Add grazing of storage organ

[JessicaNeedham]

Description:

Add grazing of storage to allow grasses with no leaves to die of carbon starvation mortality.

Collaborators:

@rosiealice @kjetilaas

Expectation of Answer Changes:

This would be answer changing with land use on.

Checklist:

• My change requires a change to the documentation.
• I have updated the in-code documentation .AND. (the technical note .OR. the wiki) accordingly.
• I have read the CONTRIBUTING document.
• FATES PASS/FAIL regression tests were run
• If answers were expected to change, evaluation was performed and provided

Test Results:

@mvdebolskiy

Attempted to run with this change using a post-AD spinup finidat but got a mass balance error on the first time step. Case on betzy here /cluster/work/users/jessica/ncsrevise_runs/noresm-fates-1850-1900-grazing-storage.2026-05-21

Possibly because grazing was happening during the spin up and this can't just be changed mid-run. Or we need additional changes. I have not investigated.

[rosiealice]

I am not sure we actually want to do this just for the sake of fixing this bug right now. It is maybe the correct ecological interpretation of what is wrong, but we will need to retune the grazing intensities if we add this change.

[mvdebolskiy]

@rosiealice @JessicaNeedham fixed the balance error and history output. storage is grazed with proportion leaf_c/total_c. Can anyone test this? scientifically.

[JessicaNeedham]

I can test this. Is the correct set up for the test:

• from 1850
• without the other h2o grazing change (H20 fix for historical. #61)
• transient land use

[JessicaNeedham]

Testing now with the grazing termination update, and the updated land use file with small transitions removed. Running from 1850 with transient CO2, climate, and land use.
Case is here: /cluster/work/projects/nn9560k/jessica/noresm_runs/noresm-fates-f19-LU-PPE-1850-1900-storage-grazing4.2026-06-02

[rosiealice]

My tests seem to show that in 2000 there is quite a big impact of this change:

In 1850 this is not nearly so pronounced, but that's because there's not much grazing then.

The impact of the y2000 change is a large +ve bias in LAI,

so I don't think we should bring this in as it is without a parameter change. I will do some more tests and report back.

[JessicaNeedham]

I also did a comparison of a f45 steady state 2000s run with and without storage grazing. Results here: https://ns9560k.web.sigma2.no/datalake/diagnostics/noresm/jessica/noresm-fates-f45-2000_storage_grazing.2026-06-05/OBS_comparison/ANN/

[JessicaNeedham]

This includes the update to the grazing rate parameter in the branch with grazing of storage. Ran for 80 years from bare ground. Grazing storage makes things colder, increases soil C, increases LAI and NPP.

[rosiealice]

An update on tuning the grazing, before I forget

I made the following set of tests changing the fates_landuse_grazing_rate on this branch.

CONT : 0.0035, 0.0, 0.01, 0.01, 0.02
MEDG: 0.005, 0.0, 0.015, 0.015, 0.03
HIGHG: 0.007, 0.0, 0.02, 0.02, 0.04

These are the LAI bises with these runs:
CONT

MEDG

HIGHG

These are changing all the grazing rates in parallel. But I think that if we were to include this change to the code, we might want to shift to something more like the MEDG numbers.

[JessicaNeedham]

Leaving open for now. To be merged with additional tuning after NorESM FATES release.