unexpected PFT coexistence / tropical trees in alpine site?

[evalieungh]

Hello good FATES community. I have a question about how FATES handles competition and coexistence of PFTs in newer model versions.

I'm (re-)running some single-site simulations with CLM-FATES for an alpine site in Norway. With older model versions I had the challenge that one PFT always seemed to dominate the output and outcompete the rest. Now, with the newer version I'm using (from NorESM / tag ctsm5.3.034_noresm_v6, updated May 2025) there is an opposite situation. All of the PFTs seem to persist with no sign of dying even after hundreds of years. Broadleaf cold-deciduous extratropical tree seems to be dominant in the canopy, which is plausible given the warm bias of my forcing data. But all the PFTs continue growing, even the tropical ones, which is very different from what I expect.

Has there been some major change in the FATES code to get this behaviour? I've been out of the loop with model developments the past year(s) so I'm not sure if this is a new feature to improve coexistence or if I am doing something wrong.

Example: Panoply plot of canopy area (FATES_CANOPYCROWNAREA_PF), PFT index (x axis) vs time (simulation year 0-1130 from bare ground)

Technical info:
single-site, NorESM / tag ctsm5.3.034_noresm_v6, cold start, GSWP3 forcing, machine Betzy, grazing enabled, all default PFTs included, driver nuopc, compset I2000Clm50Fates.

[rosiealice]

Hi Eva.

Is it possible that this tag is setting nocomp mode on by default? That would explain the behaviour you are seeing. Check the lnd_in file in the run directory...

[evalieungh]

Interesting! As far as I can see, I am not using nocomp mode. lnd_in has use_fates_nocomp = .false.

The compset longname is 2000_DATM%GSWP3v1_CLM50%FATES_SICE_SOCN_MOSART_SGLC_SWAV_SESP so there isn't a nocomp specification there either. Are there any other places a similar behaviour may sneak in? I have enabled grazing of ALL the PFTs so could that be disturbing the competition somehow?

[evalieungh]

Adding a possible explanation suggested by @rosiealice :

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Rosie: So we now have external seed rain (until the last two tags, when we turned it off) which might promote coexistence if the previous version didn’t have it.

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Eva: FATES_SEEDS_IN_EXTERN_EL (external seed influx rate in kg element per m2 per second) is quite a bit higher than FATES_SEEDS_IN_LOCAL_EL (within-site, element-level seed production rate in kg element per m2 per second).

There is some allocation to seeds (FATES_SEED_ALLOC), but only around 2.8324576E-10 in the growing season. This is not given per PFT but some shrubs and cold-decid tree seems to do OK so I suspect they could produce seeds.