Temperate forests blow hot and cold
During periods of intense heat, the forests on the one hand and the grasslands and croplands on the other behave very differently than had been believed to be the case up until now. In the initial phase the forests contribute more to a rise in the ambient temperature. It is only after a few weeks of a heatwave that they play the role of mitigating the heat whilst at this smae point in time the grasslands and croplands reinforce the rise in temperature. This surprising conclusion has been drawn by some fifteen European science institutions, including Gembloux Agro-Bio Tech, thanks to the existence of a European observation network.
Ask the average person in the street what they think of a walk in the woods during a heatwave and you will not have to wait long for a reply: a forest soothes people taking a walk as it cools them down. Simple common sense would lead us to take the argument further: so pleasant under the tree cover, this coolness is probably spread throughout the surrounding area and contributes more or less to cooling down the atmosphere.
This vision of things needs some rectifying! A study recently published in Nature Geoscience (1), to which Gembloux Agro-Bio Tech has contributed, shows that intuitive reasoning on this occasion is far from being in step with scientific observation. Produced on the basis of observations carried out through turbulence covariance over some thirty sites scattered throughout Europe – including two situated in Wallonia: one at Lonzée, close to Gembloux, the other at Vielsalm in the Ardennes -, this study shows that at the beginning of a heatwave, forests in temperate regions heat up the atmosphere more than grasslands and croplands do. It is only later, after long weeks of heat, that the forest can eventually play a temporising role, contributing to a toning down of the heating up of the atmosphere, whilst the grasslands and croplands for their part at this stage disseminate more heat. It had instead been believed up until now that the trees, due to their generally deeper rooting in comparison to crops, could more strongly bring into play the water resources of the subsoil and play a very active role in bringing down the heat through evapotranspiration.
(1) Teuling A. J., Seneviratne S. I., Stöckli R., Reichstein M., Moors E., Ciais P., Luyssaert S., van den Hurk B., Ammann C., Bernhofer C., Dellwik E., Gianelle D., Gielen B., Grünwald T., Klumpp K., Montagnani L., Moureaux C., Sottocornola M., Wohlfahrt G. (2010) Contrasting response of European forest and grassland energy exchange to heatwaves. Nature Geoscience, 3 (10), 722-727