When we think of Grevillea, we usually think of spiny-leafed shrubs growing in dry environments. However, there are a number of Grevillea species that grow as trees in northern Australia. The White Oak, or Bailey’s Oak, Grevillea baileyana, is one of these and grows in or on the margins of tropical rainforests of far north-eastern Queensland and New Guinea. It is a member of the family Proteaceae, and you can see the similarity to other tropical Australian rainforest Proteaceae, such as the Ivory Curl Tree (Buckinghamia celsissima) and Macadamia nut trees.

Grevillea baileyana has become quite a popular garden tree in recent years and the very large, lobed leaves with golden bronze undersurface are sought after by florists. The timber is dense, grained, white to pale pink and is used for doors, for cabinet making and for wood turning. However, its popularity, especially in SE Queensland, comes with a caution that it may become an invasive species, particularly in disturbed, wetter areas adjoining rainforests, wet sclerophyll forests and swamps. Look for White Oak on the northern side of Building E8C.

Tropical rainforests are usually characterised by having buttressed roots supporting tall leafy trees, shading the forest floor where few plants grow. They are usually found on very poor soils, harvesting their mineral requirements by nutrient cycling from litter on the forest floor. They are unique in their great species diversity. Within any given area, there may be a vast number of trees, but very few individual trees of the same species.

About the end of the Cretaceous (66 mya) and the beginning of the Paleogene (for old timers, this is the new term for the early part of the Tertiary), Australia was warmer and wetter than it is today, and about 85% of the continent was covered by rainforest; these forests included the ancestors of our present day Proteaceae.

When Australia separated from Antarctica and drifted north, the climate became drier, the rainforests retreated, covering only about 22% of Australia at the time of colonisation in 1788, and much less now as a consequence of logging and clearing for agriculture. As the continent dried, Grevillea species adapted to the dry conditions by growing as shrubs rather than trees and by reducing the size and the shape of their leaves, often to sharp, tough, needle- like leaves that not only minimise water loss but also deter insect and marsupial predators.

Photograph of flowers: K Townsend, Atlas of Living Australia: http://bie.ala.org.au/species/GREVILLEA+BAILEYANA Map modified from Atlas of Living Australia: http://bie.ala.org.au/species/GREVILLEA+BAILEYANA

Alison Downing, Kevin Downing, Brian Atwell Department of Biological Sciences Monday, 19th May 2014