Timing of major forest compositional changes and tree expansions in the Retezat Mts during the last 16,000 years

This paper focuses on the Late Glacial (LG) and Holocene regional and local establishment times of several tree and shrub species in the Retezat Mountains (South Carpathians) using pollen, stomata and plant macrofossil records from four lakes situated at different altitudes. We used the empirical and rational pollen percentage limits to infer the local presence and population expansion/recession times. Our results show that Pinus mugo and P. sylvestris spread first at 15,100 cal yr BP in the mountain. P. mugo arrived at high altitudes (1740 m) already at 13,600 cal yr BP. The expansion of Larix decidua was weakly recorded in the pollen diagrams, but the plant macrofossil and stomata records suggested its Late Glacial expansion mainly on the northern slope and local presence since 14,900 cal yr BP at 1740 m. The species likely went extinct in the Late Holocene (around 3330 cal yr BP). The expansion of Picea abies and Ulmus commenced earlier on the southern slope, from 15,200 to 14,400 cal yr BP respectively, compared to the northern slope, where these species expanded from 14,300 to 13,850 cal yr BP. The following step was the expansion of Quercus at 13,900 cal yr BP. Following the Younger Dryas decrease of temperate deciduous taxa and P. abies, their expansion on the northern flank was in the order of P. abies (11,750), Ulmus (11,600), Quercus (11,100), Corylus (10,600), Carpinus betulus (7100), Abies alba (5100) and Fagus sylvatica (5000 cal yr BP). Timing of the Early Holocene tree expansions on the southern flank is ambiguous due to uncertainties in the Early Holocene age-depth model of Lake Lia; however, the mid-Holocene expansion of C. betulus and A. alba were well-dated in both southern slope pollen records to 7700 and 6100 cal yr BP, respectively, and we inferred several hundred years earlier expansion on the southern flank that can be explained by climatic differences. Higher solar radiation results in longer growing seasons and milder winters on the southern flank, which likely facilitated the earlier spread of these trees. For several woody taxa studied in this paper, last glacial maximum refugial populations can be inferred either locally in the mountain (P. abies, P. mugo, P. sylvestris, L. decidua) or in the region (Quercus, Ulmus, C. betulus, F. sylvatica) on the basis of their pollen presence since at least 15,700–16,000 cal yr BP and early increase during the LG. We compared the pollen inferred population expansion times of several tree taxa along a south-north transect in the Romanian Carpathians, and found that the Early Holocene high altitude expansion of A. alba (10,650–9400 cal yr BP) is exceptional in the region. We also demonstrated that the Holocene mass expansion of Quercus and C. avellana started earliest in the Retezat, and at least two centuries later in other sites in the Romanian Carpathians. We also demonstrated that certain tree species had preference for survival and/or early expansion on the northern slopes (A. alba, L. decidua, F. sylvatica), while others on the southern slopes (C. avellana, C. betulus).