Temporary Floodplain Ponds Shape Vegetation Mosaic in a Natural River Valley: Evidence from SAR and Optical Remote Sensing

Temporary floodplain ponds (TFPs) are short-lived water bodies forming in microtopographic depressions after flood recession and represent an important but poorly quantified component of floodplain hydrology. This study investigated the spatial and temporal dynamics of TFPs and their relationship with vegetation patterns in the natural floodplain of the Biebrza River, Poland. High-resolution TerraSAR-X data and Sentinel-2 multispectral imagery were combined with field-based vegetation surveys and statistical modeling. Threshold-based SAR classification showed that TFPs occupied more than 32% of the floodplain surface shortly after spring flood recession and stored, on average, over 250 L m−2 of surface water, but disappeared within one month. Random Forest classification demonstrated that combining SAR and multispectral data improved overall vegetation mapping accuracy from 64.5% to 81.7% (Kappa from 0.574 to 0.780). A global chi-square test revealed a strong association between vegetation patterns and TFP occurrence (χ2 = 224.9, p < 0.001, Cramér’s V = 0.40). Multinomial logistic regression identified TFP depth as the strongest predictor of plant community distribution. Rorippo-Agrostietum, Caricetum gracilis and Glycerietum maximae increased with TFP depth, whereas Alopecuretum pratensis and Phalaridetum arundinaceae declined. These results show that TFPs act as a fine-scale hydrological filter structuring floodplain vegetation mosaics and that SAR–optical data fusion is effective for detecting these transient habitat patterns.