Update GetNutrientTargetCNP for Deciduous PFT

[sharma-bharat]

Title: Preventing Declines in Carbon Flux During Leaf-Off Period in Deciduous PFTs

Description:

During the leaf-off period in deciduous plant functional types (PFTs), the target leaf biomass was being reset to zero which meant the target fine root biomass also goes to zero that casuses fine-root mass to decline. This led to a decline in carbon fluxes over time (Fig. 1).

Solution:
We modified the elongf_leaf, elongf_fnrt, and elongf_stem parameters to a value of 1.0 in the parteh/PRTAllometricCNPmod.F90 module.

Key Changes:

call bleaf(dbh,ipft,crown_damage,canopy_trim, 1.0_r8, leaf_c_target)
call bfineroot(dbh,ipft,canopy_trim,l2fr, 1.0_r8, fnrt_c_target)
call bsap_allom(dbh,ipft,crown_damage,canopy_trim, 1.0_r8, sapw_area,sapw_c_target)
call bagw_allom(dbh,ipft,crown_damage, 1.0_r8, agw_c_target)
call bbgw_allom(dbh,ipft, 1.0_r8, bgw_c_target)

This modification stabilizes carbon fluxes (Fig. 2).

Figure 1: Carbon fluxes using default ELM-FATES-CNP under transient run

Figure 2: Carbon fluxes with modification described in this PR under transient run

Please note: Sudden decline in Fig. 1 and 2 towards the end is due to logging event.

Collaborators:

Anthony Walker, ORNL (@walkeranthonyp)

Expectation of Answer Changes:

This update is expected to impact model outputs related to seasonal carbon fluxes and biomass allocation in deciduous PFTs.

Checklist

If this is your first time contributing, please read the CONTRIBUTING document.

All checklist items must be checked to enable merging this pull request:

Contributor

• The in-code documentation has been updated with descriptive comments
• The documentation has been assessed to determine if updates are necessary

Integrator

• FATES PASS/FAIL regression tests were run
• Evaluation of test results for answer changes was performed and results provided

Documentation

• Technical Note update:
• User's Guide update:

Test Results:

CTSM (or) E3SM (specify which) test hash-tag:

CTSM (or) E3SM (specify which) baseline hash-tag: v2.1.0-13087-ge9afb7cb13

FATES baseline hash-tag: sci.1.68.2_api.31.0.0

Test Output:

[mpaiao]

Thanks for the pull request @sharma-bharat! I went through it and it looks good if the intent is to use the target biomass independently on the phenological state (I have not used the CNP code).

My only really minor suggestion is to remove the commented chunk of code.

[mpaiao]

Thanks for spotting this bug @sharma-bharat! If the goal is to be independent on phenological state, then your changes are correct. My only minor suggestion is to actually delete the commented out lines because they are incorrect.

[sharma-bharat]

Thanks Marcus. Yes that is our goal! I have removed the commented code.

[rgknox]

@sharma-bharat, these changes make sense. We don't want to make the nutrient storage target a proportional to leaf flushing status.

[sharma-bharat]

This is great news! Thanks!