New insight into forest diversity development

The duration and intensity of ice ages strongly determine the humidity in the atmosphere and soil. This availability of water appears to be a very determining factor for the tree diversity of the forest. This is the conclusion of an international research team led by Utrecht University palaeo-ecologist Dr Timme H. Donders. They obtained these new insights by studying fossil pollen in the sediments of ancient Lake Ohrid, on the border of Albania and North-Macedonia. The research, published today in Proceedings of the National Academy of Sciences of the USA, may provide tools for forest management to increase forest resilience to future climate change.

‘Forest vegetation in Northern Europe is relatively species-poor compared to North America and East Asia. This is due to differences in climate evolution in these areas,' Timme Donders explains. In Europe, many tree species barely survived the ice ages in areas around the Mediterranean Sea. ‘These refugia in Italy, Spain and the Balkans were areas with milder climate conditions during glacials. In warmer periods, the forest expanded northwards again, but regularly populations did not survive these major changes and died out in Europe,' says Donders.

Research on sediment core

With a respectable age of 1.4 million years, Lake Ohrid is not only Europe's oldest lake, but also most species-rich. Previous research on the lake's 568-metre-long sediment core has already shown that the development of the lake determines the local species richness. The fossil pollen now show that the area around the lake was also a determinant factor for the survival of tree species during successive ice ages. New analyses show in detail how the diversity of the forest is related to the coming and going of ice ages of varying intensity. Donders: 'From the emergence of the lake, the forest was still very diverse and fairly stable in composition with species that are now no longer found in Europe, such as pecan and swamp cypress. After about 0.9 million years, however, the forest diversity around Lake Ohrid declined sharply and not all species survived the longer and colder ice ages. Also, short term climate fluctuations varied more and more, which contributed to the decrease in forest diversity'.

It is remarkable that both the decline and the phases of recovery of forest diversity took place during cold periods and that it was precisely during these periods that the humidity was decisive for the survival of many tree species. The increasingly deeper and larger lake may have contributed to the relatively long period of high diversity in the area. The deep lake buffers to some extent the local climate, which resulted in locally milder climatic conditions. For this reason, this area remained a very effective refugium for a long time, until around 0.9 million years ago the ice ages became so severe that several tree species disappeared for good," explains the palaeo-ecologist. Such a long series of forest histories from Europe is very rare and the study, published in the leading scientific journal Proceedings of the National Academy of Sciences of the USA, will therefore be of great importance for future biodiversity research.

Publication

Timme Donders, Konstantinos Panagiotopoulos, Andreas Koutsodendris, Adele Bertini et al., ‘1.36 million years of Mediterranean forest refugium dynamics in response to glacial–interglacial cycle strength’, Proceedings of the National Academy of Sciences of the USA (2021) vol. 118, no. xx, e2026111118, https://doi.org/10.1073/pnas.202611