Post-clearing recovery of coastal dunes invaded by Acacia longifolia: is duration of invasion relevant for management success?

1. Invasive plants destroy the structure and function of many ecosystems but removal is expensive, so the likelihood of success should be assessed before a major control programme commences. Trial clearings can clarify how best to design and perform broad-scale clearing programmes. Such trials need to consider the range of conditions that might affect the outcome of control operations, not least the duration of invasion before clearing commences, a parameter that has rarely been considered previously. 2. The recovery of plant communities was monitored over 6 years in a Portuguese coastal dune system after the removal of invasive Acacia longifolia plants, together with the underlying litter in half of the cleared areas but not in the remainder. Areas that had been invaded for > 20 years (long-invaded) as opposed to < 7 years (recently invaded) were compared. 3. After the removal of A. longifolia, recently invaded areas had more exotic plant species but also higher native species richness, plant cover, initial diversity and species turnover rates than long-invaded areas. Generalist native species were initially very abundant in cleared areas but progressively gave way to species more typical of dunes. Six years after clearing, many species that usually occur on dunes were still missing. Therophytes were the most abundant life form immediately after clearing, but nanophanerophytes, chamaephytes and A. longifolia increased with time. Seedlings of A. longifolia were most abundant in long-invaded areas, but litter removal along with plants promoted increased plant species richness and cover and decreased susceptibility to reinvasion. 4. Synthesis and applications. A number of variables can affect the outcome of clearing invasive plant species. For A. longifolia on Portuguese dunes, recently invaded areas should be prioritized and thick litter layers should be removed along with the invader. Even so, recovery of native flora was not complete in this study, and other management actions are needed to supplement clearing operations, for example propagation of native species or prescribed fires to deplete the invasive species seed bank. Although complete restoration of the ecosystem will almost always be impossible, it should be possible in the long term to create an ecosystem with a structure and function that resembles the original habitat.