Notes from @inh599 email (16/4/24):
I/we have made a little further progress on the topic of understanding what is going on with LUC in the CABLE-POP runs. In short – a better understanding is emerging as to why some of the grid cells show different qualitative behaviour between CABLE-POP and ESM. This is emerging as a topic that is going to need careful consideration as the configuration of CABLE4 emerges. Jurgen and I will keep thinking/discussing as this will play an important role in TRENDY for 2024 and some of the current BIOS3 work.
More detail - In essence there is ambiguity around how to deal with transitions in/out of rangelands in the POPLUC context since POPLUC does not use the potential vegetation distribution that LUH2 assumes. From some emails that Vanessa, Peter B and Stephen Sitch had back in 2018
- Following LUH2 simple guidelines (on their website): "all natural vegetation should be cleared for managed pasture, and only cleared for rangeland if it is forested”. “
This guidance resulted in a decision to discount (set to zero) any transition from primary (and secondary) non-forest vegetation to rangeland in the POP simulations.
The ambiguity arises because POPLUC assigns each grid cell an amount of primary forest (a hard-wired fraction of primary forest+primary non-forest vegetation) independently of LUH2. Indeed POPLUC can assign a grid cell some primary forest even if LUH2 says cover was all primary non-forest. What do you then do about the transitions from primary vegetation to rangeland? How do you keep your land cover states in step with the transitions?
The way it has been done at the moment leaves any primary forest in POP that was on land assigned as primary non-forest in LUH2 untouched – so is (possibly) unrealistic.
Since the ESM1.5 is based solely on the states (and changes in them over time) this issue around co-handling the mapping between land use states/transitions and the land cover states/transitions doesn’t apply in the same way. However there is the equivalent (parallel) problem of determining the initial (pre-industrial) land cover distribution in a way that is consistent with both LUH2 and the present day (MODIS informed) land cover.