Yule Brook and the Greater Brixton Wetlands

Our speaker was Hans Lambers, Professor of Plant Biology at the UWA School of Biological Sciences. Hans has worked in several European and Australian universities. His main interests are phosphorus nutrition in native plants, food crops and pasture plants, as well as mine-site rehabilitation.

Hans began by reminding us that the south-west of WA is a global hotspot for plant diversity, and that the term “hotspot” means not only that it contains a very large number of species but also that a great many of them are under threat of extinction. Surprisingly, he said that Greater Perth is one of the world’s most bio-diverse cities. An area that contributes greatly to that diversity is the Yule Brook and Greater Brixton St Wetlands, in the south-east corner of Greater Perth, near Kenwick. For example, the Greater Brixton Wetlands has an area of 126 ha but contains 558 native species, many of them on the threatened list. (This can be compared with the Fitzgerald River National Park, at 300,000ha with “only” 1478 species.)

The Yule/Brixton area contains some Declared Rare Species, including Andersonia gracilis and Grevillea thelemanniana. On its claypans there are many orchid species, and more species of carnivorous plants there than in the whole of Europe! Hans showed us an image of a leaf of Drosera glanduligera, the Pimpernel Sundew. The leaf’s “tentacles” were alternately sticky and non-sticky, and the plain ones were used to catapult insects into the sticky centre to be captured. Frogs and fungi are plentiful too, and the invertebrate fauna includes the Peacock Spider and the rare Toad Bug. Our Club carried out the preliminary fungi survey.

The question arises as to why such diversity occurs in this area, especially as these soils are amongst the worst in the world. One of the answers seems to be that very thing: that the greatest diversity of plants usually occurs on the poorest of soils. Hans says that this occurs around the world. Even in rainforests, the highest diversity occurs on the poorest soils. The Yule/Brixton area has severely impoverished soils, consisting of highly leached soils of the Bassendean Dune System, the “Muchea Limestone” (which is not true limestone!) and claypans (which have a little bit of clay over sand). Another reason for the high diversity in the South West is the legacy of having had a stable climate and lack of geological disturbance for a very long time, giving time for prolific speciation to occur.

Diversity at this particular location, however, is especially high because of the diversity of habitats and also because of the special hydrology of the area, having wetlands that depend not so much on surface water but on groundwater. The plants depend on minerals that arrive with this groundwater.

Across the south-west, diversity always increases with decreasing phosphorus concentration in the soil. Plants in the family Proteaceae favour phosphorus-deficient soils, such as the Bassendean Sands, because their cluster root systems are highly efficient in extracting phosphorus from soils, so they have a competitive advantage over other plants in those areas.

Hans talked about Grevillea thelemanniana, as an example of a rare and vulnerable plant in the wetland. This species, which is not found at all outside the study area, was found to be unusual among the proteaceous plants in having a higher phosphorus concentration. It also has a higher calcium concentration, and stores it in its epidemal cells. But it has a lower sodium concentration than other proteaceous plants because it excludes sodium when supplied with calcium. It grows on the ‘Muchea Limestone’ because that soil is high in calcium. Grevillea thelemanniana also needs abundant water. Using isotope signatures, it was found that their roots access groundwater at depths of 1.5m to 4.3m in winter and become stressed if water is not available, which sometimes occurs in summer. So soil type, hydrology and climate are crucially important to this species. It is rare and vulnerable because it needs wetlands with groundwater available all year round, and because it needs more phosphorus and more calcium than is available in most of our south-west soils.

In summary, the flora in the Yule/Brixton has very high diversity. It includes many species of special conservation value as well as several threatened ecological communities. This flora crucially depends on groundwater and it will be impacted negatively by a drying climate, but the sensitive hydrology of the area is poorly understood. Hans says that a 100m buffer zone is needed around the reserve. The proposed Yule Brook Regional Park will connect these wetlands with Lesmurdie Falls and the Canning River. Hans has edited a new book on this reserve to be published by the WA Naturalists’ Club and the Kwongan Foundation called A Jewel in the Crown of a Global Diversity Hotspot. Its cover has a beautiful illustration by Philippa Nikulinsky.

Mike Gregson