Functional Plant Diversity: Traits, Evolution, Biodiversity Conservation and Ecosystem Restoration

Plant functional diversity, the variety and distribution of morphological, physiological and biochemical traits within and among plant communities, links biodiversity to ecosystem processes, resilience and the delivery of benefits to people. Field trait surveys cannot capture this diversity continuously across landscapes, and remote sensing is therefore increasingly expected to supply spatially explicit, repeatable information for biodiversity monitoring, conservation planning and restoration assessment. Expectations have, however, advanced faster than consensus on what optical and structural signals actually measure. This critical narrative review examines how plant functional diversity is inferred from leaf and canopy reflectance, imaging spectroscopy, multispectral satellite data and laser ranging, and evaluates the strength of evidence connecting these observations to trait-based ecology and to conservation and restoration decisions. Peer-reviewed literature published from January 2002 to July 2026 was identified through multidisciplinary scholarly indexes, citation searching and authoritative institutional sources, and was appraised for conceptual relevance, methodological adequacy, validation design and ecological context. The synthesis shows that leaf-level spectroscopy provides robust, repeatable information on several functional traits and carries phylogenetic signal, whereas canopy-level retrieval is conditioned by vegetation structure, illumination, background and model transferability. Evidence for the spectral variation hypothesis is positive on average but highly heterogeneous, and relationships between spectral heterogeneity and taxonomic or functional diversity frequently weaken or reverse with coarse spatial grain, open or managed vegetation, phenological change and inconsistent metric choice. Trait-based approaches that estimate optical traits before computing diversity, and approaches that combine spectral with structural information, appear more interpretable than raw spectral heterogeneity, although independent validation remains scarce outside a small number of well-instrumented regions. Global trait maps also disagree substantially, which indicates that upscaling choices can dominate ecological signal. Applications to conservation prioritisation and restoration monitoring are promising but rarely evaluated against management outcomes. Priorities include standardised validation designs, uncertainty propagation, multi-temporal observation, benchmark simulation, better representation of tropical, dryland and non-forest systems, and explicit links between remotely sensed functional diversity indicators and policy targets. Remote sensing can strengthen functional biodiversity monitoring, but its outputs should currently be treated as conditional, scale-dependent indicators rather than direct substitutes for field observation.