Kick-starting forest regeneration: the role of bird perches in facilitating native forest seed deposition into New Zealand retired pastures

Abstract

Why do some retired pastures adjacent to patches of New Zealand native forest resist forest recolonisation? Given that 70% of New Zealand native woody species are fleshy-fruited and bird-dispersed, one ecological limiting factor may be that seeds are not arriving into pasture from a forest source. Birds perch between foraging bouts to process seed in the gut and avoid carrying energetically expensive seed ballast. In tropical and other temperate ecosystems, natural (trees) and artificial perches in retired agricultural lands have facilitated seed deposition. Yet this approach has not been trialled in New Zealand, and, with the reduction in available seed-dispersers in New Zealand forests, this type of study is urgently needed. My research investigated fleshy-fruited seed deposition under artificial and natural bird perches in pastures at three sites in the Auckland region. I identified perch visitors, forest seed sources, forest and pasture seeds in seed traps, and seed species in the soil seed banks. Results indicated that, without perches, bird-dispersed seed from woody species did not arrive into pastures. In addition, there seemed to be a trend for greater seed deposition under natural perches than under artificial perches. The lack of fleshy-fruited forest seeds germinating in pasture soil samples confirmed that seed deposition had not occurred in pastures previously. Seed species found in traps were characteristic of early successions, including Coprosma spp., Cordyline australis and Piper excelsum, and also included weed species such as Phytolacca octandra, Solanum mauritianum and S. nigrum. Perch visitor identification suggested tui (Prosthemadera novaeseelandiae) and fantail (Rhipidura fuliginosa) were the primary seed dispersers, although surprisingly, predators such as sacred kingfisher (Todiramphus sanctus) dominated artificial perches. However bird detection methods lacked sensitivity and it is assumed other dispersers such as blackbirds (Turdus merula), silvereyes (Zosterops lateralils) and starlings (Sturnus vulgarus) were also visiting perches. I conclude that there is potential for ecological restorations to overcome resistance to forest regeneration via the use of artificial perches or single trees planted in pasture. This will harness natural dispersal processes and reduce reliance on labour-intensive plant propagation and planting strategies. Future research should investigate seedling recruitment under perches in retired pastures to identify any further barriers to forest recolonisation, such as competition from exotic pasture plants or soil compaction.