1.36 million years of Mediterranean forest refugium dynamics in response to glacial–interglacial cycle strength

Abstract

The sediment record from Lake Ohrid (Southwestern Balkans) represents the longest continuous lake archive in Europe, extending back to 1.36 Ma. We reconstruct the vegetation history based on pollen analysis of the DEEP core to reveal changes in vegetation cover and forest diversity during glacial–interglacial (G–IG) cycles and early basin development. The earliest lake phase saw a significantly different composition rich in relict tree taxa and few herbs. Subsequent establishment of a permanent steppic herb association around 1.2 Ma implies a threshold response to changes in moisture availability and temperature and gradual adjustment of the basin morphology. A change in the character of G–IG cycles during the Early–Middle Pleistocene Transition is reflected in the record by reorganization of the vegetation from obliquity- to eccentricity-paced cycles. Based on a quantitative analysis of tree taxa richness, the first large-scale decline in tree diversity occurred around 0.94 Ma. Subsequent variations in tree richness were largely driven by the amplitude and duration of G–IG cycles. Significant tree richness declines occurred in periods with abundant dry herb associations, pointing to aridity affecting tree population survival. Assessment of long-term legacy effects between global climate and regional vegetation change reveals a significant influence of cool interglacial conditions on subsequent glacial vegetation composition and diversity. This effect is contrary to observations at high latitudes, where glacial intensity is known to control subsequent interglacial vegetation, and the evidence demonstrates that the Lake Ohrid catchment functioned as a refugium for both thermophilous and temperate tree species.

Genomic evidence of speciation by fusion in a recent radiation of grasshoppers

“…The magnitude and duration of glacial‐interglacial pulses increased during the last stages of the Pleistocene (Head and Gibbard 2005 ; Hewitt 2011 ), which led to severe vegetation shifts and repeated phases of reduction in forest extension in favor of grasslands and shrub‐like habitats throughout Europe (Donders et al. 2021 ). Thereby, it is plausible that during this period C. binotatus and C. saulcyi experienced marked distributional shifts and some of their populations came into secondary contact and hybridized, resulting in the establishment of a hybrid population that rapidly underwent geographic isolation.…”

The geodynamic and limnological evolution of Balkan Lake Ohrid, possibly the oldest extant lake in Europe

“…The catchment of Lake Ohrid is dominated by deciduous oak and hornbeam forests below 1250 m a.s.l., mixed mesophilous and montane forest between 1250 and 1800 m a.s.l., and sub‐alpine grassland above (Donders et al . 2021 and references therein).…”

“…2020; Donders et al . 2021). Moreover, the continuous species‐level fossil record of diatoms from Lake Ohrid along with environmental and climate indicator time series gave new insights into the evolutionary and community dynamics of an isolated ecosystem since its formation (Wilke et al .…”

“…2020; Donders et al . 2021) and stable isotopes provided information on lake water sourcing and changes in the evaporation vs. precipitation ratio (e.g. Lacey et al .…”

The late Early Pleistocene flora of Oriolo, Faenza (Italy): assembly of the modern forest biome

“…During the most severe parts of the glacial periods, European temperate trees became restricted to local refugia in mountains and moist valley-bottoms in southern Europe (Willis 1996). The loss of temperate trees and shrubs from the Pliocene (Zanclean and Piacenzian) and Pleistocene floras of Europe and the dynamic change from glacial to interglacial phases has been the subject of numerous studies (e.g., Martinetto 2001, Bertini 2010, 2013, Leroy et al 2011, Martinetto et al 2012, 2015, Pini et al 2014a, Combourieu-Nebout et al 2015, Pross et al 2015, Magri et al 2017, Denk et al 2021, Donders et al 2021). One explanation for the successive disappearance of warmth-loving temperate woody taxa from the European flora suggested that extirpation from the European flora was determined by climate requirements still seen in the congeners of these extirpated taxa in the modern floras of East Asia and/or North America (niche conservatism; Svenning 2003).…”