Mild climate, lots of CO₂ and constant summer light: around 50 million years ago, favorable environmental conditions in the northern polar regions enabled the deciduous forests there to achieve surprisingly high productivity. This emerges from modeling of the photosynthesis performance of trees in the area of two locations near the poles. The productivity may even have exceeded that of today's forests in our latitudes. According to the researchers, the results also have current significance: They could provide clues about how plant growth in the far north will develop in the wake of climate change.
Where today there are cold deserts or barren tundra landscapes, there was growth around 50 million years ago: as fossil finds show, lush deciduous forests thrived in the Eocene era right up to the northern polar regions. This was due to the era's strong greenhouse climate: carbon dioxide levels in the atmosphere were almost twice as high as today, and mild conditions allowed trees to grow even in the polar regions. But in one aspect the northern Eocene forests differed from today's counterparts with similar temperature conditions: in the high latitudes there is constant darkness in winter, while in summer the sun shines for an extremely long time and sometimes even does not set at all. Forests that grow on the basis of such a combination of a mild climate, high CO₂ levels and extreme lighting no longer exist today.
Exotic northern forests in sight
This is exactly what aroused the interest of researchers at the University of Tübingen and the State Museum of Natural History in Stuttgart. They wondered what plant productivity conditions in northern Eocene forests might have led to. In addition to the availability of nutrients from the soil, it is based on photosynthesis performance: with the help of light energy, plants convert water and carbon dioxide into organic substances from which they build their biomass. To investigate this process in the northern Eocene forests, the researchers use data from two polar sites of deciduous tree fossils from the Eocene: They came from Ellesmere Island in Canada and the Norwegian archipelago of Spitsbergen.
“For comparison, we used the northwestern Odenwald near Darmstadt. It's not at a high latitude, but at a medium latitude, but the deciduous forest there today has similar climatic conditions to those in the far north in the Eocene," says first author Wilfried Konrad from the University of Tübingen. In the model calculations of the productivity of the former Arctic forests, the researchers included information on the atmospheric CO₂ values at the time and the climate conditions at the locations.
The so-called gingerbread tree (Cercidiphyllum japonicum) served as a plant model because its characteristics are similar to those of the trees that once characterized the Arctic Eocene forests.
Surprisingly high productivity
As the team reports, the results of the model calculations revealed an unexpectedly positive influence of the former conditions on photosynthesis performance: “Overall, we came up with a surprisingly high productivity of the forests,” says Konrad. “If one takes the current plant physiological data on the photosynthesis apparatus as a basis, the photosynthesis performance is likely to have been at least 30 to 60 percent higher than at a current location in temperate mid-latitudes. We therefore assume that the climatic conditions in the Eocene increased the photosynthesis performance of the trees and thus possibly also the productivity of biomass,” says the scientist.
The productivity of Arctic Eocene forests may even have been in the range of modern tropical forests, write Konrad and his colleagues. However, as they point out, it remains unclear to what extent nutrient availability and soil properties have influenced plant growth in northern ecosystems.
Finally, Konrad looks at the changes currently taking place in the Arctic as a result of climate change: “The currently rising carbon dioxide levels could also lead to an increase in the photosynthesis performance of northern forests. However, since other factors also have a major influence on plant productivity, the statements cannot be generalized,” emphasizes the researcher. “Nevertheless, findings on the paleoclimate could contribute to improving the models and their predictions,” says Konrad.
Source: University of Tübingen, specialist article: Paleoceanography and Paleoclimatology, doi: 10.1029/2023PA004685