Cyathea medullaris, commonly known as mamaku or black tree fern, is a large tree fern up to 20 m tall. It is distributed across the south-west Pacific from Fiji to Pitcairn Island. Its other Māori names include katātā, kōrau, or pītau.


The trunk is black and covered with distinctive hexagonal stipe bases. The fronds may be up to 5 m long, and arch upwards from the crown. Dead fronds are shed except in very young plants. The primary pinnae are from 40 cm to 1 m long, and the undersides have scales with spines along their margins. The stipes are thick, black, very rough to the touch, and are similarly covered in black scales with marginal spines. C. medullaris can be readily distinguished from related species by the hexagonal stipe scars on the trunk, and by the scales with spines on their margins.


C. medullaris is common in lowland forest throughout the North Island of New Zealand. In the South Island its distribution is more localised. It is fairly common in wetter coastal areas, but rare in the drier eastern parts and absent in Canterbury and Otago. In New Zealand it also occurs on the Three Kings Islands in the far north, on Stewart Island/Rakiura in the far south and in the Chatham Islands. Its distribution also includes Fiji, the Marquesas Islands, Tahiti, the Austral Islands, and Pitcairn Island. It is not present in the Kermadecs.