Hydrochory (water dispersal) is a major driver of plant invasions in riparian landscapes but both the diaspore traits that determine flotation and the contribution of water-mediated transport to the colonization success of non-native species remain insufficiently understood. Echinocystis lobata, an alien vine widely naturalized along European rivers, provides a suitable model for testing how seed morphology governs buoyancy and, consequently, hydrochorous dispersal potential. The main aim of this study was to examine how functional traits influence the buoyancy of E. lobata seeds, which can be classified as normal seeds, freely released from fruits, and trapped seeds, retained within fruits.

We investigated three hypotheses under controlled laboratory conditions: (H1) seed mass, particularly in interaction with thickness, determines sinking probability; (H2) surface area affects flotation mainly through interactions with mass; and (H3) trapped seeds differ functionally from normally released seeds. Using a generalized linear mixed-effects model, we demonstrated that buoyancy is governed by non-linear interactions among mass, thickness, and surface area, with distinct patterns for trapped versus normal seeds. Thin normal seeds floated longer as mass increased, whereas in trapped seeds higher mass accelerated sinking. Surface area influenced flotation only when combined with mass and trapping status. Normal seeds remained buoyant for up to 14 days, while trapped seeds sank sooner (≤11 days).

These findings highlight that variability in diaspore traits generates a broad spectrum of dispersal outcomes, supporting both local deposition and long-distance hydrochoric transport. Understanding how trait-vector interactions shape dispersal enhances invasion risk prediction and informs management strategies, such as removing reproductive plants near waterways before fruit maturation. Overall, our results demonstrate that hydrochory, amplified by diaspore heteromorphism, may be a key driver increasing the invasiveness of E. lobata in European river valleys.