Opportunities and limitations of exotic Pinus radiata as a facilitative nurse for New Zealand indigenous forest restoration

Background: We investigated the long-term potential of non-harvest Pinus radiata plantations for the facilitation and restoration of a natural forest community dominated by indigenous woody species. We investigated the relationship between indigenous regeneration and light levels and the hypothesis that proximity to indigenous seed sources is critical. We studied nine Pinus radiata stands of different ages located within Kinleith Forest, which is a large (ca. 66 000 ha) commercial exotic plantation forest located in New Zealand’s central North Island. Methods: We constructed a chronosequence of P. radiata plantation stands aged 2–89 years to represent long-term natural forest regeneration following plantation establishment. We surveyed structural, compositional and contextual aspects of this secondary succession and compared these results with an old-growth indigenous forest reference site located within the study area. Results: The exotic P. radiata canopy facilitated a regeneration trajectory characterised by shade-tolerant indigenous forest species. We found that the structure and composition of P. radiata understories were strongly influenced by stand age and proximity to indigenous forest. Stand age was important from the perspective of creating shaded conditions for the establishment of shade-tolerant woody forest species. Our results suggest that proximal indigenous forest was required for the consistent natural establishment of larger-fruited, bird-dispersed mature forest canopy species in P. radiata plantations. Conclusions: Our results showed that, even at ecologically isolated sites, the microclimate conditions created by plantation Pinus radiata stands supported a suite of readily-dispersed indigenous forest plants. Based on these results we suggest that non-harvest P. radiata stands provide an important opportunity for the restoration of indigenous forest communities in New Zealand’s production landscapes. Where restoration of forest composition similar to old-growth is the restoration objective, however, interventions might be necessary to direct and accelerate the secondary forest succession. Further replicated study is required into the relationship between native forest proximity and understorey regeneration patterns.