Many forests are damaged by drought and bark beetle infestation – reforestation can also provide more rainfall.
(Photo: picture alliance/dpa/dpa central image)
If it is too dry, the forest suffers, also in Germany. On the other hand, more trees could also provide more rainfall. Researchers therefore advise planting more forest in Europe to protect against drought – reforestation could increase the amount of rain in summer by several percent after all.
Reforestation could increase rainfall in large parts of Europe and dampen some of the effects of climate change, according to a study. Researchers led by Ronny Meier of the Swiss Federal Institute of Technology in Zurich (ETH) report in the journal "Nature Geoscience" that this could prevent summer drought in particular, following statistical analysis of weather data. Afforestation could increase rainfall in the summer by an average of 7.6 percent – that would correspond to 0.13 millimeters per day.
Julia Pongratz of the Ludwig Maximilian University of Munich (LMU) speaks of a very important study: It proves the connection between forests and precipitation on the basis of a broad observation base and considers not only the effects at the location of a possible reforestation, but also how precipitation could change in the downwind regions. However, many interrelationships are very complex, says the holder of the Chair of Physical Geography and Land Use Systems, who was not involved in the work. "It is unclear, for example, to what extent the correlations found still apply as climate change progresses."
Forecasts for Germany: drier summers
The extent to which drought can affect trees is shown by the German forest condition surveys of the last few years. Forecasts for Germany predict more precipitation in winter and drier summers in the future. It is clear that forests can mitigate the effects of climate change – for example, by protecting soils from evaporation. In addition, studies suggest that forests themselves can promote precipitation.
Meier’s team now used precipitation data to test how converting agricultural land – fields and pastures – into forest land affects precipitation. It compared climatically similar areas in meteorological databases that differed in terms of agricultural and forest areas. In addition to the influence of vegetation, the researchers also considered different climatic regions of Europe and diverse terrain types.
They assigned a total of more than 1500 such pairs to five European regions. In all regions, high forest cover was accompanied by more precipitation. The only exception was southern Finland in April through July. The effect was particularly strong near the coast – for example, in the British Isles – but weakened as the climate became more continental.
Scenario: 14 percent more forest cover
In general, the difference between farmland and forestland was 5 to 15 percent in winter, and less pronounced at 0 to 10 percent in summer. For Europe as a whole, the researchers use a so-called "realistic scenario" The team assumes that a good 14 percent of the area studied would be suitable for further forestation without any loss of food supply or biodiversity.
This could increase precipitation by more than 10 percent over 27 percent of Europe’s land area, they write. The effect would be particularly strong in the British Isles, western and southwestern France, Italy, the eastern Adriatic coast south to Greece, and parts of the Iberian Peninsula. However, parts of Germany would also benefit.
"Potential to offset part of climate change impacts"
"The changes in precipitation triggered by realistic forestation have the potential to offset some of the consequences of climate change.", writes the team, referring in particular to the Mediterranean region. "Forests could play a crucial role in adapting to increased risks of summer droughts due to climate change."
The researchers explain the effect on precipitation as follows: Due to the rougher surface compared to fields and meadows, forests slow down the movement of air masses and also create more turbulence, which increases the tendency to precipitation. In addition, evaporation is increased above forests, which increases the amount of precipitation, especially in summer when the wind is blowing. Such effects have been proven for the tropics and the Sahel zone.
Causal relationships only with reservations
However, the team admits that the correlations are based on observations, which means that causal relationships can only be assumed with reservations. Still, with increased drought predicted for the coming summers, more attention needs to be paid to vegetation impacts, they said.
LMU expert Pongratz also emphasizes this: She considers the statements for the effects of forests on precipitation to be robust. "It’s clear that climate and land cover are linked," she says, she says. "This is a closely coupled system." Nevertheless, measures at the regional level must be examined very closely for their possible consequences, he said.
"Measures like reforestation are important to remove carbon dioxide from the atmosphere and can mitigate global climate change", says Pongratz. "At the same time, each region must adapt to climate change. So if reforestation can counteract decreasing rainfall, the forest serves a dual purpose, globally and locally."