While riparian zones are often only a very small portion of an entire landscape, often these areas are still critical wildlife habitat and so difficult to ignore. It is for this very reason that the original architects of TELSA decided to represent spatial units using polygons rather than rasters – the tesselation step in TELSA allows one to preserve existing stand boundaries, including those representing sliver-like riparian areas, throughout the simulation process. Other landscape models that "rasterize" a landscape tend to wash out these small but important features.
Because of their importance to most of our users we find that modellers tend to separate riparian areas from neigbouring stands. However these areas can share the same natural vegetation dynamics as upland areas, if this make ecological sense. This would help you avoid creating a new "model" for these areas. What typically differs between riparian and upland areas is the management that is applied – in most landscapes the riparian zones are protected to some degree. By assigning the riparian areas to a different Path stratum, for example, you could then apply different management to the riparian zones. For example in the riparian strata you could set your treatment targets to 0 for management; alternatively you could also use multipliers to turn off all management in these areas.
Hope this helps, Colin.