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.

中文翻译：

---

是什么决定了欧洲夏季对造林的蒸散响应的迹象？

对造林的蒸散响应的不确定性是基于模型的森林潜在气候效益研究之间存在分歧的主要原因。在热带地区，森林的蒸散率通常高于草原，但在中纬度地区是否也是如此。为了更详细地探讨这个问题以及森林和草地蒸散速率变化的背后潜在的物理过程，针对欧洲进行了具有理想造林情景的区域模型研究。在第一个实验中，欧洲森林得到了最大程度的保护，在第二个实验中，所有森林都变成了草地。该模拟研究的结果显示出与观测研究中记录的相同的森林和草地蒸散特征，但是通过附加的敏感性模拟（其中森林的表面粗糙度降低为草地），这些蒸散速率变化的背后机理是可以被揭露。由于森林较高的表面粗糙度，太阳辐射被更有效地转化为湍流的显热通量，从而导致其表面温度（植被顶部）比草地低。因此，取决于地表温度的植被和大气之间的饱和度不足在整个森林中得以减少。减少的饱和度不足抵消了森林促进蒸腾的特性（较深的根，与草原相比，叶面积指数，LAI和反照率值更高）。如果减少的饱和度不足的影响超过了促进森林蒸腾特性的影响，则与草地相比，蒸散量会减少。如果不是这样，森林的蒸散率就会更高。这两个抵消因素的相互作用取决于一个地区的纬度和盛行的森林类型。