Abstract. Uncertainties in the evapotranspiration response to afforestation
constitute a major source of disagreement between model-based studies of the
potential climate benefits of forests. Forests typically have higher
evapotranspiration rates than grasslands in the tropics, but whether this is
also the case in the midlatitudes is still debated. To explore this
question and the underlying physical processes behind these varying
evapotranspiration rates of forests and grasslands in more detail, a
regional model study with idealized afforestation scenarios was performed
for Europe. In the first experiment, Europe was maximally forested, and in the
second one, all forests were turned into grassland. The results of this modeling study exhibit the same contradicting
evapotranspiration characteristics of forests and grasslands as documented
in observational studies, but by means of an additional sensitivity
simulation in which the surface roughness of the forest was reduced to
grassland, the mechanisms behind these varying evapotranspiration rates
could be revealed. Due to the higher surface roughness of a forest, solar
radiation is more efficiently transformed into turbulent sensible heat
fluxes, leading to lower surface temperatures (top of vegetation) than in
grassland. The saturation deficit between the vegetation and the atmosphere,
which depends on the surface temperature, is consequently reduced over
forests. This reduced saturation deficit counteracts the transpiration-facilitating characteristics of a forest (deeper roots, a higher leaf area index, LAI, and
lower albedo values than grassland). If the impact of the reduced saturation
deficit exceeds the effects of the transpiration-facilitating
characteristics of a forest, evapotranspiration is reduced compared to
grassland. If not, evapotranspiration rates of forests are higher. The
interplay of these two counteracting factors depends on the latitude and the
prevailing forest type in a region.