New look on interactions between clouds and transpiration by trees in European forests
New research shows a clear link between different cloud types and the transpiration in European forests. An international team of researchers has estimated how changes in cloudiness affect the amount of moisture released by trees into the atmosphere – something that also influences Earth’s water cycle.
A large team of international researchers has investigated the role of clouds in tree sap flow and transpiration. For the first time, the researchers used ground-based measurements to show how different cloud types affect tree sap flow and transpiration in forests across Europe. The study was led by Sini Talvinen at the Department of Environmental Science at Stockholm University.
“You may have tasted birch sap on a spring day? Both sap flow inside the tree and transpiration can feel like natural wonders and are part of the hydrological cycle. They are regulated by a number of interacting factors and processes. In this study, we show that cloudiness is one of the factors influencing this regulation,” says Abhay Devasthale, research leader in meteorological research at SMHI, and part of the international team.
The analysis shows that total cloud cover reduces maximum sap flow by up to 40 percent. However, under certain cloudy conditions, sap flow can even be greater than under clear skies. This reflects the different radiative effects of different cloud types. One important conclusion is that low and high clouds affect sap flow in different ways.
Zoom imageNew research shows a clear connection between different cloud types and the amount of sap flow and transpiration in European forests. An international network of researchers has calculated how this affects the amount of moisture that trees release into the atmosphere when cloudiness changes, which also affects the water cycle on Earth. View of southern Sweden on 19 July 2025 with data from Copernicus Sentinel-2.
Cloud cover and cloud type affect moisture fluxes
Low clouds suppress transpiration by limiting incoming radiation, while high clouds have a negligible impact even under cloudy conditions.
“These results highlight how changes in cloud cover and cloud type can alter moisture fluxes to the atmosphere through vegetation feedbacks, affecting regional and global water cycles,” says Abhay Devasthale, who contributed to the analysis of cloudiness over forests using satellite data. He used satellite observations from EUMETSAT’s CLARA-A3 dataset in the analysis.
Tree sap flow and transpiration
Tree sap flow and transpiration are influenced by factors including season, soil moisture, vapour pressure deficit (the difference between the actual amount of water vapour in the air and the maximum amount the air can hold), and radiation. The importance of these factors varies between seasons.
Clouds affect all of these factors. Variations in cloudiness lead to changes in solar radiation, precipitation and soil moisture. Clouds also modify vapour pressure deficit through their direct and indirect radiative effects.
Tree transpiration is an important source of atmospheric moisture and accounts for around 40 percent of total precipitation over land.
Satellite data provide a broader perspective
After establishing the relationship between cloudiness and transpiration using ground-based measurements, the researchers used satellite data to investigate historical trends in cloudiness over forests. This allowed them to estimate how much moisture forests have added to or removed from the atmosphere as a result of these changes.
For example, satellite data from the studied Nordic ground stations show that low cloud cover has decreased by around 0.48 to 0.66 percent per decade during the summer months over the past 40 years.
“This has likely led to increased transpiration corresponding to a release of approximately 40 to 120 cubic metres of water per hectare and decade into the atmosphere,” says Abhay Devasthale.
The wide range depends on the type of trees growing in the forest – whether it is birch, pine or spruce.
“Another way to express the result is that the reduction in low clouds leads to an increase in transpiration corresponding to about 0.6–1.2 mm of precipitation per year. Or, for each decade, adding 3–7 additional 58-year-old trees – the average age of a tree – to the roughly 600 trees found in every hectare of forest,” Abhay explains.
The research is an example of how ground-based and satellite observations can complement each other to understand and quantify important climate processes, and how the strengths of the two observational systems can be combined.
Since the scientific article was published, Sini Talvinen has changed position and is now based at the University of Eastern Finland.
