Tuesday, 26 June 2012

Wednesday wildflower: she-oak

A planted grove of Casuarina cunninghamiana at Sunnynook Park, Auckland (Karly Garnock-Jones—photo)
I don't think there's such a thing as a he-oak, but there are she-oaks; they're a largely Australian plant family, Casuarinaceae, found also in Malesia (Malaysia, Indonesia, New Guinea) and many Pacific islands.  They're in the same order as the beeches, oaks, and chestnuts, but they don't look a lot like them.  The name she-oak is a reference to the hard figured oak-like timber; nothing else about the plant is oak-like.  Two species of Casuarina are naturalised in New Zealand, C. glauca and C. cunninghamiana.

They are flowering plants, but they look very like pines until you get close.  The branches are slender and have whorls of tiny leaves, so that each branchlet looks like a pine needle. Nearly all the photosynthesis probably takes place in these green slender grooved stems.

 The fruits are woody and arranged in clusters that look like little pine cones.  The female flower clusters even look a bit like young female pine cones in their first year.

The flowers of Casuarina are wind pollinated and unisexual, in clusters on separate male and female branches, or even on separate trees.  The fruits are small 1-seeded nutlets, but they're clustered together in woody pine-cone-like aggregations (the one below is about 10 mm diameter).
Casuarina cunninghamiana fruit cluster, after the nutlets have been dispersed.
Casuarina used to be in New Zealand as a native tree: here's a fossil fruit cluster from Bannockburn in Otago, in sediments of Miocene age.
It's one of a suite of Australian plants that flourished here when the climate was warmer, topography was flatter, and soils were infertile, rather like Australia today.  Mountain-building in the Pliocene and Pleistocene put an end to that version of New Zealand. Acacia, Eucalyptus, Casuarina, and many Proteaceae went extinct, and habitats were formed that suited the modern alpine flora.

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