Vegetation succession - or vegetation development, as may be more accurate - is a key element of the restoration process, and one that scientists have been trying to understand for many years. One debate, which is significant for revegetators, is over the relative importance of relay floristics and initial floristic composition (IFC). Relay floristics describes the process whereby one group of plants replaces another, usually through colonisation, in an orderly and predictable fashion. Each subsequent stage of plants is by nature dependent on the previous stage to establish. IFC is simply the idea that propagules of all or almost all of the species that develop over time at a site are present at the site from the start. It is the different environmental tolerances and life histories of each species that determines the order and timing of their emergence. Several studies of vegetation development after fire, at rehabilitated mines and at abandoned fields have found that the IFC model seems to be more relevant than the relay floristics model (e.g. Egler 1954; Grant & Loneragan 2001; Koch 2007). If this is in fact true, it is important for the revegetator to manipulate what seeds and propagules exist at the start of the restoration, allowing time and opportunity to guide the development of native vegetation at the site. Careful management is still necessary, because the seeds of undesirable species can be deposited by birds (e.g. blackberry) or invade the site by wind or water (e.g. exotic grasses, herbs).
Developing functional diversity, redundancy and resilience
Even when the IFC model is operating, field studies of restoration sites at different ages suggest that it is difficult or impossible to predict the trajectory of succession and species composition at restoration sites (Choi 2004). It must be borne in mind that it is unlikely that any restoration will succeed in fully replicating the pre-disturbance ecosystem in function or species composition. There are several reasons for this, including - but not limited to - incomplete knowledge of pre-disturbance ecosystems, and social, economic and technological barriers. Even with complete knowledge, unlimited funding and the blessing of the community, the dynamic nature of ecosystems, disturbance and climate means "soil and vegetation development takes place today under vastly altered conditions (e.g. air and water quality, chemical contamination, contiguity to seed sources) than when the original ecosystems developed" (Wali 1999).
Functional diversity, redundancy and resilience are all terms that are often poorly understood and therefore difficult to achieve in practice. Functional diversity and a degree of redundancy are necessary components of resilience - i.e. the ability of a system to absorb external disturbances. In a resilient system, not only are all the species that perform key functions in the ecosystem present (functional diversity), but ideally there will be several different species performing any given function (redundancy). Understanding the key functions and the species that perform those functions is an excellent first step toward building a resilient, self-sustaining ecosystem.
Developing species richness and the understorey
One way to enhance the likely resilience in the revegetated area is to maximise its species richness (Tilman 1996). For many years, revegetation in Australia has been conducted using almost solely tree species, and the occasional shrub species. The assumption was that native understorey would eventually colonise the site (relay floristics) once the canopy had formed and shaded out the weeds. With the exception of tropical systems, however, most studies looking at vegetation development at revegetation sites have concluded that the vegetation follows the IFC model described above (e.g. Purdie & Slayter 1976, Koch 2007, Munro et al. 2008). It is much more common for native understorey to not come back unless specifically planted as part of the restoration project. The greatest likelihood of colonisation by native understorey (woody and herbaceous) will be at sites adjacent to high-quality remnant vegetation. This of course depends on the means of seed dispersal by the desired species.
Another reason that herbaceous understorey plants are often not included in revegetation activities is the scarcity of seed for these species. This scarcity is not only logistical, but also because they typically have not been used in revegetation. This has been a self-perpetuating problem, although recently, there has been a greater effort to increase seed production of herbaceous species. Forbs and grasses make up most of the species diversity in many native Australian plant communities, serve several useful functions, and therefore should be an important part of patch- to landscape-scale revegetation plans.
Getting real: matching resources to the restoration
Some mining companies such as Alcoa spend a great deal of time, money and effort to rehabilitate their mine pits after mining operations have ceased. For example, in Western Australia Alcoa uses topsoil from an adjacent or nearby site (about to be mined) for a site that is in the process of rehabilitation. It also direct seeds with a seed mix of between 80-110 species (compared with 10-20 species typical of many revegetation projects), and replaces rocks and logs for animal habitat. For the so-called 'recalcitrant' species, which do not return from the seeding or topsoil operations, Alcoa grows these plants from cuttings, from scarce seed quantities, or by tissue culture (Koch, 2007) . Alcoa also has what is arguably the longest monitoring program and most comprehensive data set for its restoration activities, not only because it is mandated to carry out these activities, but because it is not operating on shoestring budgets and three-year contracts (unlike many restoration projects outside the mining sector).
Clearly, most restoration activities lack the time, budget and personnel to carry out this calibre of restoration. Different revegetation options [link here to pros/cons table] come with different price tags, and consequently, different results. Careful consideration of the objectives and the scale of the revegetation project will help guide the set of actions taken.
What this means on the ground
If the idea of IFC is a more accurate model of reality, then this is good news for the revegetator. It means that, to the degree possible, revegetation activities can be based on maximising the number of species to be introduced at the start. The main considerations, then, are logistical in nature. For example, any direct seeding activities should occur before tubestock planting. The downside of many ecosystems following the IFC model, however, is that the weed seed bank must be controlled prior to seeding or planting. This is especially true when a native ground layer is important to the project's objectives. Two or more years of herbicide treatment, cover crops, fire or scalping may then be required to fully control weeds.
A thorough understanding of the species involved in revegetation is extremely important. Using the Alcoa model, knowing which species already exist in the seedbank, which species germinate easily and which require special treatments, which species re-sprout from roots or rhizomes, which species require heat or smoke to regenerate, and which species have mycorrhizal relationships will lead to greater success on the ground. Research into native Australian species is ongoing, but information does exist for many species. Consult your local herbarium or the Florabank website to find information for specific plants.
Every revegetation site will experience some plant mortality. Except in projects that only use tubestock planting, controlling the spacing of plants is almost impossible. A better strategy than trying to mimic spacing of plants is to try to mimic the relative abundance and spatial patterns of plants in the plant and seed mix. In nature, many more seeds, often by several orders of magnitude, are created than germinate and survive to adulthood. Although the germinability of many Australian species is still poorly understood, an observation of the number of seeds typically generated by a species, compared with the number of plants of that species in a reference system, will give a rough idea of the germinability, survivability, and competitiveness of a particular species' seeds. It may be easier to thin over-represented species at a revegetated site than to try to add them to an area.
Finally, the complexity and species diversity of a revegetation project should depend on the scale of the project. A project that encompasses many hundreds of kilometres will be carried out and assessed at a much larger spatial grain than a project of a few hectares. Very large projects such as Gondwana Link achieve diversity and resilience through heterogeneity at a landscape scale, through many different smaller projects. In contrast, revegetation for biodiversity at the scale of a few hectares will achieve diversity and resilience through structural complexity and species richness at the patch scale.