Abstract
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.
Highlights
Afforestation is frequently discussed as a potential strategy to mitigate the effects of human-induced climate change (e.g., Sonntag et al, 2016; Harper et al, 2018; Roe et al, 2019; Davin et al, 2020)
According to our present knowledge about the biogeophysical effects of forests and grasslands, this increased forest evapotranspiration is caused by deeper roots (Schenk and Jackson, 2003) and a higher leaf area index (LAI; e.g., HendersonSellers, 1993) than in grassland, whose influence can be attenuated by the reduced photosynthetic activity of forests and an associated stomata closure (Leuzinger et al, 2005)
To investigate the processes determining the sign of the evapotranspiration response to afforestation in the midlatitudes, simulations with the regional climate model COSMO-CLM (Rockel et al, 2008) coupled with the land surface model (LSM) VEG3D (Breil and Schädler, 2017) are performed for Europe
Summary
Afforestation is frequently discussed as a potential strategy to mitigate the effects of human-induced climate change (e.g., Sonntag et al, 2016; Harper et al, 2018; Roe et al, 2019; Davin et al, 2020). One benefit of afforestation is that forests are generally able to take up more CO2 than grasslands (IPCC, 2019) Another advantage is that forests can have a cooling effect on the land surface due to increased evapotranspiration rates compared to grasslands (e.g., Bonan, 2008; Bright et al, 2017; Duveiller et al, 2018). While several observation-based studies show the higher evapotranspiration rates of forests at midlatitudes (e.g., Zhang et al, 2001; Li et al, 2015; Chen et al, 2018; Duveiller et al, 2018), some studies exhibit an opposite behavior of forests with reduced evapotranspiration rates compared to grasslands
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
