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

Timme Donders,Anna Maria Mercuri,Alexander Francke,Katerina Kouli,Laura Sadori,Assunta Florenzano,Odile Peyron,Andreas Koutsodendris,Sébastien Joannin,Bernd Wagner,Adele Bertini,Alessia Masi,Paola Torri,Ilias Kousis,Konstantinos Panagiotopoulos,Nathalie Combourieu-Nebout

doi:10.1073/pnas.2026111118

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.

Highlights

Identification and protection of past forest refugia, supporting a relict population, has gained interest in light of projected forest responses to anthropogenic climate change [1,2,3,4]
The DEEP pollen record was divided into pollen superzones that reflect G-IG cycles, following the approach of Sadori et al [37, 41]
These G-IG variations are characterized by alternations between arboreal pollen (AP %) and herbs

Summary

Introduction

Identification and protection of past forest refugia, supporting a relict population, has gained interest in light of projected forest responses to anthropogenic climate change [1,2,3,4]. Understanding the past and present composition of Mediterranean forest refugia is central to the study of long-term survival of tree taxa and the systematic relation between forest dynamics and climate [5]. The Quaternary vegetation history of Europe, studied for over a century, is characterized by successive loss of tree species [6,7,8]. Species loss was originally explained by the─repeated─migration across E-W oriented mountain chains during glacial-interglacial (G-IG) cycles [9]. Mediterranean mountain regions are considered to serve as forest refugia over multiple glacial cycles and frequently coincide with present-day biodiversity hotspots [14].

Objectives

Methods

Results

Conclusion