Evaluating the impact of biomass limiting factor on root growth in ArchiSimple

In order to make a cereal root generated by ArchiSimple match the biomass of that one generated by FSPM-GroIMP, the biomass limiting factor function in ArchiSimple 9.1 was evaluated. The file that contains the maximum root biomass value allowed for growth in ArchiSimple is called biomrac.txt and it is given in grams at each time step, daily in this case. Figure 1 shows the evolution of a cereal root dry biomass generated by FSPM-GroIMP for a period of 94 days with the Beta factor and without it (no limitation).

The driven data for plant growth, i.e., the potential evapotranspiration (m.s-1) was set constant in 2.0×e-8 m.s-1 in order to speed up the performance of Min3P – ArchiSimple and reduce CPU time but without impacting the final result. Potential evapotranspiration only impacts root water uptake calculated from Min3P, which is not being evaluated in this simulation. This simulation only evaluates the impact of the variable biomrac on dry root biomass generated by ArchiSimple.

Figure 1. Cereal root dry biomass generated by FSPM-GroIMP for a period of 94 days with the Beta factor and without it (no limitation).

The impact of the Beta function on dry root biomass generated by FSPM-GroIMP is in the order of -50% at the end of the simulation on day 94. In reality, Figure 1 shows a delay in root growth, i.e., while the root system starts growing substantially around day 20 without water limitation being considered, the same is only observed 30 days later when water scarcity in the soil is taken into account.

Both values of dry root biomass were from FSPM-GroIMP were used to limit plant growth in ArchiSimple through the biomrac variable, and the results are shown in Figure 2. In these simulation the soil variables in ArchiSimple were set to: Croiss = 6.0; Ramif = 1.0; iCMeca = 0.07; and Orientation = 1.

Figure 2. Simulation of cereal root growth for 94 days without Beta function and with Beta function. The Beta function is exclusively implemented by the dry biomass limitation through biomrac.txt file.

The Beta function presents little or no impact on root system growth with depth. However, the Beta function impacts root surface density and the ‘spread’ of the root system throughout the horizontal layers of the soil.

Figure 3 shows the integrated root surface density (m-2.m-3) of a cereal root for 94 days without Beta function and with Beta function. It is possible to notice that the impact of the Beta function of root surface density is in the order of -70%.

Figure 3. Integrated root surface density (m-2.m-3) of a cereal root for 94 days generated by ArchiSimple without Beta function and with Beta function controlled by the biomrac variable.

This sensitivity analysis indicates that for this set of variables used in FSPM-GroIMP without soil water limitation, the root biomass generated is larger than the one generated by ArchiSimple and, therefore, the Croiss variable had to be set much higher than 1.0 in order to generate a larger root system. As well as the root system was set to grow to a larger size, the soil mechanic constraint had to be set to a higher value as well, once the cereal root was growing too deep (deeper than 2 m) and giving unrealistic biomass vertical profiles.

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Evaluating the impact of soil parameters on root biomass in ArchiSimple

Models of root system architecture are often used for studying plant-soil interactions. These models are required to simulate the structural and spatial distribution of the root system, the integration of root level processes (e.g., elongation, branching) with soil properties, and root interaction with the airborne part of a plant. Only a few of them, however, have been integrated into larger crop or ecosystem models because they are too difficult to parameterise and they require large amounts of computational power.

ArchiSimple has been designed to enable the representation of the architectural diversity coming from various plant species interacting with environmental conditions (Pagès et al., 2014). It is a dynamic architectural and functional model, in which the root system is represented as a set of small segments and meristems. The root system is modified by functions describing their development, including emission of new roots from the shoot, elongation of existing roots, acropetal branching, radial growth, and self-pruning following root decay. Model parameters can be estimated independently from observations or from the literature. Calibration and evaluation for 6 species (Musa spp., Pisum sativum, Prunus persica, Teucrium botrys, Thlaspi perfoliatum, and Zea mays) are described in Pagès et al. (2014).

In order to make a cereal root generated by ArchiSimple match the biomass of that one generated by FSPM-GroIMP, a group of variables related to the soil, as described in ArchiSimple, were evaluated in a sensitivity analysis.

The file that contains the description of soil properties within ArchiSimple has a group of 4 variables: Croiss (favours growth); Ramif (favours ramification); iCMeca (intensity of constraint); and Orientation (0 – iso; 1 – vertical).

Croiss (favours growth)

Croiss = variable; Ramif = 1.0; iCMeca = 0.00; Orientation  =  0 (iso).

Figure 1a) Effect of variable Croiss on cereal root system in the 60th day of development.

Figure 1b) Dry root biomass (g) along 60 days with Croiss varying from 0.1 to 1.0 in intervals of 0.1.

Ramif (favours ramification)

Croiss = 0.5; Ramif = variable; iCMeca = 0.00; Orientation  =  0 (iso).

Figure 2a) Effect of variable Ramif on cereal root system in the 60th day of development.

Figure 2b) Dry root biomass (g) along 60 days with Ramif varying from 0.1 to 1.0 in intervals of 0.1.

iCMeca (intensity of constraint)

Croiss = 1.0; Ramif = 1.0; iCMeca = variable; Orientation  =  0 (iso).

Figure 3a) Effect of variable iCMeca on cereal root system in the 60th day of development.

Figure 3b) Dry root biomass (g) along 60 days with iCMeca varying from 0.01 to 0.10 in intervals of 0.01.

Orientation  =  0 (iso) and 1 (vertical)

Croiss = 0.5; Ramif = 1.0; iCMeca = 0.00; Orientation  =  variable.

Figure 4a) Effect of variable Orientation on cereal root system in the 60th day of development.

Figure 4b) Dry root biomass (g) along 60 days with Orientation varying between 0 (isotropic growth) and 1 (vertical growth).

References

Pagès, L., Bécel, C., Boukcim, H., Moreau, D., Nguyen, C., & Voisin, A. S. (2014). Calibration and evaluation of ArchiSimple, a simple model of root system architecture. Ecological Modelling. https://doi.org/10.1016/j.ecolmodel.2013.11.014