Past, present and future distributions of Oriental beech (Fagus orientalis) under climate change projections

Caucasus region. Since it is assumed that future climatic conditions will cause an increase in temperature and drought, the mountainous regions of Caucasia will be preferred by Oriental beech for possibly more suitable temperature and moisture conditions. The RF algorithm sug- gests a slight expansion south of the Caspian Sea, along the southwestern Caspian coast. This supports the idea of potential refugia for F. orientalis in the future in this region and empha- sizes the importance of this region as a long-term refugium for temperate tree species. Both regions are characterized by high mountain ranges. The Alborz Mountains, located just south of the Caspian Sea, providing a barrier for humid air masses thereby creating warm and humid conditions from sea level to high elevations. This leads to a large number of suitable habitats under a changing climate as plants can move vertically. A similar situation is met in the western parts of Caucasia where humid air masses from the Black Sea create markedly humid conditions along the coast and on the slopes of the greater and lesser Caucasus (e.g [93]). Towards the east, climatic conditions become drier, but the great vertical gradient remains, thus providing a dynamic environment for tree species to respond to changing envi- ronments by vertical movements. A recent study by Martin-Benito et al. [21] supports the notion that due to little drought response and positive effects of spring-summer warmth, mid- to high-elevation sites in the Caucasus might be potential major climatic refugia for Fagus orientalis in the future.

Comparison with SDMs of European beech (Fagus sylvatica) Process-based and statistical modelling approaches have been widely used for modelling past and future distribution of a close relative of F. orientalis, Fagus sylvatica. For instance, Gisecke et al. [27] used the process-based bioclimatic model to understand the distribution of F. sylvatica during the MH. In agreement with our findings, they also compared their model results with pollen records and found some mismatches, in particular in the northern and western parts of the distribution. They emphasized the need for testing models against palynological data to increase their predictive value. Likewise, a recent study modelling the modern and past distribution of deciduous oaks (Quercus robur L.) also showed a mismatch between the modelled LGM distribution and the one inferred from pollen data [32]. This study suggested that the more restricted distribution data from the pollen record might be due to competition of deciduous oaks with pines and Mediterranean evergreen oaks. Saltre´ et al. [94] applied a process-based approach coupled with migration models to assess whether the present distribution of F. sylvatica is controlled more by climatic conditions or the migration ability during the MH in Europe. Their findings suggested that the northeast boundary of F. sylvatica distribution is limited by climatic conditions whereas the north- west boundary is limited by migration ability. This is very similar to the situation in F. orien-

talis, where, clearly, the (north)eastern boundary is controlled by climate whereas the west- ern boundary is limited by migration ability. However, in contrast to F. sylvatica, the westward expansion of Oriental beech is limited because it is replaced towards the west by its sister species F. sylvatica. Kramer et al. [95] investigated F. sylvatica distribution under climate change with both process-based and statistical modelling approaches. They found that statistical species distribution models exhibited a better goodness-of-fit for future pro- jections, with the southern limit of the F. sylvatica distribution shifted towards the north. On the other hand, future projections of Dyderski et al. [8] for twelve European forest tree species including European beech suggested that the distribution ranges in western and southern Europe will decrease. This seems plausible in view of the predicted climate change towards summer-dry Cs climates in western and southwestern Europe during 2070–2100 ([96]; Global_Shift_A1FI_1976–2100_30m.kmz). PLOS ONE Species distribution modeling of Oriental beech (Fagus orientalis) PLOS ONE | https://doi.org/10.1371/journal.pone.0242280 November 17, 2020 12 / 19