Forests are the most biodiverse terrestrial ecosystems. National forest inventories (NFIs) are the main source of information on the status and trends of forests, but they have traditionally been designed to assess land coverage and the production value of forests rather than forest
biodiversity. The primary international processes dealing with biodiversity and sustainable forest management, the Convention on Biological Diversity (CBD), Forest Europe, Streamlining European Biodiversity Indicators 2010 of the European Environmental Agency, and the Montréal Process,
all include indicators related to forest biodiversity. The scope of this article is to review and present possibilities offered by NFIs to harmonize estimation of indicators useful for international forest biodiversity monitoring and reporting. We summarize key findings from Working Group
3 of Action E43 (“Harmonisation of National Forest Inventories in Europe: Techniques for Common Reporting”) of the European program Cooperation in Science and Technology (COST). We discuss definitions and techniques for harmonizing estimates of possible biodiversity indicators
based on data from NFIs in Europe and the United States. We compare these possible indicators with indicators selected by international processes. The results demonstrate that NFIs can report comparable or harmonized estimates of indicators for multiple biodiversity features (forest categories,
deadwood, forest age, forest structure, and forest naturalness), but for others (ground vegetation and regeneration) NFIs should invest more in harmonization efforts. On the basis of these key findings, we recommend that NFIs should represent a main component of a future global biodiversity
monitoring network as urgently requested by the CBD.