Empirically Calibrated Phytoindication Scales Resolve Complementary Edaphic and Topographic Components of the Moisture Regime

The moisture regime is one of the principal determinants of plant distribution; however, traditional phytoindication systems generally treat it as a single ecological gradient, despite its complex environmental controls. This study examined whether vegetation responses could be used to integrate climatic, edaphic, and topographic determinants of moisture availability into ecologically meaningful indicator scales. European GBIF occurrence records for the regional flora were combined with climatic, edaphic, and topographic predictors, and the resulting moisture indicators were evaluated using vegetation plots, Sentinel-2 NDWI data, field soil-moisture measurements, and permutation-based null-model analyses. Constrained canonical correspondence analysis, using annual precipitation, topographic wetness index (TWI), and soil texture as environmental predictors, identified complementary moisture components that were used to derive empirically calibrated Edaphic Moisture Index (EMI) and Topographic Moisture Index (TMI). The edaphic component explained 32.7% of constrained variation and showed strong agreement with traditional moisture scales (R2 = 0.75–0.80), whereas the topographic component explained an additional 24.1% of constrained variation and represented a complementary dimension of moisture variation. Species-level EMI and TMI values were subsequently evaluated in relation to traditional Didukh and Ellenberg phytoindication systems using multiple complementary approaches, including vegetation plot analyses, landscape-scale assessment of spatial agreement with environmental moisture gradients, and permutation-based null-model testing. Both EMI and TMI captured vegetation-associated moisture variation and enabled spatial characterization of moisture conditions, although improvements over traditional indicators were scale-dependent rather than universally greater. These findings indicate that vegetation-based moisture assessment benefits from considering complementary edaphic and topographic components of the moisture regime rather than representing moisture conditions as a single undifferentiated gradient. The framework extends traditional phytoindication by providing empirically calibrated indicators that integrate vegetation responses with environmental predictors for ecological assessment.