The model can simulate a wide variety of crops and grasslands by altering a number of crop-specific parameters (see the crop parameters ). CENTURY is not designed to be a plant production model and some parameters may have to be calibrated for specific environments.
The plant production model ( Figure 3-7 ) has pools for live shoots and roots, and standing dead plant material. Potential production (g C / m2 / month) is a function of a genetic maximum defined for each crop ( crop parameter PRDX(1) ) and scalars with values ranging from 0-1, reflecting the effects of soil temperature, moisture status, shading by canopy and dead vegetation, and seedling growth.
The maximum potential production of a crop, unlimited by temperature, moisture or nutrient stresses, is primarily determined by the level of photosynthetically active radiation, the maximum net assimilation rate of photosynthesis, the efficiency of conversion of carbohydrate into plant constituents, and the maintenance respiration rate ( van Heemst, 1986 ). Thus, the parameter for maximum potential production ( PRDX(1) ) has both genetic and environmental components. However, in CENTURY, the seasonal distribution of production is primarily controlled by the temperature response function rather than the seasonal variation in photosynthetically active radiation, so the maximum potential production parameter should reflect aboveground crop production in optimal summer conditions. This parameter will frequently be used to calibrate the predicted crop production for different environments, species, and varieties. In the CENTURY model formulation the potential production is based on aboveground production, therefore root-shoot allocation must also be taken into account. The value used should be set according to estimates of potential crop production. In general, C4 species have higher potential growth rates than C3 species because of higher maximum net assimilation rates ( van Heemst, 1986 ). The range of potential production from less than 200 to 580 kg DM / ha / day corresponds to 150 to 700 g C / m2 / month.
The growth of most plant species exhibits a response curve to root temperature which is sigmoidal up to an optimum temperature, has a band of optimum temperatures over which there is relatively little effect on growth, and a rapid decline above the optimum ( Cooper, 1973 ). Plant growth rates will depend on the combined temperature response of photosynthesis and respiration. For most temperate species the lower limit at which the rate of development is perceptible is between zero and 5 C. Development increases in rate up to an optimum of 20 to 25 C and then declines to an upper limiting temperature between 30 and 35 C. For tropical species the base, optimum and maximum temperatures are approximately 10 higher ( Monteith, 1981 ). In the CENTURY model the temperature response curve can be parameterized for each crop using a generalized Poisson density function ( PPDF(1...4) ) as shown in Figure 3-8a and 3-8b .
The moisture status effect reduces growth when
The slope of the linear relationship is dependent on the available soil water holding capacity, which varies with soil texture ( Figure 3-9 ). This effect of soil texture has been observed in field data ( Sala et al., 1988 ) and accounts for the "reverse texture effect" ( Noy-Meir, 1973 ), in which the greater infiltration rate and hence lower bare soil evaporation rate in coarser textured soils results in higher production in arid environments. Site parameter NLAYPG is the number of soil layers that control plant growth and can be less than or equal to the total number of soil layers.
This moisture ratio above is used in the calculation of potential production. The effect of soil water upon potential production is parameterized by fixed parameters PPRPTS(*) in the following equation:
Moisture Ratio - PPRPTS(3) Production = 1 +
PPRPTS(3) - PPRPTS(1) - PPRPTS(2) * Soil Water
Soil water is calculated as the amount of plant-extractable water over the rooting depth of the soil. The usual values of PPRPTS(*) are 0, 1.0, and 0.8.
The shading effect on potential growth rate is a response surface dependent on the amounts of live and dead vegetation. This function, which was originally developed for the tall grass prairie, was found to be too restrictive for no-till cropping systems. Therefore, the magnitude of the effect has been greatly reduced for crops by increasing the value of parameter BIOK5.
A scaling factor for crops growing from seedlings ( PLTMRF , FULCAN ) reflects the partial interception of light with less than a full canopy present ( Figure 3-10 ). This factor takes effect after a PLTM (planting month) event, but not after a FRST (first month of growth) event.
Root growth is proportional to potential shoot growth, but the allocation of carbon to root growth can be made a function of time since planting ( FRTC(1...3) ) ( Figure 3-11 ) to reflect the dominance of root growth in seedling cereal crops or the initial dominance of shoot growth in root crops. To account for winter dormancy the root - shoot ratio does not change in months when soil temperature is below 2 C ( RTDTMP ). In an alternative formulation (FRTC(1) = 0.0) developed for Great Plains grasslands, the root-shoot ratio is controlled by annual precipitation ( Parton et al., 1987 ) as shown in Figure 3-12 .
The actual production is limited to that achievable with the currently available nutrient supply with plant nutrient concentrations constrained between upper and lower limits set separately for shoots and roots. Invoking Liebig's Law of the Minimum, the most limiting nutrient (ELIMIT) constrains production. The limits of nutrient content for shoot growth are a function of plant biomass in order to reflect the changing nutrient content with plant age ( Figure 3-13). The user specifies the effect of live shoot biomass on maximum and minimum nutrient content (BIOMAX, PRAMN(*,*), PRAMX(*,*)). For instance, for the aboveground biomass, the C:E range is calculated as:
C:E minimum, maximum =
PRAMN(E,0) + (range of PRAMN) * 2.5 * AGLIVC / BIOMAX
The conversion factor is equal to 2.5 for above and below ground biomass.
This formulation does cause some anomalies when growth is limited by nutrients, as a nutrient limited crop can have a higher nutrient concentration than an unlimited crop of the same age with greater biomass. The limits on nutrient content of roots are a function of annual precipitation ( PRBMN(*,*) , PRBMX(*,*) ). CENTURY also incorporates a function to restrict nutrient availability in relation to root biomass ( Figure 3-14 ). For legume crops the potential rate of symbiotic nitrogen fixation is specified in terms of grams N fixed per gram C fixed ( SNFXMX(1) ). It is assumed that plant available soil N will be preferentially used by the crop. All other potential limitations to growth, including P and S supply, are taken into account before calculating symbiotic N2 fixation.
Fertilizer addition is specifed as an amount (fertilization parameter FERAMT ) per fertilization event.
At harvest, grain is removed from the system and live shoots can either be removed or transferred to standing dead and surface residue. For grain crops a harvest index is calculated based on a genetic maximum ( HIMAX ) and moisture stress ( HIWSF ) in the months corresponding to anthesis and grain fill ( HIMON(1,2) ) as shown in Figure 3-15 . Moisture stress is calculated as the ratio of actual to potential transpiration in these months. The fractions of aboveground N, P, and S partitioned to the grain are crop-specific constants ( EFRGRN(*) ) modified by the square root of the moisture stress term, resulting in higher grain nutrient concentrations when moisture stress reduces the harvest index. At harvest a proportion of the aboveground nitrogen is lost to volatilization ( VLOSSP ). The crop harvest routine also allows for the harvest of roots, hay crops or straw removal after a grain crop (see the harvest parameters ). The crop may be killed at harvest, as for cereal grain crops, or a fraction of roots and shoots may be unaffected by harvest operations and growth may continue.
The crop model allows for the death of shoots and roots during the growing season. Shoot and root death are functions of available soil water in the whole profile and the plant root zone respectively ( Figure 3-16 ). Both are multiplied by crop specific maximum death rates ( FSDETH(1) , RDR ). Shoot death rates may be further increased ( FSDETH(3) ) due to shading if the live biomass is greater than a critical level ( FSDETH(4) ). Root death is only allowed to occur when roots are physiologically active, defined by soil temperature being greater than 2 C ( RTDTMP ). In months nominated as senescence months the shoot death rate is set to a fixed fraction of live biomass ( FSDETH(2) ). Standing dead material is transferred to surface litter at a crop specific relative fall rate ( FALLRT ).
Plant lignin contents ( FLIGNI(*,*) ) are specified for shoots and roots, and may be constants or a linear function of annual precipitation ( Parton et al., 1992 ). They should reflect the lignin content of senescent plant material.
The effects of grazing and fire on plant production are represented in the model
by using data from
Holland et al. (1992)
and
Ojima et al. (1990).
The major impact of fire is to increase the root to shoot ratio
(FRTSH,
fire parameters),
increase the C:N ratio of live shoots and roots (fire parameter
FNUE(*)),
remove vegetation and return nutrients during the years when fire occurs
(Ojima et al.,1990
). Grazing removes vegetation, returns nutrients to the soil, alters the root
to shoot ratio, and increases the N content of live shoots and roots (
Holland et al., 1992).
The model has three options (grazing parameter
GRZEFF
= 0, 1, 2) for dealing with the impact of grazing on the system. For option 1
(GRZEFF = 0)
there are no direct impacts of grazing on plant production except for the
removal of vegetation and return of nutrients by the animals. Option 2
(GRZEFF = 1)
is referred to as the lightly grazed effect
(Holland et al., 1992)
and includes a constant root:shoot ratio (not changing with grazing) and a
linear decrease in potential plant production with increasing grazing intensity.
Option 3 (GRZEFF = 2)
is referred to as the heavy grazed
(Holland et al., 1992)
option and includes a complex grazing optimization curve where aboveground
plant production is increased for moderate grazing and decreasing sharply for
heavy grazing levels (greater than 40% removed per month). The root:shoot ratio
is constant for low to moderate grazing levels and decreases rapidly for heavy
grazing levels. In all three options the nutrient content of new shoot will increase
in relation to the residual biomass
(BIOMAX,
PRAMN(*,*),
PRAMX(*,*)).
Leaf area index is calculated from leaf biomass and a biomass-to-LAI conversion factor. This factor is the average amount of biomass needed to have an LAI equal to 1.0. Century uses 80.0 g biomass for this factor.
Plant Production Submodels: Overview
Forest Submodel
Savanna Submodel