Vienna (OTS) – An international research team with the participation of BOKU used extensive tree ring analyzes to prove that the atmosphere in Europe has become significantly drier in recent decades due to greenhouse gas emissions compared to pre-industrial times. This worsens droughts, increases the risk of forest fires and is risky for forests and agriculture. The study has just been published in the current issue of NATURE Geoscience.
One measure of air dryness is vapor pressure deficit (abbreviated VPD). This physical quantity describes the difference between the actual and maximum possible water content of the air, so to speak, the “water thirst” of the air. Water-thirsty air with a high VPD draws more water from the soil and plants, tree growth is reduced, and trees may even die. Dry vegetation and dry soils increase the risk of wildfires. VPD is known to increase in warmer climates. However, little was known about the spatial extent and long-term fluctuations until pre-industrial times without human influence.
Oxygen isotopes in tree rings report past climate
An international team of 67 researchers led by Kerstin Treydte from the Swiss Federal Institute for Forestry, Snow and Landscape Research (WSL) collected oxygen isotope data based on two EU projects (ISONET, MILLENIUM) and a Swiss collaborative project (iTree). rings from all over Europe. Michael Grabner and Rupert Wimmer from BOKU's Institute for Wood Technology and Renewable Resources sampled different tree species from the Lainzer Tiergarten, the Vienna Basin and Dachstein. Isotopes are different atomic variants of the same element, such as oxygen. Water contains lighter and heavier versions of oxygen atoms. Trees absorb water mainly through their roots and release some of it back into the air through their leaves. The remaining part of the water plays an important role in building new wood cells. The ratio of light and heavy isotopes changes during soil evaporation, leaf evaporation, and wood formation. It is precisely these changes that are largely controlled by the VPD. This means tree rings can provide information about past and present air dryness. With this method, changes in the VPD over a 400-year period could now be reconstructed for the first time on a large scale across Europe.
Man-made and strongest in Central Europe
Using additional model simulations, the research team was able to independently test the findings from the tree ring data. The models also come to the conclusion that air dryness in the 21st century is exceptionally high compared to pre-industrial times! Furthermore, they show that current VPD values could not have been achieved without greenhouse gas emissions. Therefore, human influence is obvious.
“The VPD is particularly important for agriculture because the higher it is, the greater the water needs of crops. More irrigation is needed and yields decrease. In forests, wood supply and carbon sequestration are at risk, leading to uncertainty regarding climate regulation and the future carbon storage of these ecosystems”, says Rupert Wimmer. “These effects on agriculture and forestry could particularly affect Austria.” This is particularly worrying in densely populated regions of Europe and shows the urgency of reducing emissions and the importance of adapting to climate change. The results of the study are intended to help refine simulations of future climate scenarios and assess the potential threat that high VPD poses to ecosystems, the economy and society.
More information
The DOI number of the article in NATURE Geosciences is 10.1038/s41561-023-01335-8. The article is available at this URL.
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