59 million years ago in the wet tropical rainforests that covered most of Australia during this time, a probably already sclerophyllous tree began to diverge into 2 lineages, the first, ending in the now tropical rainforest genera Allosyncarpia, Stockwellia and Eucalyptopsis, and the second ending in the more arid adapted Arillastrum, Corymbia, Eucalyptus and Angophora. The last of these, Angophora, is a genus containing 10-13 species of tree that can be found across the east-coast of Australia.
In the more recent geological past, a common ancestor of all Angophora's, which likely had a broad distribution, was separated, probably by environmental factors, into 2 isolated populations. These 2 populations began to adapt to their local environment and eventually speciate from one another. Further isolation events, over however many millions of years, eventually resulted in the evolution of the Angophora species we see today. In one of these speciation events, a population of Angophora's was isolated in a more open and exposed woodland, which naturally imposed less competition for sunlight from neighbouring trees. Under the pressure of this environment, individual trees that grew shorter survived better, maybe because they were less exposed to sunlight and therefore preserved more water by reducing transpiration. Since being shorter was an advantage, after countless generations a dwarf form of Angophora slowly began to emerge, which we now call Angophora hispida (dwarf apple). A. hispidia grows to 6m in height, which is remarkably small compared to other Angophora species:
A. costata = 20m
A. bakeri = 20m
A. leiocarpa = 20m
A. floribunda = 30m
A. melonoxylon = 15m
A. subveluntia = 25m
A. woodsiana = 20m
Although, 2 other species of Angophora could also be regarded as 'dwarf', A. inopina and A. exul grow to 8m, and A. robur grows to 10m (A. robur is very similar to A. hispida with A. hispida having adult leaves with a rounded apex and A. robur having adult leaves with an acute apex). The explanation for dwarf forms of these species may be similar to that of A. hispida.
A. hispida now occurs naturally on scrubby ridges (heath, woodland and dry sclerophyll forest) on parts of the Hawkesbury sandstone in and around the Sydney basin bioregion. River systems from the mountains of great dividing range empty into the flood plain that is the Sydney basin. During the Permian/Triassic, sand accumulated in the basin, maybe as a result of increased sea levels and river deposition, resulting in the formation of the Hawkesbury sandstone. A. hispida (and a lot of other species of Angophora) prefer these dry sandstone/sandy soils poor in nutrients and grows from sea level to about 300m.
However, the uniqueness of this species continues... A. hispida is neotenous, meaning it retains the juvenile foliage as a mature adult plant (2 other Angophora species, A. melanoxylon and A. subveluntia are also neotenous, or at least partly - leaf venation is used to determine neoteny). This condition can be recognised on A. hispida by the presence of a dense layer of red 'hispid' trichomes covering the stems, upper branches and inflorescences/fruit, which usually only occur on juvenile plant parts. Upon closer inspection, these trichomes are actually oil glands or 'bristle glands' - essentially protruding oil glands with a multicellular wall, think of eucalyptus oil glands but elongated on a cellular stalk. The need to retain these bristle glands past the juvenile stage could be a defensive mechanism, as the glands are full of volatile oil, possibly to ward off insects from the chewing leaves or laying eggs in flower buds. Does this suggest an abundance of insects where it evolved, or maybe just 1 prolific (or new) species of insect that demanded the need for such an extensive defence? Alternatively the glands might have something to do with the role of enhancing fire, especially around the fruiting pods to release the seed (A. hispida is heavily fire adapted like the rest of the eucalypts).
