Restoration of native forests in New Zealand has previously been undertaken by planting young trees raised in nurseries and tending to those trees for several years to protect them from environmental risk factors. More recently, there have been studies into restoration methodologies using seed rather than seedlings. One such project was the 3-year Wakatipu Beech Seeding Project (WBSP), which trialed different methods of collecting, processing, treating and broadcasting seeds into areas of controlled (sprayed) wilding conifer forest around the Wakatipu with the aim of facilitating the restoration of exotic conifer stands back into native forest.
This was a joint venture between the Wakatipu Wilding Conifer Control Group and the Wakatipu Reforestation Trust, with funding from the Ministry for the Environment’s Community Environmental Fund. Further support was provided by Scion, the University of Otago, Ahika Consulting Ltd, and a dedicated team of keen volunteers.
Several challenges were encountered along the way, including an extremely dry spring, lack of locally available viable seed, weed inundation and dangers associated with working beneath dead trees. Despite these challenges, by Year 3 a range of locally sourced native tree species had been broadcast into several areas of controlled wilding conifers and seedlings had begun to emerge.
Coprosma propinqua and Mānuka seeds proved to be the easiest to collect, process, store and propagate. The germination strike rate of pittosporum tenuifolium and griselinia littoralis was also very good, but it was a bit more difficult to collect, process and store this seed. Mountain beech seed was relatively easy to collect (during the mast season) but germination strike rate was relatively low compared to the other species and then there is also the added complication of how to inoculate the seedlings with beneficial mycorrhizae at a large scale.
The germination strike rate of all species was improved greatly by removing the thick layer of decaying pine needles on the forest floor and exposing a more suitable growth medium. Seedling survival rate was improved by excluding herbivores from the trial sites.
Through this trial, it became apparent that when it comes to restoring controlled wilding stands back into native forest, there is a “sweet spot” between the control work being undertaken and the point where dead trees have decayed sufficiently to allow light-loving weeds to proliferate. It is during this sweet spot that seeding operations should be focused. Ensuring that the sweet spot is not missed would require planning of the restoration strategy prior to the wildings being sprayed, rather than this being an afterthought.
It also became apparent which factors must be considered for any seed-based restoration project to be successful. These include:
- Undertaking a thorough assessment of the site first and foremost to identify what the risk factors are (weeds, pests, exposure etc.) and to identify optimal microsites in which to focus efforts.
- Selecting quick-growing, colonising species that are suitable for the site rather than picking species based purely on personal preference. It may be necessary to develop a long-term restoration strategy involving the establishment of a suitable nursery crop to create more favourable conditions into which more desirable species can be sown later on.
- Ensuring that sufficient viable seed is going to be available locally. This is far more likely during a mast season. Field conditions are going to be less suitable for germination than in a nursery and so the strike rate will be significantly lower. Some seeds won’t make contact with a suitable growth substrate, will be ingested by animals, will blow away or may dry out. There needs to be enough seed broadcast to account for the relatively low strike rate. Consider how feasible it is to collect and process vast quantities of seed for the chosen species with the limited resources available - a different species may need to be targeted as a result.
- Ensuring that there is an absence of stock and other grazing animals such as pest species throughout the formative years of the project. Part of the initial site assessment should include an identification of which wild animals are likely to be present (e.g. rabbits, hares, goats, deer, pigs or wallabies) and determining appropriate control methods (e.g. poison, shooting, fencing).
- Undertaking adequate site preparation to remove weeds and any other ground cover so that broadcast seeds make contact with the soil e.g. spraying with herbicide and screefing, bearing in mind that clearing the site also allows weeds to colonise more rapidly.
- Controlling competing weeds and grasses during critical growth times (i.e. spring / summer) for several years to allow seeds to germinate and to allow the seedlings to grow big enough to out-compete the weeds. This may require pre-site treatments such as weed-suppressing mulches or the use of grass-specific herbicides.
- Having optimal climatic conditions during the first few years of plant growth. This is not something which can be easily controlled, but by undertaking a thorough site assessment prior to seeding, it is possible to identify the most suitable microsites where seedlings will be less vulnerable to adverse climate conditions.
Another key success factor is how to manage people’s expectations. Natural native forest restoration may take decades. However, there are growing expectations that restoration objectives should be achieved much more quickly than this. This may be due to a combination of funding timeframes (usually 3 years), election cycles and the ever-growing human cultural traits of impatience and the need for instant gratification. The human pace of life may have sped up, but native species have not evolved at the same pace.
Restoration projects which involve planting nursery-raised seedlings can provide instant gratification when the seedlings are planted in the ground, but seed-based restoration projects require a lot more patience. It is vitally important that the expectations of funders, landowners, volunteers, peer groups, employees and stakeholders are managed effectively with ongoing education.
By focusing restoration efforts on “seed islands” rather than diluting efforts across the whole site, localised success is far more likely, which helps to keep everyone involved engaged. It’s more rewarding to take people to an island where a blanket of emerging seedlings can be seen, rather than scouring the whole site for the occasional seedling emerging here and there. By keeping people engaged, more seed islands can be added in subsequent years when seed becomes locally available, therefore accelerating and maintaining momentum of the restoration project.
Full results of the Wakatipu Beech Seeding Project and a several helpful resources to assist other groups seeking to embark on seed-based restoration projects are available on both the Wakatipu Wilding Conifer Control Group website and the Wakatipu Reforestation Trust website.
Article submitted by Hilary Lennox