They may be heading for the tipping point: the Mediterranean forests could quickly turn into steppes as a result of climate change, researchers report. This emerges from an examination of sediment cores from Greece. The analysis results reflect the development of the vegetation there in combination with moisture values over the last 500,000 years. It turns out that the forests disappeared within a few decades when a certain threshold was undershot. According to forecasts, a corresponding drought can already be expected in the Mediterranean region in the near future, say the scientists.
They still form valuable elements in the Mediterranean environment: the forests of the region are home to a diverse plant and animal community and also provide important ecosystem services. They protect the soil from erosion, regulate the water balance and the regional climate and act as a source of food and wood. However, the climatic conditions in the Mediterranean region, which tend towards extremes, pose a considerable challenge for the forests. "In view of the CO2 emissions caused by humans and the associated global warming, there is growing concern about their continued existence," says first author Andreas Koutsodendris from the university Heidelberg. So far, however, it has remained unclear what tolerance range the Mediterranean forests could actually have with regard to drought.
In the mirror of fossil pollen grains
Koutsodendris and his international colleagues have now pursued this question by taking a deep look into the past: In order to understand how Mediterranean forests once reacted to climatic changes, they examined drill cores that come from a special sampling location in north-eastern Greece: In Tenaghi Philippon formed underground a terrestrial archive with detailed information from the past. The cores of the researchers covered the period of the past 500,000 years.
Fossil pollen grains, which can be assigned to specific plants, could provide information about the vegetation in the datable layers. The scientists were able to obtain information about the amounts of precipitation occurring at the same time from certain geochemical traces in the sample material. As the team reports, the disappearance of the pollen typical for tree species in connection with the precipitation values is clearly evident: in the past, the Mediterranean forest landscapes turned into steppes as soon as precipitation fell below certain threshold values.
Critical level in sight
"In the past, under natural conditions, a drop in rainfall of 40 to 45 percent was enough to herald the sudden transition from a forest to a steppe landscape," sums up Koutsodendris. The developments then took place quite quickly – within a few decades. "Our results show that the switch from forest to steppe biomes over the past 500,000 years occurred quite abruptly and not as a gradual decline in forest cover," the researchers write.
The results of their investigation thus indicate that if the drought persists – as current climate models predict – desertification of the forests in the Mediterranean region can be expected in the near future. "The water deficit, combined with increasing human pressure on Mediterranean ecosystems, raises concerns that moisture levels are falling below the required level for forests to survive," the scientists write. In order to at least take countermeasures and increase the resilience of the Mediterranean forests to the consequences of climate change, measures should now be taken to protect them.
Source: University of Heidelberg, specialist article: Nature Communications, doi: 10.1038/s41467-023-37388-x