TDV-optimization exemplified: Application to coastal cliff vegetation in Cabo Verde

Aims : We illustrate the application of Total Differential Value (TDV) optimization – a vegetation classification technique – and introduce methodological innovations for identifying syntaxa at different ranks and their diagnostic species. This approach produced a new syntaxonomic synthesis for the coastal cliff vegetation of the Cabo Verde archipelago. Study area : Cabo Verde archipelago. Methods : We analysed 167 relevés sampled on coastal cliffs using a five-step approach: (1) Initial characterization of communities based on dominant species using formal logic rules. (2) Analysis of each physiognomic group for patterns of differential species using TDV-optimization, supporting the identification of preliminary associations. (3) Validation of associations by confirming the presence of differential species across them. (4) Application of TDV-optimization to the synoptic table to establish preliminary higher-rank syntaxa. (5) Validation of higher-rank syntaxa with characteristic species by assessing species fidelity across all 484 known relevés from Cabo Verde using the Differential Value (DiffVal) index. Results : The five-step classification procedure revealed 17 physiognomic-floristic units at the association level, supported by differential species. TDV-optimization successfully identified higher-rank syntaxa – two alliances, one order, and one class – based on exclusive species and with clear biogeographic and ecological interpretations. Partial DiffVal scores proved useful for distinguishing characteristic species at higher ranks. Conclusions : Based solely on presence-absence data, TDV-optimization and the DiffVal index proved effective for vegetation classification at different scales, revealing co-occurrence/co-absence patterns. The identified syntaxa emphasize Cabo Verde’s distinctness from Mediterranean and Eurosiberian communities, confirming its affinity with the Warm deserts and semi-deserts subbiome and its biogeographical placement within the Tropical Sahara region. Further research should expand knowledge of Cabo Verdean and other African vegetation and numerically validate its syntaxonomy to support evidence-based conservation policies. Taxonomic reference : Plants of the World Online database (POWO 2025), except for Campylanthus glaber subsp. spathulatus (Brochmann et al. 1997), Fagonia latifolia (Pinto Basto 2002), Launaea arborescens subsp. melanostigma and Tetraena vicentina (Rivas-Martínez et al. 2017), and Lotus bollei (Chevalier 1935). Syntaxonomic references : Mucina et al. (2016); Rivas-Martínez et al. (2017) for the endemic Cabo Verde vegetation. Abbreviations : DiffVal = Differential Value; IAP = Internal Assignment Precision; pDiffVals = partial values used in the calculation of DiffVal; TDV = Total Differential Value.