Agricultural and Forest Meteorology 106 2001 1–21
The simulation of canopy transpiration under doubled CO
2
: The evidence and impact of feedbacks on transpiration in two 1-D
soil-vegetation-atmosphere-transfer models
Jon C. Gottschalck
a,∗
, Robert R. Gillies
b
, Toby N. Carlson
c
a
Division of Meteorology and Physical Oceanography, University of Miami, Miami, FL 33149, USA
b
Department of Plants, Soils, and Biometeorology and Department of Geography and Earth Resources, Utah State University, Logan, UT, USA
c
Department of Meteorology, The Pennsylvania State University, University Park, PA 16802, USA Received 24 May 1999; received in revised form 27 June 2000; accepted 29 June 2000
Abstract
Increasing levels of atmospheric CO
2
concentration [CO
2
] has caused a debate in the scientific community over how veg- etation responds to this increase — specifically in the stomatal resistance increase SR
increase
and the transpiration decrease TR
decrease
. This paper presents results in changes of stomatal resistance and transpiration from two canopy model simulations under varying environmental conditions for corn and soybeans for presentday and doubled atmospheric [CO
2
] conditions. The two canopy models used in the simulations were the off-line version of the land surface exchange parameterization of the
GENESIS general circulation model LSX and the Penn State University Biosphere-Atmosphere Modeling Scheme PSUB- AMS. Simulations were performed with each model’s original stomatal resistance parameterization SRP and then with a
field-derived SRP developed from corn and soybeans field measurements taken during 1993–1995.
For corn using the original SRP, the percentage ratio TR
decrease
SR
increase
× 100 calculated ranged from 20 to 37 for both models and values of Ω a non-dimensional ‘decoupling coefficient’ that relates how closely the TR
decrease
is correlated with SR
increase
ranged from 0.63 to 0.80. The results for corn, using the field derived SRP, showed that the percentage ratio and Ω ranged from 18 to 25 and 0.75 to 0.82, respectively. For soybeans, the values of the percentage ratio and Ω for the
original SRP ranged from 20 to 39 and 0.61 to 0.80, respectively, while for the field derived SRP they were 12–20 and 0.80–0.87. The model derived values for the percentage ratio were substantially less than those reported in the literature for
experiments conducted at the leaf scale. It is also demonstrated, using these models, how inter-canopy LSX and PSUBAMS and mixing layer PSUBAMS feedbacks modify transpiration beyond those initiated by changes in stomatal resistance alone.
© 2001 Elsevier Science B.V. All rights reserved.
Keywords: Atmospheric CO
2
concentration; Stomatal resistance; Transpiration
∗
Corresponding author. Tel.: +1-305-361-4745; fax: +1-305-361-4696.
E-mail address: jgottschrsmas.miami.edu J.C. Gottschalck.
1. Introduction
As a consequence of human development, mainly the burning of fossil fuels for energy, the atmospheric
carbon dioxide concentration [CO
2
] has increased to 358 mmol mol
−1
during the past 35 years from
0168-192301 – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 1 9 8 - 2
2 J.C. Gottschalck et al. Agricultural and Forest Meteorology 106 2001 1–21
315 mmol mol
−1
Keeling et al., 1995. International reports have suggested that [CO
2
] will double by the middle of the next century Watson et al., 1990. An
important aspect of the rise in [CO
2
] is its effect on plant function and physiology. In an elevated [CO
2
] environment, more [CO
2
] enters the leaf through the stomata, increasing the leaf interior [CO
2
] which manifests itself as an increase in the stomatal resis-
tance and a decrease in transpiration Mott, 1990. Transpiration changes initiated by increased [CO
2
] have impacts on both agriculture and meteorology.
Agricultural scientists are interested in any alteration in the water usage by field crops that might affect
yield and result in future economic repercussions see reviews by Morison, 1985; Eamus, 1991. Meteorol-
ogists, however, are more concerned with how plant responses to increased atmospheric [CO
2
] might alter the surface energy balance and affect climate at the lo-
cal, regional, and global scales Jacobs and DeBruin, 1992; Avissar, 1993; Pollard and Thompson, 1995;
Henderson-Sellers et al., 1995; Jacobs and DeBruin, 1997.
Transpiration at the scale of the single leaf in ele- vated levels of atmospheric [CO
2
] has been actively researched for the past 35 years. In these studies, a
decrease in transpiration TR
decrease
was reported and when applicable, the change was expressed as a per-
centage ratio
1
TR
decrease
SR
increase
between the de- crease in transpiration and the increase in stomatal
resistance SR
increase
. The magnitude of the transpi- ration decrease and the percentage ratio varied con-
siderably. It depended on many factors such as plant characteristics and environmental conditions, such as
photosynthetic photon flux density, humidity, and air temperature of the test. These leaf scale studies as-
sessed the decrease in transpiration and the percent- age ratio in three ways magnitudes shown as de-
crease in transpiration; percentage ratio
1
: direct mea- surement Rogers et al., 1984 57; 71; Rozema
et al., 1990 15–48;
2
NA; Kirkham et al., 1991 25–35; 43, indirect calculation from a combi-
nation of measurements which were study dependent Pallas, 1965 21–68 NA; Egli et al., 1970 25
1
In some studies, the ratio could not be reported as stomatal resistance, was not directly measured — alternate means to arrive
at a value for the change in the transpiration were used.
2
NA: not available, refer footnote 1.
NA; Akita and Moss, 1972 23; NA; Morison and Gifford, 1984 21; 36; Idso et al., 1984 27;
NA; Valle et al., 1985 5; NA; Idso et al., 1987 9; NA; Nijas et al., 1988 51; NA, and by ap-
plication of numerical models to simulate the energy balance for a single leaf Owensby et al., 1993 21;
40; Zhan and Wilks, 1994 23; NA. A review of 46 observations of doubled [CO
2
] effects on tran- spiration was reported by Kimball and Idso 1983
who found on average that transpiration decreased by 34. Furthermore, a review article by Cure and Acock
1986 compiled all published data for known doubled [CO
2
] experiments for major crops and reported that transpiration was reduced on average by 23 for all
crops and the percentage ratio on average was 76. When scaling from the single leaf to a full canopy,
other factors affect the extent to which doubled [CO
2
] changes transpiration. For instance, inside canopies
plants tend to modify the microclimate, which may inhibit water loss from the plants. Moreover, air
temperature and humidity above the canopy are fre- quently changing in the face of atmospheric forcing
from above, altering the atmospheric demand for heat and water from the canopy. In addition, a change in
canopy transpiration may result via a change in wind speed above the canopy or in the height and density
of the vegetation, all of which alter aerodynamic re- sistances. Changes in these quantities that alter the
transfer of water vapor in and through the canopy will impact what the vegetation ‘feels’ through the guard
cells of the leaf Aphalo and Jarvis, 1993. These inter-canopy and boundary layer effects interact to
adjust the transpiration Baldocchi, 1994; Field et al., 1995; Jacobs and DeBruin, 1997; Raupach, 1998;
Steduto and Hsiao, 1998a–c.
A few canopy field studies have been performed on the impacts on transpiration in elevated [CO
2
]. The free-air carbon dioxide enrichment FACE program
Hendrey and Kimball, 1994 conducted increased ambient + 200 mmol mol
−1
[CO
2
] studies on canopy sized plots of cotton and wheat. Hileman et al. 1994
observed that in an enriched [CO
2
] environment, sea- sonal transpiration was unchanged despite an increase
of 13–44 in stomatal resistance; this was due to an increase in plant foliage. Besides this field study,
there are some canopy modeling studies that have researched the impacts on transpiration in a simulated
doubled [CO
2
] environment Friend and Cox, 1995;
J.C. Gottschalck et al. Agricultural and Forest Meteorology 106 2001 1–21 3
Henderson-Sellers et al., 1995; Pollard and Thomp- son, 1995; Carlson and Bunce, 1996; Bunce et al.,
1997; Brown and Rosenberg, 1997; Grossman-Clarke et al., 1999. Friend and Cox 1995, using a single
column model, found a decrease in evapotranspiration of 22 for doubled CO
2
experiments over an Ama- zon tree canopy. Henderson-Sellers et al. 1995 and
Pollard and Thompson 1995 used off-line versions of land surface parameterizations in the GENESIS
LSX and CCM1-Oz BATS general circulation models, respectively, to diagnose doubled [CO
2
] ef- fects on transpiration. These two models in their
off-line form prescribe atmospheric conditions so that no dynamic interaction occurs between the sur-
face fluxes and the mixing layer. For both studies, the stomatal resistance was doubled to parameterize
doubled [CO
2
] for wet grassland canopies and re- ported a decrease in transpiration of 28 and 18 for
Pollard and Thompson 1995 and Henderson-Sellers et al. 1995, respectively. Carlson and Bunce 1996
performed a sensitivity test in a corn and soybean study by representing the effects of doubled [CO
2
] on stomatal resistance by increasing the stomatal re-
sistance from 0 to 150 and observed a subsequent decrease in transpiration ranging from 0 to 25.
Grossman-Clarke et al. 1999 simulated a 5 de- crease in total seasonal canopy transpiration under
elevated [CO
2
] 370–550 mmol mol
−1
using a wheat crop model and, Brown and Rosenberg 1997 found
decreases in evapotranspiration of 3–10 and 4 for corn and soybeans respectively for a 40 stomatal
resistance increase.
The results from these canopy studies are interesting and important because the results expressed either as
TR
decrease
or TR
decrease
SR
increase
are much less than those observed for research conducted at the scale of
the single leaf. The discrepancy between the results in the literature for studies conducted at the scale of a sin-
gle leaf compared with those from canopy models in- dicate that other factors besides the stomatal resistance
are interacting to alter the transpiration at the canopy scale. Feedbacks in ambient humidity, wind speed, ra-
diation, and vegetation structure significantly impact the flux of water vapor from the canopy Jarvis and
McNaughton, 1986; McNaughton and Jarvis, 1991; Aphalo and Jarvis, 1993; Baldocchi, 1994; Jacobs and
DeBruin, 1997; Raupach, 1998; Steduto and Hsiao, 1998a–c.
This paper presents the results of a modeling ap- proach used to investigate how plant inter-canopy and
boundary layer feedbacks alter the decrease in transpi- ration when perturbed in doubled [CO
2
] under differ- ing environmental conditions. The results are reported
both in terms of the percentage ratio
3
, the decoupling coefficient, Ω Jarvis and McNaughton, 1986 as well
as the stomatal resistance to total aerodynamic resis- tance ratio r
s
:r
a
.
2. Methodology
