The evaluation of forest fuel loading is required by most fire management activities. However, the consideration of shrub biomass for forest planning purposes has been limited by the inability to predict its growth and accumulation. The main objective of this study was to model shrub biomass over time under a tree canopy to be able to include shrub management in fire risk mitigation plans. Data for this purpose was obtained from the 4th and 5th Portuguese National Forest Inventories. Five biologically realistic models were built to describe shrub biomass accumulation in Portuguese forests. The selected model indicates that maximum biomass is affected by stand basal area and the percentage of resprouting shrub species in the stand. Biomass growth rate was clearly affected by the regeneration strategies of shrubs in combination with climatic conditions (mean annual temperature). The model can be used in the accumulation form for initialization purposes or in one of the two alternative difference forms to project observed shrub biomass. The acquired ability to estimate shrub biomass facilitates its inclusion in forest growth models and simulators and will contribute to more accurate estimates of fire behaviour characteristics and stored carbon. This is instrumental to improve decision-making in forest management plans that integrate fire risk, namely to schedule understory fuel treatments.