Grassland PFTs die under semi-arid conditions

[luyssaert]

In r7272, many of the grassland PFTs still died in the (semi-)arid regions. These are the regions were grasses should be able to grow. One obvious suspect is the phenology. If Phenology is decoupled from the wet season, the PFTs may have a hard time to restore their reserves for the next season. Another problem could be the choice to simulate grasses as deciduous whereas in many regions of the world (but may be not the driest regions) they behave closer to evergreens. Last, in reality grasses could produce seeds to survive a dry period. This functionality is absent in r7272. Rather than using seeds we have to replant. Dying and replanting may not be a serious problem as long as we would take the N from the soil rather than the atmosphere. It should be checked how much C enters the system through the replanting.

[aducharne]

Following today's discussions and the possible link of dieback with a lack of water, thus a poor representation of vertical water fluxes in the soil (not enough infiltration and/or excessive drainage), I refer to this recent paper by Natasha focused on semi-arid sites including grassland ones:

MacBean?, N., Scott, R. L., Biederman, J. A., Ottlé, C., Vuichard, N., Ducharne, A., Kolb, T., Dore, S., Litvak, M., and Moore, D. J. P.: Testing water fluxes and storage from two hydrology configurations within the ORCHIDEE land surface model across US semi-arid sites, Hydrol. Earth Syst. Sci., 24, 5203–5230, ​https://doi.org/10.5194/hess-24-5203-2020, 2020.