Plant lifecycle
There are several functions that govern the lifecycle of
plants. The total life span is determined by age of the plant and the maximum
plant age. A distinction between the growing season and the dormancy period
affects leafing and litter fall and finally plant growth stages during the
growing season affect allocation patterns. All those functions are represented
in Figure 5.2.
Figure 5.2. Lifecycle of plants on the northern hemisphere (annual and perennial). Growth stage = green, Growing season / dormancy = blue, plant maximum age and old/new biomass allocation = no colour.
A plant can either exist from the beginning of the simulation, or it can be sown during the course of the year. As the simulation proceeds the plant age is counted for every existing plant. If the plant existed from the beginning of the simulation, the initial plant age is given in the table Initial Conditions of plants, and the age is increased from that value and onwards as time goes by in the simulation. All plants celebrate their birthdays on day 1 i.e. New Years Day (or day 180 for the southern hemisphere) irrespective of whether they were sown the same year or not. When the plant age equals the maximum plant lifetime given in table Plant Behaviour, the plant dies. For annual plants it is therefore advisable to choose a maximum plant lifetime of 1. At the year shift (or day 180 for the southern hemisphere) the new biomass from the previous year is transformed into old biomass.
Some perennial plants go into dormancy during the winter. Deciduous plants prepare for the dormancy by loosing their leaves. When litter is formed the plant withdraws nutrients from the dying parts and store them in their remaining tissues. When the growing season starts the stored nitrogen and carbon can be used to build up new leaves during leafing. A dormancy period can optionally be simulated (see switch Winter regulation). The dormancy period begins when the air temperature is less than –5 °C for three consecutive days. Similarly, the growing season starts when the difference between the air temperature and the threshold temperature for emergence exceeds 0°C for three consecutive days.
A growth stage is an indicator of where in the lifecycle the plant is at present, and the allocation patterns for carbon and nitrogen is highly dependent on this. The growth stages in the model are labelled 0-4 and are listed in the table below. Each growth stage represents a different allocation pattern.
Each simulated plant must have an initial growth stage, which is given in the table Plant Behaviour. Annual crops will normally start at a growth stage index (GSI) between –1 to 1 whereas perennial plants such as trees often start at a GSI of 1 i.e. the plant has already emerged. By the passing of time the plant moves from growth stage to growth stage until the plant maximum GSI is reached (see Plant Behaviour). A maximum GSI of 2 means that grain will not be developed whereas a maximum GSI of 4, results in grain development. When the plant maximum GSI is reached, the plant retunes to the plant minimum GSI. Typical minimum and maximum GSI values for crops are –1 and 4, and for trees 1 and 2 respectively, which means that the GSI will vary for crops but will be constant for trees.
A value of –1 means that no plant exists. Sowing takes place when GSI=0 and the start of growth or emergence occurs when GSI=1. Sowing or emergence day number (if the plant starts at a GSI of –1 or 0) is given for each plant in the parameter tables Start of growth. If 0 is given as day number, the day number will be calculated from temperature sums, whereas values between 1-365 will be interpreted as a fixed date.
The temperature sums as degree-days are calculated by adding the temperature excess over the threshold values. These sums are used for estimating most of the different plant development stages.
The plant is in the leafing phase between GSI 1 and 2. During this period carbon and nitrogen in perennial plants is allocated from the mobile pool to the leaves as an additional source. The mobile pool contains carbon and nitrogen that was retained when the plant lost biomass as litter fall the year before.
Most plants develop grain in order to reproduce themselves. Grains are normally of outmost importance for agricultural crops, but are often not of interest when looking at trees in forest ecosystems, even though these species also develop fruits. Therefore the inclusion of grain development is optional.
The start of grain filling, Gi, is calculated as a function of day-length and temperature:
where gstepday, gthresday, gsteptemp and gthrestemp, are parameters, D is day length and Ta is the air temperature.
The function for the grain filling start, Gi, is multiplied by a parameter gstep, to calculate GSI. The grain filling starts when GSI has reached the value of 2. When GSI has reached 3 the grain filling is finished and the grains will mature before they are ready to be harvested.
For plants with a maximum GSI of 4, harvest occurs when the grain has matured (i.e. when GSI = 4) or at a specified harvest day number (see Harvest of plants). Again temperature sums will be used to estimate the harvest day number if the harvest day number is given as 0. If the maximum GSI is less than 4 a harvest day number can still be specified at which harvest will take place, which means that leaves, stems and roots are harvested at that date.
After harvest the GSI for all grain crops (i.e. plants with a maximum GSI of 3 or more) will be put to the minimum GSI specified for the plant.
A plant dies at the year shift the year when the plant age exceeds the maximum plant lifetime, given in the parameter table Plant Behaviour, or after ploughing. When the plant dies the GSI is automatically put to the minimum plant GSI. For plants with a maximum leaf lifetime of 1 year i.e. deciduous plants, specified in the parameter tables Plant Behaviour, GSI is also returned to the minimum plant GSI at the year shift.