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 prior to side-arm reconnection. Riparian forest vegetation was surveyed on 20 May 2024 using three 10 × 10 m plots per site, with species cover estimated using the Domin cover-abundance scale and reported as plot means. Aquatic vegetation was surveyed on 30 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 checked against Plants of the World Online (POWO). 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.