Fragments of rainforest survive along the banks of Ourimbah Creek. Prior to the 1880s, it grew wherever there was deep, fertile soil, that is, right across the floodplain. This is where the roots of trees could access water during drought. Fortunately, when this forest was cleared, it was not entirely eradicated. In fact, during World War II, farm labour became so scarce that some previously-cleared land was re-invaded by the bush.
The hills surrounding the Ourimbah Creek valley were also logged, but timber getting did not eradicate so much as clearing for agriculture did. For this reason, the hills are still covered by native forest. However, this is, for the most part, not rainforest, but a more open forest dominated by eucalypts. In the heat of summer, you would keep cooler in the deeper shade of the valley rainforest rather than in this gum forest.
In bushfire weather, you would also be safer in the rainforest. Eucalypts have evolved to encourage fire. They cast branches and bark and this accumulates on the forest floor year after year until a bushfire arrives to consume it. Rainforest trees also lose branches, but their wood rots quickly and isn't so inflammable. Litter under rainforest doesn't present anything like the fire danger that it does under gum forest.
Fields on the fertile soils near the creek only have to be neglected for a few years for some sort of forest to invade again.Two quick-growing eucalypts can compete with the rainforest species in this - the blue gums, Eucalyptus saligna and E. deanii. These gums grow into huge trees but their seedlings can't grow under a rainforest canopy - it's too shady.
In contrast to these eucalypts, the rainforest species can tolerate shade as seedlings, although to varying extents. Red ash (Alphitonia excelsa), pencil cedar (Polyscias murrayii) and bleeding heart (Omalanthus populifolius) are at one end of the tolerance spectrum, in that their seedlings survive best where there is a gap in the canopy. On the other hand, species like maiden's blush (Sloanea australis), brown beech (Cryptocarya glaucescens) and native tamarind (Diploglottis cunninghamii) not only tolerate shade, but require it. Their seedlings can all too easily be killed by being exposed to the sun on a hot day in midsummer.
Just as different species of rainforest trees tolerate shade to varying extents, so they tolerate floods differently. The alluvial soils along Ourimbah Creek were formed from the silt that is deposited by each flood. These floods continue. Palms can survive for several days with their roots under water, but pencil cedar (Polyscias murrayii) and the kangaroo apple (Solanum aviculare) are much more sensitive and more likely to die. These kind of differences, with a host of others factors, ensure that each patch of forest ends up having a different mix of species. Superficially, it may look uniform, but in fact it is far from being so. And then, every tree species has a different and specific set of insects that it supports. In fact, all the living things that the vegetation supports, whether fungi, frogs or furry animals, form a complex and varying mosaic throughout the forest.
Bleeding heart (Omalanthus populifolius) needs a gap in the canopy for its seedlings to develop. It is killed by frost, however and in the frosty environment of Ourimbah Creek it needs just enough cover to keep any frost off its leaves. Its fruit attracts brown pigeons (Macropygia amboinensis), often with a dominant bird constantly having to drive off rivals.
Bangalow palms (Archontophoenix cunninghamiana) can form almost pure stands where floods are frequent. Cattle can exterminate them, however. Palms will come back to protect the creek banks from erosion if the creek is fenced. Our Catchment Management Authority can arrange grants to farmers who wish to protect the creek in this way.