28 P.D. Jamieson et al. Agriculture, Ecosystems and Environment 82 2000 27–37
Terrestrial Ecosystems GCTEs International Wheat Network. The data generated in those experiments
have been used to validate assumptions incorporated in several models working at various levels of de-
tail Grant et al., 1995; Grossman et al., 1995, and to test the capabilities of the models to predict the
performance of wheat crops in scenarios where atmo- spheric [CO
2
] is increased and conditions are warm Kartschall et al., 1995; Tubiello et al., 1999. In
this paper, the performance of three models, Sirius 99 Jamieson and Semenov, 2000, AFRCWHEAT2
Porter, 1993 and FASSET Jacobsen et al., 1998 are tested against data from 2 years of FACE experiments
where [CO
2
] and N fertiliser were varied. The mod- els incorporate different assumptions about the effect
of N shortage on canopy development and growth, and different levels of detail on processes within the
crop. The major characteristic that the three models have in common is that their major computational
time-step is 1 day, although some processes are cal- culated using a shorter-time step in AFRCWHEAT2.
The major objective of this paper is to test whether the assumptions incorporated into the three models are
supported by experimental evidence, and whether or not these models are suitable for assessing the impacts
of global atmospheric change on wheat production.
2. Model descriptions
The models used in this simulation study are des- cribed in detail elsewhere. Only brief descriptions of
major processes are given here, with an emphasis on the differences among the models. The descriptions
are given in the order of Sirius, AFRCWHEAT2 and FASSET.
Sirius 99 Jamieson and Semenov, 2000 is an updated version of Sirius Jamieson et al., 1998a
that uses a new description of N distribution and re- sponses. Shoot biomass accumulation is calculated as
the product of intercepted photosynthetically active radiation PAR and light use efficiency LUE, in-
cremented daily. The LUE is constant at 2.2 g MJ
−1
, except in extreme water stress, a condition that did
not occur in these experiments. Light interceptance is related to green area index GAI via Beer’s law,
with an extinction coefficient of 0.45 Jamieson et al., 1998a. Variations in N supply have no effect on the
LUE. CO
2
effects are simulated by increasing the LUE linearly so that at double current [CO
2
], LUE is increased by 30. Hence for an increase in [CO
2
] from 350 to 550 ppm, LUE is increased by 16 to
2.55 g MJ
−1
. Note that no biomass is assigned to roots, giving a lower LUE than otherwise. Any N in roots
is considered to be part of the unavailable soil pool. GAI is calculated at the canopy level as a function
of thermal time, modified by stress. In the new N description Sirius 99 uses, green area leaf laminae,
exposed leaf sheaths, exposed stem surface is as- sumed to require 1.5 g of N per m
2
. New increments of GAI can expand to their potential size only if
enough N is available. Non-leaf tissue enclosed leaf sheaths, interior of true stem, interior of thick leaves,
but excluding grain, collectively identified as “stem” is maintained above a minimum N concentration
[N] of 0.5, but may store excess N up to 1.5. If the minimum N requirement for a new increment of
“stem” cannot be met from storage or from the soil, then green area may be sacrificed to supply it. Crop
demand for N on any day is calculated from the need to expand GAI and grow “stem”.
Partitioning of biomass to grain is calculated assum- ing that all new biomass from the beginning of grain
growth is grain, and a pool of 25 of anthesis biomass is translocated to the grain at a constant rate in thermal
time, so that it is all transferred by the potential end of grain filling. Should grain growth be prematurely
curtailed, not all the pool is transferred. Grain demand for N is set at the beginning of grain fill so that 80
of the N in the plant at that time will be transferred to the grain, again by the potential end of grain filling
Jamieson and Semenov, 2000. The source of N for this is first, N in storage in “stem” some of which is
released into the “stem” by naturally senescing GAI, second, any mineral N available in the soil, and third,
from premature senescence of green area. This last effect can cause an early end to grain filling through
more rapid removal of GAI. Grain growth is assumed to end when GAI has reduced to zero.
In contrast,
daily biomass
accumulation in
AFRCWHEAT2 Porter, 1993 is calculated using a photosynthesis equation applied at hourly intervals
during daylight to unit GAI layers of the canopy, and summed over GAI layers and the day. This is the
one exception to the daily time-step, which is used to calculate everything else, although even in this
P.D. Jamieson et al. Agriculture, Ecosystems and Environment 82 2000 27–37 29
case simulation still uses daily weather data as input. The rate of photosynthesis depends on the light level
at each layer, calculated from Beer’s law using an extinction coefficient of 0.6 [CO
2
] and stomatal resis- tance calculated as a function of the vapour pressure
deficit VPD. Respiration losses are also calculated so that the biomass increment is the difference be-
tween what is gained and what is lost. This means that late in crop life total biomass may fall.
The calculation of GAI is also done in more de- tail than in Sirius. The current sizes of leaves on the
mainstem and tillers is calculated each day, and GAI is calculated by summing these per unit ground area.
Leaves have finite lifetimes, and expand and contract at constant rates in thermal time. Hence, changes in
GAI reflect the balance of births and deaths of both leaves and tillers. Drought reduces leaf extension rates
and lifetimes.
Accounting for N is done differently from Sirius. N demand is set assuming that there is a maximum
[N] on a whole plant basis, and this decreases from 4.5 at emergence to 1.5 at the beginning of grain
filling as a function of development stage. Minimum required [N] is set at 1.5 reducing to 0.3 in the
same fashion, and optimum [N] is assumed to be mid- way between the maximum and minimum levels. N
in excess of the optimum concentration is labile N in storage. If [N] is above optimum levels, then growth
processes are unlimited by N. Below this level, a set of stress factors based on the ratio of current to opti-
mum [N] is used to reduce tiller production rates, leaf expansion rates and lifetimes, and to reduce photosyn-
thesis. Thus, AFRCWHEAT2 treats N-stress in very much the same way as it treats water stress.
Both biomass and N demand per grain are constant in thermal time, so that grain growth and N uptake de-
pend on grain population and temperature. The sources of biomass for grain growth are current assimilate, and
a pool of carbohydrate that is initially calculated as 20 of the shoot biomass at anthesis, and increased by
a fraction of the biomass accumulated between anthe- sis and the beginning of grain filling. Grain growth is
unrestricted by assimilate provided this latter exceeds demand, in which case excess assimilate is added to
the carbohydrate pool. The existence of the pool means grain growth can continue after the canopy dies.
FASSET is a whole farm simulation model Jacobsen et al., 1998, which includes a soil–plant–
atmosphere model that simulates N turn-over and crop production as affected by availability of water and N
Olesen et al., 1996. The soil model has a one-dimen- sional vertical structure and simulates the daily
movement and plant availability of N. The down- ward movement of water and N in the soil profile
is simulated using the concepts of the SLIM model Addiscott and Whitmore, 1991 that is also incor-
porated into AFRCWHEAT2. Soil temperature is simulated using an extended heat flow equation. The
turn-over of soil organic matter in the soil is simu- lated using the structure of the DAISY model Hansen
et al., 1991. A simplified version of the DAISY model is used to simulate crop water and N uptake.
The crop model has three biomass pools: shoot biomass, below ground biomass and biomass in grains.
The daily increase in biomass is proportional to the intercepted radiation. The LUE is reduced by drought,
by low temperatures and by low N status in the crop according to the function by Sinclair and Horie 1989.
The N status is the current N uptake scaled between a minimum and a maximum N uptake, which depend on
both crop biomass and GAI. The crop GAI is assumed to increase up to the date of anthesis as a function of
vegetative biomass and the nitrogen status. This func- tion was estimated using data from growth analysis
experiments in winter wheat in Denmark. The senes- cence of the GAI follows crop development, but is
hastened by drought stress. The extinction coefficient for light interception is 0.44. Crop phenology follows
the Sirius approach with modifications as described by Olesen et al. 1999. Partitioning to grain is similar
to Sirius.
3. Experimental
