Acacia longifolia invasion impacts vegetation structure and regeneration dynamics in open dunes and pine forests

Acacia spp. are among the most serious plant invaders worldwide, and Acacia longifolia specifically causes problems in Portugal. In this study, we evaluated the impacts of A. longifolia invasion on community structure, light climate, plant diversity and regeneration in pine forests and open stabilized dunes in northern and southern Portugal. Having the growth form of a small tree or shrub, between 1 and 8 m tall, A. longifolia tended to dominate the middle stratum of forests and to share dominance in the upper stratum of open dunes. The presence of A. longifolia was associated with a decreased canopy cover in the lower stratum of all studied habitats, and at some sites with a significantly increased leaf area index and reduced light intensity in the understory. Species number and diversity were reduced in some habitats by up to 50% in invaded compared to non-invaded areas. Furthermore, in forest habitats, A. longifolia seedlings were facilitated by proximity to an adult A. longifolia while the establishment and growth of native seedlings was negatively impacted. The replacement of drought tolerant native species by the water spending invader, A. longifolia, may have serious implications for ecosystem functioning, especially during the prolonged drought periods predicted to occur in Portugal in the future.