Plant Growth
When nitrogen and carbon flows are not simulated, the plant exists only as a driving force for heat and water dynamics. In this case the plant can have shape characteristics like height, leaf area index and root depth that are used to estimate transpiration. These characteristics can be given in a table or be read from a file. The resulting plant is therefore only “virtual” and does not consist of any biomass. Simulating carbon and nitrogen flows together with vegetation means that the plant will have a real biomass (i.e. storages of carbon and nitrogen in the plant) that will increase when the plant grows. The shape characteristics of the plant are simulated from this biomass. These simulated values are always used in the biotic section of the model, whereas in the abiotic section the use of simulated values is optional. Hence, it is possible to have for example one leaf area index generated from parameters that determines transpiration and another simulated leaf area index that determines photosynthesis (growth).
Growth and plant development are simulated if the switch Growth” is set to any of three alternative options for plant growth (i.e. this switch must not be turned “off”). Subsequently there are three different basic approaches to calculate the plant growth (leaf assimilation) in the CoupModel. The simplest approach is to assume that the plant growth and the nitrogen uptake are described by a logistic growth function (see Logistic growth approach). This means that the potential growth is a function of time (in terms of day-number) and not a function of weather. Another approach estimates the growth from a water use efficiency parameter and from the simulated transpiration (see Water use efficiency approach). Alternatively, light use efficiency can be used to estimate potential growth rate, limited by unfavourable temperature, water and nitrogen conditions (see Light use efficiency approach). A biochemical model after Farquhar et al. (1980) can be used if hourly values of photosynthesis and transpiration is of interest (see Farquhar approach).
This section also describes how the assimilated carbon is allocated to different parts of the plant; see Allocation to different parts of the plant. The carbon uptake gives rise to an uptake demand of nitrogen in the soil, see XE "root uptake demand" Root uptake demand”, and the plant also loses some carbon to the atmosphere by respiration, see Respiration.
Allocation to different parts of the
plant