During heatwaves, forests initially have a warming effect on surrounding air temperatures. However, in the longer-term they have a cooling impact, according to a recent study. This new information about plant contributions to temperatures will help further improve climate modelling.
Climate change is expected to make summers in Central and Western Europe drier, which means that heatwaves are likely to become more severe and frequent in future. In this study, carried out in collaboration with the FLUXNET1 project, the researchers compared how soils in forests and grasslands contributed to temperature during the European heatwaves of 2003 and 2006, compared with normal summer conditions.
They used data from satellites on land surface temperatures in combination with data from towers built over forests and grasslands located across Central and Western Europe. These towers measure, among other things, the exchange of heat between the forests and grasslands and the atmosphere. Changes in heat transfers during a heatwave are strongly influenced by the amount of moisture in the soil and this is affected by the type of land cover.
At the start of a heatwave, the researchers found that average surface heating above forests is twice that above grasslands. In the 2006 heatwave, for example, forests were hotspots of heat, with the atmosphere above forests being heated at a rate up to four times greater than over grasslands.
Grasslands and croplands were more efficient at cooling surface temperatures at the beginning of a heatwave because the heat accelerated evaporation of soil moisture. However, once the soil moisture was reduced, and if there was no rain, temperatures above grasslands rose rapidly, over a period of one week. This process is thought to have occurred in the later stages of the August 2003 heatwave in France.
In contrast, forest cover conserved water. Leaf stomata (pores) close in response to hot and dry air, reducing the rate at which water is evaporated from the plants. In addition tree roots are typically 25 per cent deeper than grass or crop roots. These two factors allow trees to evaporate soil moisture over extended periods of time. During the height of the 2003 heatwave, for example, the temperature over forests in some regions was 3.5°C cooler with respect to the long-term average than over grasslands. Therefore, over the longer-term, forests had a more moderating effect on heatwaves than grassland or croplands.
In the 2003 heatwave, the highest temperatures in France (over 40°C) were reached in August after an already hot and dry summer, when soil moisture levels had been depleted. In comparison, during the heatwave in July 2006, the heatwave lasted for a shorter time and soil moisture levels were not depleted to the same extent as occurred in August 2003.