Senescence-driven canopy thinning in Rosa rugosa thickets does not mitigate negative effects on grey dune vegetation

Invasive shrubs can modify ecosystem structure and functioning in ways that depend on their ontogenetic stage, yet this dimension of invasion impacts is rarely examined. We investigated how the ageing of Rosa rugosa thickets influences vegetation, soil properties and microbial functioning in grey dunes of the Baltic coast. By sampling the centres and edges of shrub patches, as well as adjacent resident vegetation and sparsely vegetated bare sand, we assessed how invasion effects vary along a demographic gradient. Ageing thickets formed partially open centres (due to shoot senescence and dieback), creating microsites favourable to late-successional and shrub-associated species. At the same time, the concurrent development of a dense root-litter mat inhibited the recovery of specialist grey dune flora, thereby stabilising a successional shift towards more widespread, mesic-tolerant assemblages. Changes in soil physicochemical properties were moderate and largely associated with organic-layer build-up and sea-spray interception, whereas microbial communities showed rapid and pronounced functional responses, particularly in respiration and enzyme activity. These results demonstrate that R. rugosa acts as an ecosystem transformer whose impacts intensify and change through ontogeny. Importantly, natural senescence of thickets does not reverse invasion effects, but reinforces successional advancement, highlighting the need for active management to protect grey dune biodiversity.