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Flora
The Flora of the Jarrah Forest
The jarrah forest is a dry sclerophyll open forest to woodland, unusual in Australia in that it is mostly dominated by a mixture of only two eucalypts over its entire extent, jarrah (Eucalyptus marginata) and marri (Corymbia calophylla). In addition there is a small-tree component, with bull banksia (Banksia grandis), sheoak (Allocasuarina fraseriana), snottygobble (Persoonia longifolia and Persoonia elliptica). The undergrowth consists of sclerophyllous shrubs to 3m high predominantly from the families Anthericaceae, Dasypogonaceae, Leguminosae, Orchidaceae, Apiaceae, Epacridaceae, Asteraceae, Proteaceae, Restionaceae, Myrtaceae and Cyperaceae.
The south-west of Western Australia is an area of exceptionally high plant diversity, and is considered one of the world’s biodiversity hotspots. In contrast to most of the world’s forests, the plant diversity of the jarrah forest is found in the understorey vegetation. Although the entire northern jarrah forest only contains approximately 20 indigenous tree species, the flora is estimated to contain at least 784 species. This estimate includes swamps, streams and rocky outcrops. In the upland areas where Alcoa’s mining occurs, the flora contains 300 to 400 non-weed plant species.
The northern jarrah forest has been classified into site-vegetation types (see the table for a description of some of these). Alcoa uses a similar system to survey all areas to be mined. Alcoa usually mines in upland areas of the forest in site-vegetation types S, P, T and combinations of these types (see explanatory table of site vegetation types and graphs).
Pre-Mining Flora Surveys
Broadscale Flora Surveys
Several years before mining activities are commenced the whole region of a future crusher is mapped on a 120m x 120m grid system. This is carried out by Botanical Consultants. The forest is mapped according to the Havel Site Vegetation Types (see introduction about the flora of the jarrah forest) and any rare or priority flora are also identified. Any threatened ecological communities are identified and recorded. Full flora species lists are produced which are used when choosing plant species to be used for rehabilitation of the areas when mining is finished.
Forest Species Control Plots
Once the broadscale flora monitoring has been carried out, detailed vegetation control plots are permanently established in forest adjacent to mining. These plots are located in the site vegetation types typical of the areas planned for mining. Within these plots all trees are identified and measured in the large 20m x 20m area and the abundance and cover of all understorey plants are recorded in twenty 2mx2m square quadrats; a total of 80 square meters within each plot. These plots are re-monitored every five years and are used to compare with identical plots in the rehabilitated mine pits. It is from these comparisons that we measure our performance against our species richness target in rehabilitated areas.
The abundance of plant species in these forest plots is used to select the important plant species for rehabilitation and to decide on how much seed to use, or what planting density to aim for in the rehabilitated areas. The most abundant species in the unmined jarrah forest are usually the highest priority species for returning to rehabilitated areas.
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