Te Uru Rākau Publishes the Forbes Ecology Ltd State of Knowledge Report on Transitioning Plantations to Native Forest

Transitioning Exotic Plantations to Native Forest - State of Knowledge (2021)

Abstract:

The Ministry of Primary Industries commissioned a review of (1) the state of knowledge on the topic of transitioning exotic plantations to native forest and (2) of the existing data and research approaches for addressing knowledge gaps for forest carbon aspects of transitioning plantations. The existing literature was reviewed, and anecdotal evidence was compiled to inform this document.

What we do know is that at the stand scale, forest age and structure, microclimate and shade tolerance are of high importance to the structure and composition of native understorey regeneration. So too are gradients in site factors such as topography and soils. At wider scales, we know that macroclimate is an important predictor of understorey regeneration and existing modelling has indicated which areas of Aotearoa are reasonably capable of natural regeneration (i.e., warmer, higher rainfall areas close to seed sources). However, empirical surveys and forestry experiments of regeneration and tree growth along key gradients are lacking. Proximal seed sources are known to be important for forest regeneration, and this too applies to regeneration in exotic plantations. Excessive herbivory by introduced mammalian herbivores (e.g., deer, goats, pigs, possums, lagomorphs and domestic stock) filters regeneration and can severely constrain the composition and structure of understorey regeneration, and weeds can be a major limiting factor. Various options exist for addressing these threats, including defending forests using fencing or professional animal pest control operations or addressing intense weed threats (e.g., old man’s beard or blackberry).

The topic of transitioning exotic plantations to native forest is relatively new and there has been
insufficient time and a lack of coordinated research to determine the actual timeframes required to transition exotic plantations to native forest. Related to this, we do not know what level of regeneration is required to successfully attain a transition to native forest or to what levels old-growth native forest canopy dominants will establish in relation to biotic and abiotic gradients. While we know that the performance of regeneration (e.g., structure, composition, growth and biomass) will differ along the main abiotic and biotic gradients, we have few empirical data from exotic plantations to demonstrate this. Work to attain empirical data is critical, as these data would allow management thresholds to be identified as well as informing carbon and forest dynamics models. The role for species other than radiata pine (e.g., exotic angiosperms), particularly when planted in polycultures designed to provide specific structural and functional elements is also unknown and is deserving of trials and investigation.

Practical considerations for transitioning plantations are likely to include defining clear objectives (e.g., carbon forestry, soil conservation, or restoring biodiversity or cultural values), implementing effective pest control, adaptively managing stand level interventions, recognising the influences of site factors and anticipating the causes of tree instability. Transitions would be most appropriate in areas of higher rainfall with good amounts of native seed sources, healthy native forest bird populations, low numbers of browsing animals and manageable plant pest issues. Transitional forestry is only appropriate where there is committed/guaranteed long-term funding and a robust plan for ongoing forest management (including good infrastructure within the forest to support this). Covenants to guarantee this approach are also required. Given the current state of knowledge, transitions should only be attempted at scales which are reasonably manageable.

Recommendations are made for a programme of research including national scale surveys and forestry experiments conducted along important biotic and abiotic gradients. These trials would better define management needs and rates of growth and biomass/carbon accumulation in transitional forests.

A second report which applies the State of Knowledge to three case studies will be published soon - contact Adam if you would like to receive a copy in due course.

Report access:

Access the State of Knowledge Report at this link: https://www.mpi.govt.nz/dmsdocument/47521-Transitioning-Exotic-Plantations-to-Native-Forest-A-Report-on-the-State-of-Knowledge-2021-22- 

Bibliographical reference:

Forbes, A., & Norton, D. (2021). Transitioning Exotic Plantations to Native Forest: A Report on the State of Knowledge. MPI Technical Paper No: 2021/22. Prepared for Te Uru Rākau - New Zealand Forestry Service by Forbes Ecology.