Update Grazing Logic (cherry-picked mirror of NorESMHub 55)

[rgknox]

Description:

This updates grazing logic. This is a mirror of @rosiealice 's updates submitted to NorESMhub:

NorESMhub#55

Collaborators:

Expectation of Answer Changes:

Checklist

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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
• FATES-CLM6 Code Freeze: satellite phenology regression tests are b4b

If satellite phenology regressions are not b4b, please hold merge and notify the FATES development team.

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:

FATES baseline hash-tag:

Test Output:

[ckoven]

Looks good, see comment. Thanks @rgknox, @rosiealice, and @JessicaNeedham for doing this.

[rosiealice]

Actually @glemieux @ckoven, @JessicaNeedham and I discussed this and I think we should keep the code working the same way as it is working in the first instance, prior to 47c0a8f
The reason is that any change at this point will require a re-tuning of the grazing rates, and I don't think the logic is more or less right/wrong either way. It makes sense that for all LUC types grazing doesn't happen if NPP is negative, and we are adding an additional case for crops where we don't allow them to be grazed to the ground. The only issue with this is that I have not yt figured out why crops need this change to avoid extinction and rangelands and pastures do not. I think it is to do with them being a distinct PFT and possibly having seed bank differences. But I never properly got to the bottom of it. But 1) they are much better in this implementation and 2) the rangelands and pastures were more productive than we would like if they were also subject to the LAI threshold. So i think in retrospect we should leave it like this until we have a chance to do further investigation.

[glemieux]

Per the software discussion today, @rosiealice confirmed we should revert back to the original behavior in (as of 783eae3). @ckoven confirmed he's good with moving forward with that decision. @glemieux to update the paranthesis to reflect this decision.

[glemieux]

Regression testing on derecho is underway

[glemieux]

Regression testing against fates-sci.1.92.0_api.44.1.0-ctsm5.4.032 is complete and B4B for all expected tests.

Results: /glade/u/home/glemieux/scratch/ctsm-tests/tests_0414-105100de

UPDATE (4/20/2026): I noticed weird run failures for FatesColdHydro. I'm going to double check this.

[glemieux]

Regression testing against fates-sci.1.92.0_api.44.1.0-ctsm5.4.032 is complete and B4B for all expected tests.

Results: /glade/u/home/glemieux/scratch/ctsm-tests/tests_0414-105100de

UPDATE (4/20/2026): I noticed weird run failures for FatesColdHydro. I'm going to double check this.

Upon re-review and retesting against new baseline fates-sci.1.92.1_api.44.1.0-ctsm5.4.033 all expected runs are B4B. The unexpected runs from last test were due to a glitch in the testing not flagging expected failures appropriately.

This is good to integrate.

Results: /glade/u/home/glemieux/scratch/ctsm-tests/tests_0420-095914de