During heatwaves forests reduce their evaporation, causing the atmosphere to warm up even more, say researchers.
During extremely long periods of heat, however, this reduction enables the forests to continue their evaporation for longer, so the net effect is ultimately one of cooling in relation to the surroundings, explained a team of scientists led by Ryan Teuling from Wageningen University in the Netherlands.
Writing in the journal Nature Geoscience, Dr Teuling worked on the investigation in collaboration with climate researchers from a number of European countries.
The study was prompted by recent heatwaves in Europe, which had raised interest in questions about the influence of land use on temperatures and climate.
Up to now, scientists had assumed that a lack of precipitation during heatwaves automatically led to a reduction in evaporation.
That reduction was thought to be less for forests, because trees, with their deeper root systems, have more water available to them. Examination of the precise role of land use, however, has been largely neglected up to now.
The study found large differences in evaporation strategies during heat waves. Grasslands evaporate more at higher temperatures and stop only when no more water is available.
Forests, in contrast, respond to higher temperatures by evaporating less, which leaves more water at their disposal.
During brief heatwaves, therefore, the greatest warming is found above forests, but during prolonged heat waves the increased evaporation of grasslands ends up causing a shortage of water.
This can lead to exceptionally high temperatures, such as those measured in France in the summer of 2003.
This mechanism might also offer an explanation for the unusually high temperatures near Moscow this summer, the researchers suggest.
In these types of extreme situations, forests in fact have a cooling effect on the climate.
The research was done on the basis of observations made above forests and grasslands in Europe by an extensive network of flux towers. For areas without towers, satellite data were used.
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