Restoring mature-phase forest tree species through enrichment planting in New Zealand’s lowland landscapes

New Zealand’s formerly extensive lowland native forests have been comprehensively cleared or modified, and large areas of secondary-growth vegetation have subsequently established. These areas are comprised of native, exotic, and mixed tree and shrub species assemblages. The mature-phase canopy and emergent tree species representative of pre-human New Zealand forests are often rare or locally extinct in these forests, indicating negative ramifications for long-term biodiversity conservation and ecosystem service provision, especially such as carbon sequestration. The successful recruitment of mature-phase canopy and emergent tree species may be prevented by biotic and abiotic filters related to dispersal (e.g. lack of seed sources or lack of dispersal agents), environmental variation (e.g. unsuitable germination microclimate or light availability), and competition (e.g. exotic weed competition). Failure of mature-phase tree species to cross through these filters may halt forest succession and cause arrested development of the ecosystem. There are also social and cultural imperatives for restoring mature-phase tree species, such as reassembling desired forest habitat and landscapes and providing lost natural heritage and cultural resources. Therefore, to restore secondary forests, depauperate remnant forests and create new forests that have complex structure, high biomass, and natural canopy tree diversity, mature-phase canopy and emergent species should be reintroduced through human interventions (i.e. enrichment planting). Experiments demonstrate that mature-phase tree species establishment can be optimised through canopy manipulation to address competition for light. Such targeted management can determine successful recruitment of mature-phase tree species, as can weed maintenance post-enrichment planting and landscape-level pest animal control. Currently political focus is emphasising planting of new early-successional native forests. However, support from scientific research and policy development is essential to actively recruit mature-phase tree species where they are now poorly represented and hence forest succession may be arrested. Afforestation and emissions trading policies need to support the reinstatement of mature-phase tree species within existing regenerating and degraded forests and newly created forests to facilitate the substantial ecological and ecosystem service benefits they provide over the long-term.