Enhancing bird-mediated seed dispersal to increase rainforest regeneration in disused pasture – A restoration experiment

Abstract

Abstract Frugivorous birds are important seed dispersers but are infrequent visitors to pasture. Therefore, the recruitment of rainforest trees into disused pasture is often limited by dispersal. We studied potential seed sources, seed-dispersing birds, seed rain and seedling recruitment in an experimental management project that aimed to catalyse rainforest regeneration adjacent to old-growth forest in north-eastern Australia. To attract birds, nine semi-natural perches and water basins were installed in each of three 0.64\xa0ha experimental plots, concurrent with herbicide control of pasture grasses. Despite a diverse avian seed-disperser community in the adjacent forest, only three species visited the plots. However, the presence of water increased seed deposition. Water basins collected twelve times as many native tree seeds as mesh seed traps (a 24-fold increase in seed deposition per unit area), including more large seeds (≥10\xa0mm), and more species, especially of bird-dispersed forest trees. This was due to the large-gaped Pied Currawong (Strepera graculina) that deposited large seeds beneath the perches, when using water to regurgitate seeds. Deposition of large native forest tree seeds was also enhanced under perches positioned close to an already established tree or large shrub, highlighting an important indirect effect of pasture trees. Such pasture trees can themselves be effective regeneration nuclei, and their function improved by actions to suppress pasture growth and encourage dispersers. However, in this study, these interventions were not sufficient to fully mitigate barriers to rainforest regeneration. Although seed deposition was enhanced by the presence of water, semi-natural perches and pasture trees, recruited seedlings in the plots were not representative of the seed deposited. Instead they were dominated by a subset of small-diaspore, pioneer species. To further test the potential for semi-natural perches, water and pasture suppression to increase rainforest regeneration in disused pasture, requires testing of water source designs and carefully designed experimental restoration to examine dispersal and other regeneration barriers.