Impact of Prunus serotina invasion on understory functional diversity in a European temperate forest

We investigated the relationships between the overstory dominance of Prunus serotina, and the functional diversity of the understory plant communities, in a managed European forest. Vegetation, habitat characteristics and disturbance history were surveyed in 32 invaded stands vs. 32 paired uninvaded stands, after a random stratification. Community specialization and functional diversity indices were compared and a RLQ analysis was run to link species traits to environmental variables. The herb layer of invaded stands exhibited significantly more specialist species and a lower trait diversity compared to uninvaded stands, with respect of species richness and vegetation cover. Light arrival to the forest floor and soil properties explained most of the variation in the RLQ analysis, but 20% of the variation strongly correlated with P. serotina dominance and associated disturbances. Traits characterizing shade-tolerant, short-living ruderals and shade-avoiders (vernal geophytes) were significantly associated to invaded stands, while those associated to light-demanding graminoids characterized uninvaded stands. The establishment of functionally close species (seedlings and saplings of native woody species) tended to be lower in invaded stands. We concluded that the invader was becoming the new ecosystem engineer, first by inducing trait convergence and community specialization, thus promoting traits that enable species to capture resources in the new environment it was creating, and second by reducing the grain of local heterogeneities.