Native seed rain in plantation forest is associated with distance from native forest edge

Abstract: Ecological restoration to mitigate human impacts on the environment is increasingly widespread, particularly regarding forest ecosystems. Exotic plantation forests have been posited as beneficial in the restoration of forest ecosystems by creating a favourable microclimate in the understorey for native vegetation to establish. Seed dispersal into plantation forests is a vital part of this process. We sampled seed rain across a 6-month period at three sites in mature (>20 years) Pinus radiata plantation forest in the Hawke’s Bay region of Aotearoa New Zealand across a gradient from native forest edge into the plantation forest. Seed traps were exponentially spaced along three 620 m transects at each site. Vegetation sampling of the understorey was undertaken to compare vegetation composition with that of the seed rain. Over the sampling period 2,061 seeds of 19 different species were captured. Distance from native forest edge had a significant negative effect on the species richness and abundance of seed rain for both dry seeded and fleshy fruited seeds, as well as seeds from woody species. However, this effect was significantly stronger at the Blowhard site, which was adjacent to a more diverse, mature native forest. Cirsium sp. and Coriaria arborea var. arborea were present in the seed rain but not in the plantation understorey vegetation. Seed rain was most abundant and diverse in the first 20 m of the plantation forest from the native forest edge. The presence of understorey vegetation along the transects, including 18 woody species that were not captured in the seed rain, suggests seed rain is penetrating the plantation forest up to 620 m, albeit at a lesser frequency beyond 20 m from the native edge. Frequent islands of native forest within a plantation forest matrix are likely to act as nuclei to improve the richness and cover of regenerating native plants in the plantation understorey.