Opened 2 years ago

Closed 2 years ago

#654 closed defect (fixed)

Trade-off between water stress and allocation

Reported by: luyssaert Owned by: kim.naudts
Priority: critical Milestone: IPSLCM6.v2
Component: Biogeochemical processes Version:
Keywords: Cc:


By setting k_root to a similar value as Bonan, the hydraulic architecture seems to work fine but the root mass is too low (~10 gC m-2). Either the value og k_root needs to be changed or the change in k_root should be compensated by tuning another parameter in the equations below.

sapwood_density = deux * pipe_density(pft) / kilo_to_unit
calculate_c0_alloc = sqrt(k_root(pft)/k_sap(pft) * tau_eff_sap/tau_eff_root * sapwood_density)

A test case that shows the issue with root biomass can be found at obelix
/home/users/sluys/ORCHIDEE.CN.CAN/config/ORCHIDEE_OL/FG2.PARAM.n in this test case the values for k_root have already been changed.

Change History (4)

comment:1 Changed 2 years ago by luyssaert

  • Owner changed from somebody to kim.naudts
  • Status changed from new to assigned

comment:2 Changed 2 years ago by luyssaert

From Tyree 1998 (Oecologia): Root conductance (Kr) can be defined as water flow rate (kg s-1) per unit pressure drop (MPa) driving flow through the entire root system. The unit through the whole root system would then be kg H2O s-1 MPa-1.

Values of Kr could be scaled by dividing by some measure of root size (root surface area, total root length, or mass) or by dividing Kr by leaf surface area. Division by root surface area (Ar) is justified by an analysis of axial versus radial resistances to water flow in roots. In the radial pathway, water flows from the root surface to the xylem vessels through nonvascular tissue. In the axial pathway, water flow is predominately through vessels. The resistance of the radial path is usually more than that of the axial path (Frensch and Steudle 1989; North et al. 1992). Most water uptake is presumed to occur in fine roots (<2 mm diameter) and fine-root surface area is usually >90% of the total root surface area (personal observation). So root uptake of water would appear to be limited by root surface area and hence it is reasonable to divide Kr by Ar yielding a measure of root efficiency.

Some roots are more efficient than others. Division of Kr by total root length (L) is not as desirable, but is justified because Ar and L are correlated approximately and L can be estimated by a low cost, line intersection technique rather than a high-cost, image analysis technique.

Scaling by root mass is justified by consideration of the cost of resource allocation. Plants must invest a lot of carbon into roots to grow and maintain them. The benefit derived from this carbon investment is enhanced scavenging for water and mineral nutrient resources. Total root dry weight (TRDW) is a measure of carbon investment into roots. Thus the carbon efficiency of roots might be measured in terms of Kr/TRDW, Ar/TRDW, or L/TRDW. Scaling by TRDW provides information of ecological rather than physiological importance.

The current value of c0_alloc is about 30 times too small which -given the sqrt- relates to a factor of 1000 for either k_root, k_sap, tau_sap, tau_root or density. A factor of 1000 hints at a problem with the units, e.g. kg that should have been g. Triple check the units of the different literature sources. Previously we only used k_root and the value contained the root and soil conductance. Now we calculate k_root and k_soil_to_root separately. k_root is much larger. The plant will allocate enough C to the roots to allocate k_root and k_soil_to_root. The sum of both should be used in allocation.

Ideally k_root (see below) and a value for k_soil_to_root are used to calculate k_belowground which is used in the allocation. Problem is that we need the root biomass to calculate k_soil_to_root and that we need k_soil_to_root to calculate the root biomass. Now allocation starts from Cs, it could be written to start from Cr but that is not an easy task. The benefits would be full consistency between allocation and plant water stress and a dynamic root allocation (as has been observed). The latter has been tried in 2014 but it turned out to be much more difficult than expected. This development needs to be thought thought more carefully. In addition ::k in hydrology is the effective soil conductivity. To use that value we would have reconsider the hydraulic architecture. The solution for the moment is to couple allocation and hydraulic architecture by defining two related parameters independently.

Last edited 2 years ago by luyssaert (previous) (diff)

comment:3 Changed 2 years ago by luyssaert

Since r6474 allocation makes use of k_belowground. Hydraulic architectures makes use of k_root and a k_soil_to_root.

Last edited 2 years ago by luyssaert (previous) (diff)

comment:4 Changed 2 years ago by luyssaert

  • Resolution set to fixed
  • Status changed from assigned to closed
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