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. This process is intensive and laborious.
More recently, there have been studies into restoration methodolgies using seed rather than seedlings. Seed can be broadcast more easily over a greater area and the costs associated with raising seedlings in a nursery are eliminated. However, many of the management strategies required to ensure success of nursery-grown seedlings apply equally, if not more so, in the management of field-germinated seedlings.
Unfortunately, many recent trials have been focused on innovative ways to broadcast the seeds (e.g. using drones) and little attention has been given to the myriad of environmental factors which must be managed if a seed-based restoration project is to be successful.
This is not a new revelation. Guiding principles are articulated in Native Forest Restoration by Tim Porteous, which was first published in 1993, as well as many other earlier papers and publications.
So, if we’ve known of these principles for a long time then why aren’t they being consistently applied? These critical success factors are discussed in more detail in the following sections.
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 nursey 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, which 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, and so there needs to be enough seed broadcast to account for this. 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). These controls measures will need to be maintained for several years. Rodents may also need to be controlled depending on the type of seed being used and the size of the rodent population.
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 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 weather conditions.
Seedlings are most vulnerable during the first 2 - 3 years of their life. The advantage of doing a restoration project using seedlings rather than seed, is that the seedlings spend the first 1 - 2 years of their life under very controlled conditions with regular care, so the strike and survival rates are higher. Seedlings raised in the field are not raised in controlled conditions and so the strike and survival rates are generally a lot lower (although those that do survive are arguable better adapted than nursery stock). It could be argued that the field management considerations listed above are, therefore, even more critical for a seed-based restoration project.
Any restoration project will span many years so careful planning is essential to ensure that all of these success factors are thought through and that adequate resources have been allocated. Having to manage these factors means that seed-based restoration projects may not feel much quicker or cheaper than using nursery-raised seedlings. However, it must be noted that when you’re dealing with a landscape that has been altered by hundreds of years of human activity, then you may no longer have a suitable habitat for certain native species. You are, essentially, trying to establish native species into what is now a foreign environment.
For example, it may seem logical that beech trees should be established on a site if that’s what used to grow there 600 years ago. However, that site will likely be very different from what it was 600 years ago. Clearing, burning, subsequent soil erosion, invasion by exotic species and their symbiotic mycorrhizae, other changes to the microbiology and chemistry of the soil, and the introduction of pest animal species are just some of the reasons why the site may now be unsuitable for the establishment of native beech trees. An initial site assessment and suitable species selection may, therefore, be two of the most important success factors for such a restoration project.
It may be that a fast growing, easy to collect nursery crop, such as mānuka, coprosma or pittosporum species, is established first to help create more suitable onsite conditions for the species which is ultimately desired. Mānuka and kānuka seed, for example, is easy to collect and disperse, has a wide ecological tolerance, vigorous growth, and is able to colonise some of the more inhospitable sites. Branches laden with seed pods can be collected at any time of the year and laid across a site that has been adequately prepared. The seed pods eventually pop open, dispersing vast quantities of seed onto the ground. The cut branches provide shelter, temperature and humidity regulation, and protection for emerging seedlings. However, a closed canopy that provides a shaded understory can be achieved more quickly with coprosma and pittosporum species and so a mix of these species as a nursery crop may be preferred.
There are some examples in areas with higher rainfall where exotic species have been used as a successful nursery crop too, such as the use of gorse as a nursery crop at Hinewai Reserve on Banks Peninsula.
Another very important key success factor for any restoration projects is managing people’s expectations. This is discussed in more detail on the Seed Islands page.