"When people think of trees, they see them as sources of oxygen and shade, as water purifiers and providers of wood, but no one really considers how amazing these living beings are."
Peter Wohlleben is a German forester and bestselling author, credited with having given back Germany its forests.
Woodlands are easy to appreciate. But urban trees too are invaluable, making concrete squares and tarmac roads look that much fresher and adding a lushness that makes them immediately more appealing.
As Europe faces another summer of extreme heat and record-breaking temperatures, trees can providing much-needed shade and natural cooling from the scorching sun.
And whether a tree stands in the city, in a forest, in an orchard meadow or at the side of a country road, it also makes a major contribution to climate protection and biodiversity conservation.
"Forests store carbon in the long term in the biomass of trees, but also in deadwood and mineral soil," says the Bavarian State Institute for Forestry and Forest Management (LWF) in Germany.
Exactly how much is stored per tree depends on many factors, such as the climate, the size, the species and where it is located.
A common misconception is that trees directly store the greenhouse gas carbon dioxide (CO2) they absorb from the atmosphere. In fact, they use carbon (C) to build biomass and release oxygen (O2) back into the air, the LWF explains.
Significant carbon storage - but declining
"Forests are an important carbon sink worldwide," says the Thünen Institute, the Federal Research Centre for Rural Areas, Forestry and Fisheries.
However, their ability to act as such a sink – that is, to draw carbon from the atmosphere and store it – has declined in recent years in the EU, for example, says the GFZ Helmholtz Centre for Geosciences, citing a study in the journal Nature.
The average annual carbon uptake by forests in the EU between 2020 and 2022 has fallen by around 27% compared to the period from 2010 to 2014.
"This is due to increased timber harvesting for energy production and the various consequences of climate change, ranging from heat and drought to pest infestations, storms and forest fires."
Although Europe has significant forest cover, with roughly 40% of EU land area covered by forests, a recent study published in the journal Science warns that "forest disturbances in Europe are highly likely to increase in the coming decades."
Researchers predict that forest damage in Europe is expected to rise by 20% by 2100 compared to recent decades, even if the world sticks to ambitious climate targets.
"In the future, Europe's forests are likely to absorb less carbon," says Christopher Reyer, scientist at Potsdam Institute for Climate Impact Research (PIK) and co-author of the study.
"If forests take up less carbon, or potentially even release more than they absorb, this increases pressure on other sectors such as transport and agriculture to reduce their emissions more rapidly. At the same time, forest management needs to focus more strongly on building resilient forests."
Cooling, shade, clean air
Trees also perform important functions in the city - for example, by providing shade. The crown of a 60-year-old plane tree can reach a width of almost 19 metres, and that of a chestnut tree 11.2 metres, according to a guide to urban trees published by the Technical University of Munich.
Scientists have listed further benefits - such as trees filter pollutants from the air, thereby improving air quality.
Or that they reduce rainwater run-off after heavy rainfall "by catching the rainwater in their crowns and slowing down the seepage of water that has infiltrated the soil." This reduces the risk of local flooding caused by the sewer system becoming overloaded.
And in the summer heat, trees can act as a kind of natural air conditioning – primarily through evaporation. A single mature deciduous tree can evaporate around 300 to 500 litres of water on a hot summer's day, says the LWF.
With evaporation of 400 litres per day, this corresponds to a cooling capacity of around 270 kilowatt-hours (kWh). But the actual evaporation rate depends heavily on site conditions.
The key factor is whether there is sufficient water available in the soil. "In dry conditions, trees close their stomata, which significantly reduces evaporation and thus also the cooling effect," the institute notes.
A haven for biodiversity
"Native trees are real hotspots of biodiversity: they provide food, shelter and habitat," says a spokeswoman for the Bavarian Society for the Protection of Birds and Nature (LBV). "Many species are specialized to very specific trees, such as butterflies whose caterpillars feed only on certain tree species."
Diversity is crucial: mixed stands of native species create significantly more habitats and are more stable than monotonous or non-native stands, which are of little use to many animals.
The LBV is particularly dedicated to the protection of orchard meadows.
These are among the most species-rich habitats in Central Europe: "Old fruit trees with hollows provide nesting sites for birds such as the little owl, whilst blossoms are an important food source for wild bees and butterflies. The interplay of trees, meadows and extensive land use also creates habitat for bats and numerous beetle species."
Whether in town or country, trees are homes. Birds such as blackbirds and titmice nest in the crowns of urban trees, while flowers and leaves provide food for insects.
