*4.1. Pinus Type Sylvestris: Ecological Niche Models and Ecological Considerations*

Our ENMs showed considerable performances in estimating the current and past distributions of *P. mugo*/*uncinata*, *P. nigra*, *P. sylvestris* and *Q. pubescens* in Europe, as also shown by model validation. AUC values such as the ones that we obtained (>0.8) are among the highest reported for published models (e.g., [44,53]) and documented a high predictive power of habitat suitability [68]. Our study was further supported by AUCdiff values (e.g., [47]).

In agreement with Caudullo et al. [43], our current models for Europe matched well the observed distribution of *P. mugo*/*uncinata, P. nigra P. sylvestris,* while, with regard to *Q. pubescens,* the models identified a larger area than that shown in Caudullo et al. [43], in our case comprising the whole of the Iberian Peninsula. *P. mugo*/*uncinata* occurs in the mountains of Central and Eastern Europe, but is especially abundant in the subalpine belt of the Eastern Alps and the Carpathians [42]. Disjunct ranges occur in the lower mountains of the Jura and the Vosges, and at high altitudes in the Mediterranean and Balkan Mountains, such as the Apennines, the Albanian Alps and the Rila-Pirin-Rhodopes in Bulgaria [43]. Indeed, in Italy, *P. mugo*/*uncinata* belongs strictly to the subalpine belt, occurring above the forest treeline in the Alps and locally in the northern-central Apennines [69]. *P. sylvestris* ranges from Scotland, Ireland and Portugal in the west, east to eastern Siberia, south to the Caucasus Mountains and north to the Arctic Circle in Scandinavia [7] while, in Italy, it spreads to the Alps and occurs as a relic in the northern Apennines [69]. The *P. nigra* group is a widely distributed Mediterranean mountain conifer with a discontinuous range extending from North Africa (35◦ N), through the northern Mediterranean, eastwards to the Black sea, finally in the western Mediterranean on the islands of Corsica and Sicily (both as *P. nigra* subsp*. laricio*). In southern Italy, *P. nigra* forests are today confined to a few relic carbonatic rocky mountains where they form open vegetation on the steep slopes between the grasslands and the broadleaved forest [27]. *Q. pubescens* ranges from the Atlantic coast of France to the shores of the Mediterranean Sea, and across peninsular Italy, the Balkan Peninsula and the Aegean regions, to the coasts of the Black Sea and most of Anatolia. Although our models showed that *Q. pubescens* might potentially occur in the entire Iberian Peninsula, the western extreme of its geographic range, to the best of our knowledge, pubescent oak lives only in northern Spain [43]. This is not surprising since ENMs do not take into account biotic interactions such as competition, nor they may include historical factors which might play a role in influencing actual distribution [70]. In Italy, according to our models, this species occurs on almost the entire territory [69].

The substantial matching between our models and the maps shown in Caudullo et al. [43] represent an encouraging piece of evidence supporting the reliability of the maps of past predicted distribution of *P. mugo*/*uncinata, P. nigra, P. sylvestris* and *Q. pubescens* in Italy. The past dynamics of *Pinus* species, and consequently their paleobiogeography, are poorly documented because pollen and charcoal studies do not allow confident species identification. Regarding the Iberian Peninsula, paleoecologists mention the occurrence of *P. sylvestris* and *P. nigra* probably because both species are currently present in the mountain areas of this region [71]. An integrated paleobotanic, genetic and modelling approach pointed at the existence of western Europe of potential glacial refugia of *P. sylvestris* up to 40◦ N [3]. The few preceding modelling studies of *P. sylvestris* past potential distribution (e.g., [3,16,26]) showed contrasting results. Habitat suitability modelling of the boreal *P. sylvestris* [3] indicated a potentially wide LGM range in southern Central Europe and eastwards, but those models covered a smaller area than the one we predicted and indicated that potential glacial refugia of *P. sylvestris* were located between ca. 40◦ N and 50◦ N with a patchy geographical distribution. Svenning et al.'s Maxent models [26], instead, showed a potential distribution that extended over southern Italy more than ours. Such discrepancies may be due to the different modelling approaches or climatic variables used in such studies. Based on our models, we predicted that in our study area and more generally over southern Italy, *P. nigra* represents the most likely candidate. In fact, neither past potential distribution of *P. mugo*/*uncinata* nor that of *P. sylvestris* reached our study region in the LGM.

Besides, based on the assumption that the ecological requirements of *P. nigra* match closely those of *Q. pubescens*, we found that the potential distribution of the latter species contributed only 30% to the potential occurrence of *P. sylvestris* vs. 70% to that of *P. nigra*, further corroborating our findings.

Two subspecies of *P. nigra* are described for Italy [72], the subsp. *nigra* and the subsp. *laricio* (*P. nigra* and *P. laricio* sensu Pignatti [69]), respectively. The former has several scattered populations from 200 to 1500 m a.s.l. in the northeastern Alps, while small patches grow on calcareous slopes in the central-southern Apennines between 100 and 1350 m a.s.l. [73]. *P. n. laricio* mostly grows on siliceous soils, in the Sila region (Calabria) between 900 and 1600 m a.s.l. and in Mt. Etna (Sicily) between 1200 and 2000 m a.s.l. [69,73]. In our area, the dominance of carbonatic substrate is a strong argument against the occurrence of *P. nigra* subsp. *laricio* which is strictly linked to siliceous substrates.

Today, *P. nigra* forests are extremely rare in southern Italy. It is an early successional species, and pure self-replacing forests are constrained to few mountainous Mediterranean areas where they can be considered an edaphic climax limited to thin soils. More often this species is part of precursor or transitional associations towards deciduous broadleaved forests. However, *P. nigra* communities are still present, with a very small population, just ca. 80 km north to our study site on a subcoastal Mesozoic limestone ridge (Mts. Picentini, Vallone della Caccia), where these stands are part of the xerophilic open vegetation that occurs on the steep slopes as transitional vegetation between the grasslands and the broadleaved forest vegetation [27]. The *Pinus nigra-Q. pubescens* association which characterises the Pleniglacial could indicate at the local scale a warm-cool bioclimate sensu Finlayson et al. [74].
