Industrial Symbiosis in Low-Density Industrial Territories: A Strategic Value-Chain Perspective for the Circular Bioeconomy

Industrial symbiosis is widely proposed as a mechanism for the circular bioeconomy, but its evidence base is dominated by dense industrial settings. This study examines how symbiosis can emerge in a low-density industrial territory and which conditions prevent technically plausible exchanges from materialising. A region in North-Eastern Portugal (26 municipalities, 10,908 km2, 28.3 inhabitants/km2) was examined through four instruments: territorial characterisation from official registers; a population of 89 firms in forestry, mushroom production and biocomposites; a structured survey; and documentary triangulation for the non-responding biocomposite sector. Forest and shrubland occupy 58.0% of the territory, and established institutional and R&D capacity is present. Conversion capacity, however, is fragmented and functionally mismatched with the intermediate processing required for higher-value material exchange, and no established inter-sectoral by-product exchange among the three chains was identified: one mushroom producer imports processed lignocellulosic substrate from Spain, while local forestry residues are shredded on site. The constraints identified are functional conversion capacity, aggregation, information and facilitation; declared willingness to transfer was present among most surveyed firms and was not the limiting condition observed, while relational readiness in its broader sense was not assessed. Potentially capable organisations exist, but no explicit by-product-exchange facilitation mandate was identified in the institutional sources reviewed, so facilitation appears unassigned rather than unavailable. The findings therefore shift intervention priorities towards the infrastructural and institutional shortfalls identified in the observed cases.