Fire Ecology of Australian Eucalypts :: Geology Nature Essays

Fire Ecology of Australian Eucalypts Eucalypts belong to the myrtle family (Myrtaceae), which are evergreen ‘tropical’ rainforest trees (Bowman 2000). Three genera are considered eucalypts: Eucalyptus, Angophora, and Corymbia. Roughly 600+ species of eucalypts exist today, and nearly all are endemic to Australia (farrer.riv.csu.edu.au). Although eucalypts began as members of the rainforest, the pressures of poor soil, increasing aridity and most importantly recurrent fires pushed them out of the rainforests and on to become the dominant species in a harsh land. Today, fire promoting traits such as volatile leaf oils, copious litter production, and highly flammable bark allow eucalypts to out-compete rainforest species for prime sites. Geologic and Vegetative History of Australia Before 250 mya, all continents were connected into a super-continent now called Pangaea. About 250 mya, Pangaea split into two continents—Laurasia (North America, Asia, and Europe) became the northern continent and Gondwana (South America, Africa, India, Antarctica, and Australia) the southern continent. At around 145 mya, the angiosperms arose in the center of Gondwana and spread outward. The continent of Gondwana began fragmenting around 120 mya, with the break-off of India; Australia began its northward trek about 45 mya. At the time of its departure, a Gondwanic rainforest dominated by araucarias (early gymnosperms) and Nothofagus (southern beeches) covered Australia; the forests also contained members of the Myrtaceae family, from which the eucalypts arose about 34 mya. Soil cores from this period show very high counts in Nothofagus pollen, and very low levels of charcoal (Kershaw et al. 2002). While the presence of fire was felt in the ancient rainforest, its affects were relatively minor and infrequently felt. This forest covered most of Australia until the mid-Oligocene, roughly 28 mya. The rainforest may have endured if the climate of Australia had not undergone dramatic climate changes during this time. As the continent moved towards the northeast at a rate of 6-7 cm yr-1, it grew increasingly more arid. A quiet geologic history resulted in a lack of tall mountain ranges, and the continent could not capture the moisture of incoming oceanic winds. Dry climate began to take its toll upon the Gondwana rainforest; soil cores from the mid-Oligocene to the mid-Miocene show a gradual decrease in Nothofagus pollen, and a gradual increase in Myrtaceae pollen counts. This is most likely explained as a move towards a warmer or more seasonal rainforest, due to the lack of grass species in Australia and low charcoal levels (Kershaw et al.