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Text S1.
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Scaling FPAR for the RPVB-case
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In SiB2, FPAR is derived from satellite observed normalized difference vegetation index
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and is vegetation type dependent. It varies between zero and one, with higher values
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corresponding to a denser canopy. In the C, RP, and RPV cases, FPAR is prescribed
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from satellite observations; it then affects the components of the surface energy, water
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and carbon balances but does not respond to them. In the RPV-case, the carbon
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assimilation rates calculated for each grid point of the C and RP-cases were averaged
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over the last 10 years of simulations and their ratios (gpp_C/gpp_RP) were applied
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directly to the maximum photosynthetic capacity Vmax to reduce its values at every grid
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point and simulate a large scale down-regulation effect [Sellers et al. 1996]. An
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assessment of the Free-air CO2 enrichment (FACE) experiments indicates that the effects
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of elevated CO2 on plants and ecosystems grown under natural conditions increase light-
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saturated carbon uptake, diurnal cycle carbon assimilation and growth. It also indicates
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that trees were more responsive than other functional types and that C4 vegetation
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showed little response. The magnitude of the photosynthetic acclimation was different
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among different C3 functional types and Vmax values were shown to be reduced to a
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greater extent in grasses and shrubs than in trees [Ainsworth and Long, 2005, see also
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Leakey et al. 2009].
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In the RPVB case, we developed a method that allows FPAR to respond to changes in
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atmospheric CO2 and the precipitation generated by the increase in CO2 concentration.
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The method rests on the idea that in an increased atmospheric CO2 concentration,
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terrestrial vegetation down- regulates its physiological activity per unit leaf area but then
1
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grows more biomass and redistributes photosynthetic capacity to a greater number of
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leaves in the canopy. The modeling consists in “down-regulating” the Vmax but at the
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same time scaling up FPAR. The effect of temperature on Vmax is neglected in this
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study [Leakey et al., 2009; Ainsworth and Long 2005]. Thus, at each grid point, the
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maximum photosynthetic capacity vmax was reduced by,
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V max new  V max{ fpar max*
gpp _ C
 (1  fpar max)}
gpp _ RP
(1)
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We hypothesize that the excess photosynthetic capacity following the reduction in Vmax
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will increase FPAR such that total carbon uptake within each grid cell approaches that of
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the RP-case.
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The ratios (gpp_RP/gpp_C) were applied to the RPVB-case, which incorporates down
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regulation; to increase FPAR proportionally. The scalar for FPAR was then obtained as:
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scale  fpar max*
gpp _ RP
wstress _ C
 (1  fpar max) *
gpp _ C
wtress _ RP
(2)
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Where fparmax is the maximum value for FPAR observed at each grid cell for the entire
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annual cycle; wstress_RP and wstress_C represent the water stress functions obtained in
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the RP and C cases, respectively.
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fraction in the model’s root zone. It inhibits carbon assimilation rates and conductance if
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the root zone’s water level is low. The increase of FPAR was modulated by the ratio of
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water stress between the C and RP runs (C/RP) to ensure that FPAR is allowed to
The water stress function is a measure of the water
2
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increase only over grid cells where water availability was not limiting photosynthesis and
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evapotranspiration in the RP-case.
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compensates for the down-regulation, and the second part augments FPAR in the absence
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of water stress in the RP-case. This water stress restriction is put in place to avoid
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situation where vegetation growth is not corroborated by the modeled root zone water
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content.
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Finally in the RPVB-simulation, scaled values of FPAR were obtained by multiplying the
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value of the original FPAR by the scaling function at each grid cell as:
The first part of the right hand side of (eq. 2)
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fparnew  scale * fpar
(3)
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The newly computed value fparnew was bound by a maximum allowable value
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fpar_max_a, as:
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0.001  fparnew  fpar _ max_ a
(4)
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Where fpar_max_a is obtained using fparmax, its corresponding value of maximum
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greenness fraction, greenmax, and the leaf area index maximum range corresponding to
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the grid cell’s vegetation type, zltmax. Zltmax is the value of LAI defining the structural
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limitation and is restricted to a maximum value that represents an FPAR of 0.95 (Sellers
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et al., 1996b).
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fpar _ max_ a  1.0  e
 park max* zlt max
green max
(5)
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Using the new values of FPAR for all grid cells, global fields of vegetation cover
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fractions, leaf area index (LAI) and greenness fractions were obtained at all grid cells.
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These fields, along with the modified Vmax fields were used as boundary conditions for
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the land surface model, SiB2, to run the RPVB-case forward.
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The scaling procedure led to an overland average increase of about 25.3 % in FPAR and
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of 58.3 % in LAI. Over the forest-dominated eastern regions of the United States, FPAR
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was increased by 24.3% and LAI by 54.8%.
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References
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Leakey, A.D.B. et al. (2009). Elevated CO2 Effects on Plant Carbon, Nitrogen and Water
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Relations: Six Important Lessons From FACE. Journal of Experimental Botany.
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60(10):2859-2876
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Ainsworth EA, Long SP, 2005, what have we learned form 15 years of fee-air CO2
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enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy
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properties and plant production to rising CO2. New Phytologist, 165, 351-372
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Sellers P.J. L. Bounoua, G. J. Collatz, D. A. Randall, D. A. Dazlich, S. O. Los, J. A.
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Berry, I. Fung, C. J. Tucker, C. B. Field, T. G. Jensen (1996) Comparison of Radiative
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and Physiological Effects of Doubled CO2 on Climate. Science, 271, 1402-1408.
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Sellers P.J., S. O. Los, C. J. Tucker, C. O. Justice, D. A. Dazlich, G. J. Collatz, and D. A.
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Randall, 1996b: A revised land surface parameterization (SiB2) for atmospheric GCMs.
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Part II: The generation of global fields of terrestrial biophysical parameters from satellite
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data. J. Climate, 9, 706–737.
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