International use of exotic plantations for native forest restoration and implications for Aotearoa New Zealand

The desire for ecosystem restoration and native forest expansion is growing internationally. Transitional forestry, where an exotic plantation forest is transitioned to a native forest, is a potential method of native forest restoration and carbon sequestration that is gaining interest in Aotearoa New Zealand. However, it is currently unknown whether this approach can produce a permanent native forest ecosystem and how representative of remnant native forest it could be. This article explores international research into the use of exotic plantations to facilitate native forest restoration, specifically the ecological processes affecting restoration, and management interventions which could be applied to improve the forest transition. We found that the composition of the landscape matrix is highly influential on native regeneration. Increasing native vegetation cover to at least 25% has a strong positive effect on native regeneration within exotic forests. Increased native vegetation cover also improves effective seed dispersal as there are more sources of seed, shorter distances for the seed to travel, and improved habitat size and connectivity for avian seed dispersers. Further research into which seeds are entering plantation forests, by what method, and the distance from the seed source would be highly useful in understanding the potential for exotic plantation forests to transition to a native forest, and what barriers may be preventing this from occurring. Canopy manipulation was also found to be a key tool in stimulating establishment and growth of native species within an exotic plantation, especially those later-successional species which rely on small-scale disturbance in their natural succession. Practical methods of gradual canopy removal while minimising disturbance is an area which requires further research. We found that pest management is likely to be critical for transitioning exotic plantation to native forest. Herbivores present a significant barrier to succession as they can prevent growth and survival of susceptible species of regenerating plants and have significant effects on species composition and forest structure due to preferential browsing. Rodents which consume seeds prevent plants from establishing, further limiting regeneration. Mammals which predate native birds also disrupt the forest ecosystem due to the importance of avian seed dispersal for many tree species. This review highlighted the need for further research into transitional forestry and the potential to revert an exotic plantation to a native forest. Research conducted must occur across a variety of macroclimates, forest types, soils, and landforms throughout Aotearoa to ensure advice represents regional differences and is as comprehensive as possible.