COMET-Farm uses data from the FIA (Forestry Inventory and Analysis) database to construct biomass accumulation equations. Unfortunately, due to lack of data, this does not include shrubs, but only trees. If you pick a small tree species and expect it to remain small, you could enter a smaller DBH in the agroforestry module. 

COMET-Planner does have the capabilities of accounting for hedgerows by entering your location, selecting woody plantings, then hedgerows.