Roles of Propagule Pressure, Biotic Resistance, Temperature and Soil Moisture in the Early Establishment of the Invasive Aquatic Plant Crassula helmsii

ABSTRACT Crassula helmsii (Kirk) Cockayne is an invasive aquatic species increasingly colonising European freshwaters and wetlands. Despite its rapid spread, the mechanisms underlying its establishment remain poorly understood. This study experimentally assessed the effects of propagule pressure, native plant competition, temperature and soil moisture on the colonisation and regeneration ability of C. helmsii . Three complementary experiments were conducted under greenhouse, outdoor mesocosms and growth chambers, respectively. Establishment success and growth performance were measured (i) across varying propagule densities (one or three propagules), in the presence or absence of native grasses ( Agrostis stolonifera and Phalaris arundinacea ), (ii) under a gradient of soil moisture (dry to wet) and (iii) under two temperatures (18°C and 23.5°C). Contrary to expectations, propagule pressure did not enhance establishment or regeneration ability of C. helmsii , suggesting that colonisation success is not density‐dependent. In some cases, higher propagule densities even reduced individual establishment, possibly due to intraspecific interactions. Neighbour grasses facilitated early establishment—likely through microclimatic buffering—but later limited C. helmsii growth through competition for resources, demonstrating a shift from facilitation to inhibition. Soil moisture strongly influenced regeneration ability, with desiccation followed by rehydration triggering delayed regrowth. In contrast, variation in temperature had negligible effects on plant performance. These results highlight the plasticity of C. helmsii and the importance of both biotic resistance and abiotic factors in regulating its invasion potential. Management strategies should integrate the role of native vegetation and moisture regimes to prevent colonisation and minimise re‐establishment following control efforts.