naturalness ones.