Environmental Engineering Reference
In-Depth Information
The tree component does not use fixed partitioning tables for timber trees because
this would prevent the modelling of the impact of branch or root pruning on the
tree growth. The tree component includes a dynamic module of C allocation for
timber trees, governed by two types of rule:
-
Teleonomic allocation rules based on allometric equations define the relative
sizes of above-ground sub-compartments, e.g. the relationship between Diameter
at Breast Height (DBH) and tree height. Such allometric relationships capture
internal constraints not explicitly dealt with in the model (e.g. architectural
model and structural stability constraints limiting amount of leaf biomass for a
given amount of wood biomass) in relation to the tree dimensions.
An optimal allocation assumption ('functional balance') between above- and
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below-ground biomass mediated through stress indices, which assume that a
plant allocates its biomass so as to maximise its growth rate under the given
environmental conditions. This approach has been extended to the ratio between
coarse and fine roots, with a dynamic allocation procedure that avoids the need
for fixed partitioning tables.
WaterUptake: Plant Water Uptake
The WaterUptake component was developed by INRA. It allows daily plant water
extraction to be estimated from each of the soil layers and for each plant species
(one or two possible) in the field from the plant water demand, the soil description,
and the root distribution in the soil. Roots are assumed to be homogeneously distrib-
uted horizontally. Two types of model have been implemented. The simple model for
cases with up to two plant species, assumes that the water demand is met as long as
water is available in the soil layers containing roots (water content above wilting point).
The second model is the more complex one used in the His-AFe model for mixed
vegetation. This model estimates the amount of water that each plant can extract from
the soil by integrating the matric flux potential for each plant in each rooted layer.
Soil Components
SoilCarbonNitrogen: Soil Carbon and Nitrogen Dynamics
The SoilCarbonNitrogen component was developed by CIRAD and INRA.
The models implemented describe N mineralization-immobilization turnover and
the interactions between C and N dynamics in decomposing plant residues and soil
organic matter (SOM). It includes above- and below-ground plant residue pools and
three soil organic matter pools (microbial biomass, young and old SOM) with different
turnover times (Fig.
4.4
). Rates of decomposition are modified by temperature,
moisture, lignin content of the residues and N availability. Stabilization of SOM is
simulated by transferring fractions of decomposed microbial biomass and young
