Many of the dominant jarrah forest understorey plant species are “obligate resprouters” and over thousands or millions of years of evolution have abandoned seed as a method of reproduction. They do not produce viable seed, or if they do it is difficult to collect, or does not readily germinate. As a result, some of these species are under-represented in rehabilitated areas compared with the forest vegetation because they do not readily establish from the natural seed in the topsoil that is returned or from applied seed.
These difficult plants are known as recalcitrant species, and they often play an important role in the jarrah forest ecosystem. For example, common dry land sedges are a favourite food source for kangaroos in the rehabilitated areas. They also resprout quickly and vigorously after disturbances such as fire, and hence give the forest resilience to such disturbances.
Several process are used to overcome this issue in rehabilitation:
Tissue culture or micro propagation is the most difficult and expensive method of propagating “recalcitrant” plant species for mine rehabilitation. This method is used when other methods such as seeds or cuttings fail. Instead of seed, these ancient species grow and spread by sending out vegetative shoots and underground rhizomes and bulbs. These vegetative adaptations are ideally suited to recovering from fires, grazing and drought, which are regular occurrences in the natural jarrah forest. However it does make them difficult to return to mined areas.
Because no-one has propagated these species before, it can take two or more years to determine the best growing methods to get each species into regular production.
Tissue culture is essentially growing plant shoots in a sterile, controlled environment, usually in sealed jars. The plant shoots are grown on media which contains nutrients for plant growth; minerals, amino acids, vitamins, hormones, sugar and water – all set in a jelly called agar. Every four weeks the plant material is divided and placed into fresh agar and within a few months thousands of plants can be produced. It is particularly effective for plants which do not grow well from seeds but which grow vegetatively.
Alcoa’s tissue culture laboratory contains:
- A media preparation room with autoclave for sterilising equipment and media.
- Two culture rooms with diffuse light and controlled temperatures for growing shoots.
- A room with laminar flow cabinets that blow clean filtered air over the work area.
Once adequate plant numbers have been grown in agar they are removed and placed in soil trays and put into a misting room. This allows acclimatization of the very delicate plants before they are put out into the open nursery, and then planted out in the mined areas.
Approximately 100,000 plants are grown from tissue culture each year for planting into rehabilitated mined areas. Many of the dryland rushes and sedges and other grass-like species are grown from tissue culture.
Tissue culture can produce many hundreds or thousands of plants from one mother plant, which means these plants are all genetically identical, or the same clone. To ensure we are not compromising genetic diversity Alcoa aims to have at least 10 clones of each species for each mine planted into rehabilitated areas each year.
For some jarrah forest plant species, seed does germinate but is difficult to collect and only small amounts can be obtained. It is more efficient to use this seed to produce seedlings in the nursery and plant them out, rather than simply use the seed in the broadcast seed mix, where the number of successful seedlings will be much lower.
Species that require complicated germination treatments that cannot be applied to large amounts of seed may also be grown as seedlings and planted out. This includes the Snottygobble (Persoonia longifolia), a small native tree that is common in the jarrah forest. Some other plant species will germinate well from broadcast seeds but die back each summer and then resprout from an underground tuber when the following winter rains arrive. We have found that if the tuber is too small the plant cannot resprout and hence most of these seedlings are lost in the second year. For these tuberous species it is best to grow them to a reasonable size in the nursery, then plant them out in the rehabilitated areas. Provided the tuber is large enough, they can resprout each winter and successfully grow in the rehabilitated areas. Old man’s Beard (Clematis pubescens), Native Gerbera (Tricocline spathulata) and some grasstrees (Xanthorrhoea) are grown this way.
Approximately 50,000 plants are grown from seed and planted each year in rehabilitated mine pits.
Approximately 50,000 plants are grown from cuttings each year at the Marrinup nursery for planting into Alcoa’s rehabilitated bauxite mines. Cuttings are used for propagation of species that do not return well from the natural soil seedbank, or where seed is unavailable or does not germinate after known treatments. Species grown from cuttings are from the Goodeniaceae family including the Blue Lechenaultia and Dampiera. Native buttercups (Hibbertia species) also grow well from cuttings.
Fresh shoot material is taken from wild populations in spring each year (with appropriate collecting permits). The shoots are immediately placed in water, quickly transported to the nursery, trimmed, dipped in rooting hormone powder and placed in soil. If all goes well these plants will be ready for planting the following winter.
The best shoots for cuttings are from plants that are resprouting after being burnt, so collections are made from recent control burns or wildfire areas.