Fragments of rainforest survive all along the banks of Ourimbah Creek. Prior to the 1880s, it grew wherever there was deep, fertile soil and where the roots of trees could access water during drought. Fortunately, while this forest was largely cleared, it was not completely 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 logged, but not cleared to the extent that the alluvial flats in the valley were. As a result, they are still covered by the original native forest. However, this is, for the most part, not rainforest, but rather a more open forest dominated by eucalypts. In the heat of summer, you would keep cooler in the deep 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 is not so inflammable, so that litter doesn't present anything like the fire danger that it does in the gum forest. On the deep soils along the creek that were cleared for agriculture, a field only has to be left to itself 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 they cannot produce a seedling generation under a rainforest canopy - it is too shady. In contrast to these eucalypts, all the rainforest species can tolerate shade as seedlings, although to varying extents. Red ash (Alphitonia excelsa), pencil cedar (Polyscias murrayii) and bleeding heart 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 their tolerance of other stresses also varies. The alluvial soils along Ourimbah Creek were formed by floods that over many years have deposited their silt from the overflowing creek bed. 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 will very likely die as a result. Differences in flood level and the fruiting of rainforest trees, with a host of others factors, ensure that each patch of forest has a different mix of species. Superficially, the rainforest may look uniform, but in fact it is far from being so. The variation in tree species is presumably mirrored by all the other living things that the vegetation supports, whether fungi, frogs or furry animals. |  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 fruits attract 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. |
The rainforest |