How Mixed Are Production-Oriented Planted Forests? A Global Review and Classification Approach

Mixed planted forests (MPF) are increasingly recognized for their potential to reconcile forest productivity with biodiversity conservation and ecosystem services, such as climate adaptation and mitigation. They can be mixed in multiple ways, but the lack of a standardized procedure to compare mixture levels has hindered the exploration of their potential benefits, rendering the term "mixed" too generic to support meaningful comparisons across studies or informed decision-making. This study aims to (i) conduct a global literature review of the characteristics of MPF established for production (i.e., excluding biodiversity-oriented forests), and (ii) develop an index to classify production-oriented planted forests according to their level of mixture. We systematically screened the literature and selected 153 studies covering 253 MPF stands, from which we extracted detailed information on management practices using 10 criteria related to species composition (e.g., number of species, genetic diversity, species origin, functional composition) and structural/spatial design (e.g., spatial distribution, age heterogeneity, understory natural regeneration). We propose an index to classify MPF according to mixture level (scaled to a 0-1 range) based on a multi-criteria decision analysis using the PROMETHEE II outranking method. The MPF diversity index serves as a classification tool for MPF designs based on their relative dominance across multiple indicators. The results revealed that most published MPF worldwide occupy low to intermediate positions along the diversity gradient (index mean = 0.48), indicating a diversity gap even among intentionally diverse production-oriented planted forests. The proposed index provides a transparent and comparative classification of MPF designs that can support policy and management decisions by highlighting differences in diversity-related design attributes.