Plant the Month for August is Callitris rhomboidea R.Br. (Oyster Bay pine), which occurs in DEWNR‘s Park of the Month, Onkaparinga River National Park. It is a conifer from the large and cosmopolitan Cypress family (Cupressaceae), which includes many important timber (e.g. Sequoia sp., the Redwoods of North America) and horticultural species (e.g. Cryptomeria, from Japan). Callitris includes approximately 16 species, 13 of which are restricted to Australia. The majority of these occur in heath and woodlands, extending into semi-arid areas. Recent molecular research suggests that Callitris evolved from rainforest ancestors in response to a drying climate over the past 30 million years (Larter et al. 2017) and in contrast to other Australian conifers, they are remarkably drought tolerant, allowing them to thrive in arid conditions.
This species is commonly referred to as the Oyster Bay pine, in reference to its occurrence on the Georges River, in the vicinity of Sydney (also known as Port Jackson pine). Callitris rhomboidea extends from coastal southern Queensland through New South Wales, Victoria and Tasmania. In South Australia, it occurs chiefly on Kangaroo Island and a few scattered localities in the Mount Lofty Ranges, including the Onkaparinga River Gorge, and the Mount Crawford region to the north. While it is widely distributed, Oyster Bay pine is only locally common, occurring on sand dunes and in rocky areas in heath and woodlands. Callitris are intolerant of fire and preferentially grow in areas that are protected by topography or slow rates of fuel accumulation. However, the Oyster Bay pine can readily regenerate from seed, which are held in the thick woody cones that cluster on fruiting branches. Like many plant species of Australia’s fire prone woodlands (for example, Banksia), fire may act as a stimulus for seed release followed by mass germination and generation of dense, even-aged stands.
There are two species of Callitris occurring within the Onkaparinga River National Park. Callitris rhomboidea is perhaps best distinguished from Callitris gracilis (Slender Cypress-pine) by the shape of the cone scales on the female cone, which are rhomboidal (hence the name, rhomboidea) and have a distinct point near the apex.
Callitris rhomboidea is ‘Rare’ within South Australia, and has declined in parts of its range as a consequence of land clearing, grazing and weed competition. While the level of decline is insufficient to warrant listing as ‘Threatened’, factors such as high frequency fires and susceptibility to ‘die-back‘ (caused by the fungal pathogen, Phytophthora cinnamomi) are potential future concerns (IUCN Red List 2017).
Contributed by State Herbarium molecular botanist Ed Biffin.