Plant ecologists have demonstrated the usefulness of functional traits in explaining vascular plant community structure and ecosystem functioning. Functional trait-based approaches have been increasingly applied to cryptogams over the last 30 years. However, the application of vascular plant-based traits to non-vascular photoautotrophs (NVP; here bryophytes and lichens) remains challenging due to their differing ecophysiological characteristics. Several elements of NVP functional ecology do not have parallels in vascular plant ecology, such as traits related to morphology (shoot, ramet or thallus architecture, and leaf structure), water relations (poikilohydry, desiccation tolerance), and reproduction (vegetative, spores), as well as the scales (e.g., colony) at which some of these traits are measured. Another issue concerns data availability, with relevant information scattered across the literature and NVP trait measurements conducted in a variety of ways that are not always comparable. Here, we propose a hierarchical trait ontology and a thesaurus for NVP to drive future research. Our literature review resulted in a final corpus of 598 articles published between 1967 and 2024. Two hundred thirty-two traits were identified, with the majority (70%) being used as response traits related to growth and reproduction in individual organisms. The number and diversity of traits used in NVP studies have increased markedly over time, resulting in substantial heterogeneity in terminology, definitions, and measurement protocols across the literature. We also found that ecological performance indices were frequently treated as traits, highlighting a widespread conceptual confusion that complicates data synthesis and cross-study comparisons. The use of the trait-based approach in vascular plant studies has been supported by the development of standardized centralized definitions, protocols, and databases, a foundation not yet developed for NVP research. Based on this review, we propose an ontology for structuring NVP trait data and definitions. Together with more standardized protocols, we hope this framework will enable the future development of consistent functional trait databases of NVP, ultimately advancing our understanding of how traits mediate functions and responses of these organisms in ever-changing environments.
