Can forest management inventory support national forest inventory to improve the municipal-level estimation of timber volume?

National forest inventories (NFIs) provide unbiased statistics for forest resource monitoring. Since NFIs primarily target large areas, they are less effective for small areas, such as municipalities, due to low sampling intensity and precision. In Finland, forest management inventories (FMIs) provide stand-level information but are biased for domain-level estimations. We evaluated a model-assisted estimator that used NFI plots integrated with FMI plots for municipal-level estimations. We used linear and tree-boosting models in 619 synthetic municipalities. The assisting models had a fixed set of nine predictor variables extracted from aerial image-based point clouds and Sentinel-2 images. Our findings indicated that the assisting models that used FMI field plots did not show improved efficiency over models fitted with the NFI plots in the 20 km buffer zone that surrounded the municipality of interest. The model-assisted estimators explained 66%–69% of the variation of the NFI field data-based mean estimates. A modest improvement was feasible by fitting a model that used both NFI and FMI plots, although the marginal precision improvements achieved and the additional effort required to harmonize the FMI plots with the NFI plots was not justified.