Comparison to other studies

4.3 Results and Discussion 83 In areas with herbaceous biomass plantations, upward surface radiation exceeds that of the afforestation scenario, which results from higher albedo of herbaceous biomass plantations compared to forests. However, the effects on net surface radiation are weak. Net surface radiation is reduced by these changes in South America, East Asia and parts of Africa. In other areas, such as western Russia, the higher albedo does not affect net radiation significantly. Three mechanisms could be responsible for the latter observation: either the long, dark, boreal winters reduce the effects of higher albedo, or the albedo changes are too small to significantly alter net surface radiation, lastly other effects might counter the albedo effects. Changes in cloud cover due to altered transpiration which might counteract alterations in upward surface radiation were excluded.

0.0 0.2 0.4 0.6 0.8

−0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06

Albedo change as a function of cover fraction

Cover fraction of HBPs []

Albedo, Additional simulation − Afforestation []

AFR AUS CAM EUR NAM NAS SAM SAS R = 0.88

Figure 4.5: Land surface albedo differences between the additional simulation and the af-forestation baseline as a function of the cover fraction of herbaceous biomass plantations (HBPs). Only gridcells in which the cover fraction of herbaceous biomass plantations ex-ceeded 0.05 were included. The black line shows the linear regression. AFR: Africa, AUS:

Australia, CAM: Central America, EUR: Europe, NAM: North America, NAS: Northern Asia, SAM: South America, SAS: South Asia

4.3 Results and Discussion 85

−150 −100 −50 0 50 100 150

−50 0 50

a) Precipitaiton, [mm/a]

−60 −36 −12 12 36 60

−150 −100 −50 0 50 100 150

−50 0 50

b) Transpiration, [mm/a]

−60 −36 −12 12 36 60

−150 −100 −50 0 50 100 150

−50 0 50

c) Evapotranspiration, [mm/a]

−60 −36 −12 12 36 60

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−50 0 50

d) Soil Water, [m]

−0.05 −0.03 −0.01 0.01 0.03 0.05

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e) Runoff, [mm/a]

−60 −36 −12 12 36 60

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−50 0 50

f) Cover fractions of HBPs, []

0 0.1 0.2 0.3 0.4 0.5

Additional simulation − Afforestation

Figure 4.6: Difference between the additional simulation and the afforestation baseline in precipitation, transpiration, evapotranspiration, soil water and runoff. Only significant dif-ferences shown (Student’s t-test 5% confidence level)

0.0 0.2 0.4 0.6 0.8

−150

−100

−50 0 50

Transpiration change as a function of cover fraction

Cover fraction of HBPs []

Transpiration, Additional simulation − Afforestation [mm/a]

AFR AUS CAM EUR NAM NAS SAM SAS

R = −0.45

Figure 4.7: Transpiration differences between the additional simulation and the afforestation baseline as a function of the cover fraction of herbaceous biomass plantations (HBPs). Only gridcells in which the cover fraction of herbaceous biomass plantations exceeded 0.05 were included. The black line shows the linear regression. AFR: Africa, AUS: Australia, CAM:

Central America, EUR: Europe, NAM: North America, NAS: Northern Asia, SAM: South America, SAS: South Asia

4.3 Results and Discussion 87 Bathiany et al., 2010; Pongratz et al., 2010; Brovkin et al., 2013). Several idealized studies suggest that deforestation in the tropics warms the climate while deforesta-tion in temperate and boreal regions cools the climate (Claussen et al., 2001; Brovkin et al., 2006; Bathiany et al., 2010). Brovkin et al. 2006, Betts et al. 2007 and Pongratz et al. 2009a found that historical land-use cooled the northern hemisphere by increas-ing albedo as forests were converted to agricultural areas. Brovkin et al. 2013 show that the biogeophysical effects of the land-uses of RCP2.6 and RCP8.5 do not influence global climate but can have a local effect in areas where total land-use change exceeds 10%. In light of these findings, the small changes on both local and global scales found in my study are surprising.

Schaeffer et al. 2006 analyzed the effects of large-scale woody biomass plantations on albedo and transpiration in the northern hemisphere as compared to afforestation.

They found changes in albedo of up to 10% in individual gridcells. My findings are of a similar magnitude but with two major differences: first, my scenario uses herbaceous biomass plantations rather than woody biomass plantations and second, my scenario encompasses all regions, not just the northern extra-tropics. Woody biomass planta-tions are harvested every 4-7 years leading to widely fluctuating leaf area indices and albedo and transpiration changes between years. Herbaceous biomass plantations are harvested annually and grow back quickly, thus their canopies, albedo and transpira-tion are more homogenous between years.

Several studies consider the biogeophysical effects of herbaceous biomass plantations on local conditions. Le et al. 2011 model the effects of Miscanthus, Panicum and Zea mais on the hydrology of the Midwestern United States of America. They find that Miscanthus and Panicum have significantly higher transpiration than Zea mais. Im-portantly, they show that overall hydrology in the Midwestern US is affected by the choice of species. However, they only compare different C4-grasses. My study shows that compared to forests, that are mainly composed of C3-species, herbaceous biomass plantations transpire less, even though C3-plants profit more from higher atmospheric CO₂-concentrations as they are more affected by CO₂-fertilization. In situations of local water limitation, planting C4-grasses or crops may therefore be preferable to planting forests.

Georgescu et al. 2011 also analyzed the effects of converting annual crops into perennial herbaceous biomass plantations. They find increases in both albedo and transpiration.

Their simulations, like Le et al. 2011, illustrate the importance of the reference. They analyze the effects of converting crops to herbaceous biomass plantations and compare the new plant type with the initial plant type. I compare the effects of two alternative choices, afforestation and herbaceous biomass plantations. Thus, compared to crops, both forests and herbaceous biomass plantations would increase overall transpiration, but, compared to afforestation, herbaceous biomass plantations decrease transpiration.

On the other hand, herbaceous biomass plantations increase albedo compared to crops and compared to afforestation.