Description:

This PR introduces a physically explicit representation of non-vascular plants (NVP; e.g., moss and lichen) in CTSM–FATES and its biophysical effect by assigning them to a dedicated vertical layer in CTSM, and a NVP PFT in FATES. The core architectural decision is to use layer 0 of the soil-snow layer structure to accommodate NVP and its growth (i.e., changes in height) when it is present. This layer integrates consistently into CLM’s radiation (under snow), thermal, and hydrology schemes, while coupling to FATES (NGEET/fates#1556) for canopy radiation (without snow) and photosynthesis (see the following call graph).
Note: The implementation is not fully ready yet and is still open for discussion.

Key implementation details in CLM:

• Adds NVP as an explicit surface layer at index 0, controlled by col%nvp_layer_active and col%jbot_sno, with geometry (dz, z, zi) updated each daily timestep via UpdateNVPLayer.
• Extends snow radiation to include NVP absorption at layer 0 with NVP specific optical parameters (when under snow).
• Integrates NVP into the thermal solver with its own thermal properties.
• Adds NVP specific hydraulic properties, water retention curve, and water balance subroutine NVPWaterBalance_Column.
• Adds interface with FATES for exchanging NVP structure properties, radiation flux, temperature and water status.

Collaborators:

@rosiealice

Expectation of Answer Changes:

This PR is expected to produce answer changes when NVP is active and NVP layer is present:

• Surface energy balance modified due to new top boundary condition at layer 0 (CLM, FATES)
• Soil temperature profiles altered due to NVP thermal properties (CLM)
• Hydrology affected by additional water storage and evaporation pathway (CLM)
• Carbon fluxes (GPP/NPP) changed due to NVP photosynthesis and modified PAR environment (FATES)

No answer changes are expected when NVP is disabled.

Description of generative AI usage

Claude code has been used for porting changes into the new CTSM version and designing the overall structure. The call graph is also generated by Claude code.

Any User Interface Changes (namelist or namelist defaults changes): Yes.

Does this create a need to change or add documentation? Yes