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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.
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During the mid-Miocene (~15 mya), soil cores show a sharp decline in Nothofagus pollen counts as the rainforests are subjected to more stresses and disturbances. By the end of the Miocene, Nothofagus pollen has almost completely disappeared from soil cores, and charcoal levels jump sharply. This occurs after the climatic change towards a drier environment, and a subsequent shift towards more flammable vegetation. Thus, unlike previous theories that state that frequent fire regimes brought about the shift to sclerophyllous plant dominance, soil cores show that climate and sclerophyllous plants facilitated frequent burning (Kershaw et al. 2002, Bowman 2000).
Events occurring during the Pleistocene and the Holocene are not as well known, due to the expensiveness of drilling in commercially worthless Quaternary sediments. However, the few soil cores show a major jump in charcoal levels between 30-40 ka without a substantial climatic change, which are indicated by switches between high and low levels of δ18O isotope values in marine sediments. High levels indicate periods of glaciation, while low levels indicate interglacial periods. It is hypothesized that this charcoal is an indicator of Aboriginal arrival in Australia, or their full inhabitation of the continent. There is actually a marked absence of charcoal between 50-60 ka, the accepted Aborigine arrival period according to archaeological evidence (Kershaw et al. 2002). Throughout the pre-European Holocene, there is a gradual increase in fire activity. This increase may be due to increased activity of the El Niño Southern Oscillation (ENSO). In Australia, the ENSO is responsible for droughts and heavy seasonal rains. Between 5-7 ka BP, soil cores show a retreat of eucalypt forests, and a return of rainforests; it is thought that an increase of rainfall and a decrease in seasonality was responsible for this phenomena. The arrival of Europeans is evidenced by a nearly four-fold increase of charcoal in soil cores, followed by a drop to lower levels than found during the Holocene (Kershaw et al. 2002).
Fire Regimes and Plant Responses
For a fire to start, two things are necessary: an ignition source, such as lightning or a firestick, and a minimum amount of sufficiently dry fuel. The interaction between ignition sources and fuel loads dictates a particular fire regime. A fire regime is a function of five components: frequency, season, extent or patchiness, type of fire, and fire intensity (Whelan 1995).
The way in which a plant species responds after fire exposure is often a reflection of the fire regime, and vice versa. Some species are considered fire ephemerals, which appear directly after a fire, bloom, and set seed before secondary succession fully begins. These species usually appear after infrequent, intense fires. The North American fireweed is a good example of a fire ephemeral, perhaps most well known from photos of post-fire Yellowstone Park. In Australia, terrestrial orchids might be considered fire ephemerals; after the Ash Wednesday fires of 1983 rare orchids blanketed the hillsides (Pyne 1991).
Some species are considered non-sprouters, and tend not to survive fires well; the few survivors experience severe growth setbacks, and recover slowly. Rainforest species, such as Nothofagus, are examples of non-sprouters. Some non-sprouters are considered obligate seeders—they require exposure to fire for seed dispersal (such as E. regnans) and survival of the species is dependent upon seed production. Non-sprouter species are found in environments which experience infrequent fires, perhaps only 100-150+ years (Bowman 2000, Hobbs 2002). These fires usually are quite intense and burn into the tree crowns, due to the heavy build-up of litter. Massive tracts of forest are fully burned to the ground—the Black Friday fires of January 13, 1939 devastated over 1 million hectares of wet sclerophyll forest (Gill 1978).
On the opposite end of the spectrum are sprouters, where most adult plants survive a fire and resprout from the roots or stems; many eucalypt species, especially those of the mallee—brushy, thicket-forming eucalypts—are considered sprouters. Seed germination is quite poor amongst sprouter species, even with the aid of the ashbed affect, and continuation of the species is dependent upon vegetative regrowth (Pyne 1991). Sprouter species are found where fire is common, usually occurring every 1-10 years. These fires typically start low to the ground, fueled by an abundance of grass, but proceed up the eucalypt tree trunks with the aid of bark streamers. Because they occur so often and consume nearly all available fuel, fires in the mallee regions are of lower intensity (in comparison to wet sclerophyll forests) and often patchy.
General Fire Protection in Plants
When considering the survival of individual plants, the intensity of the fire is the most important determinant. Some plant tissues, such as meristems and seeds, are susceptible to high temperatures even at very short durations. But relatively low temperatures can be just as detrimental when sustained for long periods of time. The most critical tissues of the plant are the cambium, which gives rise to the xylem and phloem, the ‘plumbing’ of vascular plants; meristems, or the actively growing tissues which allow the plant to grow upwards and outwards; and seeds, which are the insurance for a genetically unique generation. Plants of all species employ several tactics in order to survive fire, and protect critical tissues (Whelan 1995).
Fire survival begins at the cellular level. It has been found that resting tissues that are in a state of dehydration (such as is found at the end of the growing season) fare better than metabolically active and fully hydrated tissues. Dried and inactive tissues do not suffer the affects of ‘boiling’ and enzyme inactivation as greatly as hydrated, active tissues. Also, it has been found that scarified seeds, where the seed coat has been broken and water has been allowed to enter, suffer greater mortality than intact seeds. The explanation used above for plant tissue mortality applies here as well (Whelan 1995).
The next lines of defense are protection of critical tissues, and the prevention from heating tissues to dangerous temperatures. Plants use thick bark and/or cork to shield the cambium and deeply buried meristems known as epicormic shoots. Tightly packed leaf tissues surrounding apical buds and meristems act as insulation. Many plants, such as trees, merely grow so tall that heat from ground fires cannot affect their critical tissues. Others enclose their seeds inside heavy fruits, or use the leafy canopy to shelter seeds. Finally, a layer of soil can insulate buried seeds, which then become part of the seed bank (Whelan 1995).
Post-Fire Plant Responses
In the aftermath of a fire, several occurrences take place in the renewal of the vegetation. Many of these occurrences are facilitated by what is known as the “ashbed effect.” This is the interaction of the removal of competitors and choking litter, trace nutrient mobilization, soil sterilization, decreased surface albedo that warms soil, massive seed production, and burial in soft ash (Pyne 1991).
Plants that survive the fire can experience increased productivity; in mallee, or eucalyptus scrub, primary production increases 6-7% per year, for up to 35 years after a fire (Pyne 1991). This is caused by decreased albedo, nutrient increases, competition removal, and the reduction of seed production so that plants can focus on vegetative growth.
Increased flowering often occurs after a fire, as is demonstrated by the grass tree, Xanthorrhoea johnsonii, a common plant in some types of eucalyptus forests. Although it will most times flower in the Australian springtime, it is much more prolific following a bushfire—greenhouses will apply a blowtorch to the plant to stimulate the production of a flower spike. Generally, increased flowering is caused by the warming of the soil, chemical changes in the soil, and the removal of leaves and other tissues, such that the plant can focus upon seed production (Whelan 1995).
Seed dispersal also increases after a fire due to better wind flow through the fire-pruned canopy, and better water flow across the ground. Some plants have become bradysporous, where plants seeds are stored in the canopy and survive the fire inside woody fruits. They are then released afterwards, usually in massive amounts—a forest of mountain ash (Eucalyptus regnans) can produce around 14 million seeds ha-1 (Pyne 1991)!
In addition, a flush of seedlings often occurs after a fire, caused by increased seed dispersal, decreased herbivory, and the ashbed effect. The thinning of the overhead canopy also allows more sunlight to reach the ground and growing seedlings.
Eucalyptus Fire Adaptations
Most eucalypt species have acquired traits which allow them to promote fires and survive them and/or rapidly take over the newly-burned environment. Earlier theories stated that fires played a direct hand in the development of these traits; however, it is now thought that eucalypts were merely pre-adapted for fire survival and promotion (Bowman 2000).
Perhaps the most notable adaptation of eucalypts is the lignotuber. A lignotuber is a specialized root/crown structure located beneath the soil surface; it contains many food-storing cells and shoot-forming structures. Thus, they serve a dual function—they act as protection for tender shoots, and provide food for shoots emerging after a fire. Some eucalypts have lignotubers throughout the lifespan, like those of those of the mallee, while others lose them once they reach adulthood. Lignotubers are of ancient origin, and probably are not strongly related to fire or drought (Bowman 2000). More likely, they are an evolutionary response to frequent shoot/crown removal by something more constant than fire, such as repeated herbivory.
Epicormic shoots are another prominent adaptation, typical of sprouter species. Strands of epicormic bud tissues are located beneath leaf axils; when a leaf falls, the epicormic shoot develops until just below the bark surface. There, shoots are held in dormancy by hormones produced by the shoots and leaves above. When shoots and leaves are removed by herbivory, fire, or wind, so too are the hormonal checks, and the epicormic shoots quickly burst through the bark. Both epicormic shoots and lignotubers are typical of sprouter species (sres.anu.edu.au).
Bark, crown structure, and leaf characteristics of eucalypts all promote the kindling and spread of fires. Some eucalypts are termed “stringybarks” or “candlebarks;” their bark hangs in long, fibrous stringers which fire can easily light and spread upwards into the canopy. Convection winds can also whirl burning stringers into the air and carry them to other dry patches (Pyne 1991). Some eucalypts also shed their bark as it burns, preventing long-term heat exposure to the underlying cambium. Sprouter eucalypts tend to coppice, or grow into dense thickets, and produce pendulous leaves. Heavy brush production promotes intense fires, and the leaf arrangement funnels hot air upwards through the canopy. The leaves of all eucalypt species contain some amount of volatile oils. In a study conducted by McArthur and Cheney (cited by Whelan 1995), it was found that although eucalyptus wood does not burn particularly hot, the leaf oils burn nearly twice as hot as the wood (Table 1).
On the other hand, non-sprouter species tend towards bradyspory. Although fire is the signal for mass seed shedding in some eucalypt species, the act of bradyspory itself probably evolved as a form of herbivore satiation—producing so many seeds such that herbivores cannot possible eat them all. In addition, many eucalyptus species possess thick, woody seed pods which protect plant embryos from intense fire. But again, it is thought that thick seed coats evolved as protection from heavy-billed parrots (Bowman 2000).
Eucalypts, Rainforests, and Boundary Controls
It is common to find pockets of rainforest and vine thickets within vast tracts of eucalypts, or see dense rainforest butting against eucalypt forest with little to no ecotone separating the two; these phenomena has puzzled botanists and biologists since the arrival of Europeans to Australia. Factors which explain this odd distribution, such as soil fertility, drought resistance, and recurrent fire, will be discussed.
Eucalypts are considered sclerophylls (“hard leaves”), usually characterized by having small, hard, evergreen leaves. It is thought that sclerophylly arose in order to survive in extremely nutrient-poor soils, such as those found in Australia; low phosphorus levels in leaves reduces overall plant loss of phosphorus from leaf loss. It has also been found that sclerophylls possess low levels of foliar phosphorus; 1000 ppm phosphorus is the accepted threshold for sclerophylly. However, 54% of the rainforest species in a study concerning leaf phosphorus levels had levels lower than 1000 ppm, and 22% lower than 600 ppm. Under the ‘1000 ppm’ rule, these rainforest species could be considered sclerophylls (Bowman 2000). This makes sense evolutionarily, as both rainforests and eucalypts have occupied increasingly nutrient-poor soils for millennia.
Contrary to what common sense would say, the rainforest tree species of Australia are considerably drought-tolerant. In a study cited by Bowman (2000), 2-year-old Nothofagus seedlings raised in a greenhouse survived a simulated 7 week drought before dying. In addition, the intermixing of rainforest species and eucalypts points to the fact that rainforests can tolerate extreme water stresses.
Finally, recurrent fire must be considered as the controlling factor of rainforest and eucalypt forest boundaries. Fire control was first postulated by Czech botanist Domin in 1911, and today, is becoming widely accepted by Australian scientists.
Species of tropical rainforests are considered relatively fire-resistant, and can survive mild, infrequent fires, but their boundaries retreat under a frequent fire regime. During long periods of fire absence, tropical rainforest invades wet sclerophyll forest. Monsoon rainforests typically occur in areas of fire protection, such as in ravines and on rocky outcrops, which act as natural fire breaks. Trees can recover from fire damage; many fires are required to induce monsoon rainforest retreat. Such forests cannot colonize long-unburned eucalypt savanna, although all environmental conditions may be right. It is thought that soil sterilization may be required to remove toxins or detrimental microbes. In temperate rainforests, Nothofagus sp. rapidly colonizes the floor of E. regnans forests, forming a dense canopy beneath the more open E. regnans canopy. The eucalypt forest cannot regenerate until fire opens the canopy, and triggers massive seed fall. Two fires within a 40-50 year period seriously set back Nothofagus regeneration (Bowman 2000).
A single fire can cause widespread retreat of rainforest, if it occurs after a serious disturbance. Cyclones, major droughts, and heavy frosts can greatly damage rainforests, creating a heavy litter load; once the litter dries, massive, intense fires can result (Bowman 2000).
Also, Aboriginal burning cannot be separated from eucalypt forest/rainforest distributions. It is known that Aborigines differentially burned the Australian landscape in a process termed ‘firestick farming.’ They used fire to fell trees for firewood, flush animals from the bush, replenish grazing grounds, and keep clear forest passageways between hunting grounds. Aborigines tended not to burn rainforest thickets as often they contained nutritious foodstuffs such as cycads, or were the homes of totemic spirits (Nicholson 1978).
The climate of Australia has altered drastically from a Gondwanic rainforest paradise to a dry, harsh land of fire. While all plant species had to adapt to an increasingly dry climate and very poor soils, many could not adapt to the increases in fire frequency. The various eucalypt species, however, entered the new arid climate pre-adapted to fire survival with such traits as lignotubers, epicormic shoots, and bradysporous habits. Volatile oils and heavy litter production allowed them to promote hot, intense fires which fatally damaged fire-sensitive neighbors. New fire regimes dictated the distribution of sprouters, non-sprouters, and fire-sensitives. Eucalypt species exploited the advantages fire gave them over fire-sensitive rainforests, facilitating their dominance of the continent. Fire also controls the boundary between rainforest and eucalypt forest, creating abrupt lines between the two. Aboriginal firestick farming may also have played a part in the persistence of rainforest thickets within tracts of eucalypt forests.
Fuel Type Heat of Combustion (MJ/kg)
Eucalyptus obliqua 19.23
E. capitellata 19.92
E. amygdalia 21.35
Pine sawdust 21.74
Pine pitch 35.13
Eucalyptus oil 37.20
Table 1: Heat of Combustion for Various Fuels
The heats of combustion of various eucalyptus species are similar to those of typical North American species. However, eucalyptus oil burns much hotter (Source: Whelan 1995).
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