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. Uncertainties in the evapotranspiration response to afforestationconstitute a major source of disagreement between model-based studies of thepotential climate benefits of forests. Forests typically have higherevapotranspiration rates than grasslands in the tropics, but whether this isalso the case in the midlatitudes is still debated. To explore thisquestion and the underlying physical processes behind these varyingevapotranspiration rates of forests and grasslands in more detail, aregional model study with idealized afforestation scenarios was performedfor Europe. In the first experiment, Europe was maximally forested, and in thesecond one, all forests were turned into grassland. The results of this modeling study exhibit the same contradictingevapotranspiration characteristics of forests and grasslands as documentedin observational studies, but by means of an additional sensitivitysimulation in which the surface roughness of the forest was reduced tograssland, the mechanisms behind these varying evapotranspiration ratescould be revealed. Due to the higher surface roughness of a forest, solarradiation is more efficiently transformed into turbulent sensible heatfluxes, leading to lower surface temperatures (top of vegetation) than ingrassland. The saturation deficit between the vegetation and the atmosphere,which depends on the surface temperature, is consequently reduced overforests. This reduced saturation deficit counteracts the transpiration-facilitating characteristics of a forest (deeper roots, a higher leaf area index, LAI, andlower albedo values than grassland). If the impact of the reduced saturationdeficit exceeds the effects of the transpiration-facilitatingcharacteristics of a forest, evapotranspiration is reduced compared tograssland. If not, evapotranspiration rates of forests are higher. Theinterplay of these two counteracting factors depends on the latitude and theprevailing forest type in a region.

What determines the sign of the evapotranspiration response to afforestation in European summer?