Baseline riparian forest and aquatic macrophyte vegetation dataset from the Danube right bank near Draž (Croatia), to support before-after monitoring of side-arm reconnection (2024-2025)

Baseline biodiversity data collected prior to hydromorphological interventions are essential for understanding and evaluating ecological change following restoration or engineering measures. In large river floodplains, interventions such as side-arm reconnection are expected to alter hydrological regimes, habitat connectivity and disturbance patterns, with cascading effects on both riparian and aquatic plant communities. However, without well-documented pre-intervention datasets, post-restoration assessments often lack a reliable reference against which observed changes can be interpreted. Along the right bank of the Danube River near the village of Draž (Croatia), planned hydro-technical measures aim to reconnect a cut-off side arm to the main channel. This reconnection is expected to modify local hydrological conditions and habitat structure in adjacent riparian forests and aquatic habitats, creating a clear need for baseline vegetation data collected prior to intervention. This data paper presents a baseline dataset of riparian forest and aquatic vegetation collected along the right bank of the Danube River near the village of Draž (Croatia), prior to planned hydromorphological restoration measures involving the reconnection of a cut-off side-arm to the main channel. Riparian forest vegetation was surveyed in May 2024 using three 10 m × 10 m plots per site, with species cover estimated using the Domin cover-abundance scale and reported as plot means. Aquatic vegetation was surveyed in July 2025 using three 10 m belt transects per site, with species cover recorded in five ordinal cover classes and reported as transect means. The dataset comprises 20 sampling events (10 forest and 10 aquatic), 251 taxon-by-event occurrence records and 331 measurements, including species cover or abundance, establishment status (native, introduced, invasive) and event-level environmental descriptors, such as canopy-cover classes, water physicochemistry, water-velocity class and CORINE Land-Cover categories. Taxonomic nomenclature was standardised according to Plants of the World Online (POWO 2026). The sampling design and data structure allow the dataset to serve as the “before” component of a before–after monitoring framework, supporting future assessments of vegetation responses to hydrological reconnection. The dataset provides a reference for restoration evaluation, invasion dynamics and comparative studies of riparian and aquatic plant communities in large river floodplains and constitutes the baseline vegetation component of the ecological monitoring programme established within the Horizon Europe project DaWetRest.