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Unimaginable Freaks of Fire: Profile of a Pyrophyte
… But enough! Where are the words to paint the million shapesAnd unimaginable freaks of Fire, When holding thus its monster carnivalIn the primeval Forest all night long?
—CHARLES HARPUR, “The Bush Fire” (1853)
… I had a full opportunity of examining this, one of the finest sights which tropical countries display … Above us the sky was gloomy and still; all round us the far-stretching forests exposed a strange and varied pageant of darkness and fire, accompanied by the crackling of flames and the crash of falling trees.
—“AN EMIGRANT MECHANIC,” Settlers and Convicts (1849)
WITH GATHERING SPEED, like a flaming maelstrom, Australia spun to its destiny as a fire continent. Proliferating fires pushed scleroforest and grassland across a biotic threshold. Bushfire became a reality with which almost—but not quite—every landscape of Old Australia had to contend. Since the Pleistocene it has generally been the case that, where biotas have changed, they have moved toward a state of more fire, not less. Some species were swept aside, while some accommodated, adapted, and learned to tolerate fire. Others thrived.
The eucalypts flourished overall. Their evolutionary history, its peculiar genetic makeup, had conditioned Eucalyptus to exploit those unsettled times. Nutrient scavengers of ravenous dimensions, woody weeds ready to colonize disturbed sites, evergreens that could adopt many growth habits and that wrapped protective coverings around critical tissues so that they could thrive in strong heat and sunlight—the eucalypt alliance amalgamated hundreds of species which were ideally predisposed to survive in an environment of increasing fire. Their scleromorphic traits were even better preadapted to fire than to drought, and the rising tide of fire soon swept them before it. Generic adaptations evolved into more fire-specific traits.
No other genus that had so far survived the voyage from Gondwana could compete with Eucalyptus for dominance within Australian forests. Yet there remained areas from which they were excluded: eucalypts shunned the frost-ridden subalpine terrain; on chronically dry sites eucalypts gave way to spinifex, mulga, and gibber desert; on perennially wet sites, eucalypts were crowded out by rainforest or were challenged by paperbark Melaleuca. But everywhere else—wherever fire was routinely possible, even over a span of centuries—eucalypts flourished and shaped whole communities of pyrophytes. The scleroforest it dominated bloomed when burned. Without fire its biophysical engines cooled, and its biotic dynamics decayed.
FRIENDLY FIRE
Most eucalypts can accommodate most fires. But they do so in ways both common and diverse.
Their defenses begin with their bark. What kills is a kind of thermal ring-barking caused by a very high temperature or a long duration of lower heating. But bark is thick, it is densest at the base where the fire burns, and it conducts heat poorly. Surface fires pass by, charring the exterior but not killing the living cambium beneath it. While it is common for heat to concentrate preferentially on one side or the other, either because fuels pile up on the uphill side of a trunk or because winds form eddies on the lee side, at worst this wounds only one side and explains why most basal cavities develop uphill or downwind.1
The thick bark, too, protects epicormic buds buried beneath it. When branches die, new buds are liberated and shoot out. Even if fire torches the crown, a new canopy rapidly emerges and clumps of epicormic sprouts clothe the bole and major branches like moss. Canopy-depleting fires, however, are abnormal in most scleroforests. Once past a juvenile stage, eucalypts shed their lower branches. Between the forest litter, which sustains the fire, and the living canopy, which maintains the tree, there is a considerable gap in fuels that is difficult to bridge by flame unless the surface fire burns with extraordinary intensity, a pilot light in a forest furnace. Even if the canopy is burned off or irredeemably scorched, the fatal fire is only a flash burn. It does not consume the live tissue or the woody fruits encased in tough, nutty caps. Eventually some sprouts become dominant and shape a renewed canopy, and seeds rain down to the waiting ashes.2
Something analogous happens below the surface as well. All but twelve or fifteen eucalypts develop a lignotuber. In place of bark, these subsurface tissues are protected by soil and the simple physics of heat transfer. Probably 95 percent (or more) of the heat released by a fire dissipates upward through radiation and convection; the remainder enters the soil, but it cannot penetrate far since soil is a poor conductor of heat and a few centimeters is ample to shield roots and microbes. An intense surface fire could well consume or lethally scorch a seedling; but if a young tree existed in an environment that burned—if, that is, adequate litter was piled around it—then it was probably not thriving anyway. Regardless, the seedling had already stored in its lignotuber most of the critical nutrients it required. A new shoot, or multiple shoots, punches through the ashy crust; within a few years one stem becomes dominant and rapidly evolves into a new tree; the lignotuber matures. In many species, even if the entire bole is destroyed, new sprouts appear. In the mallee habit, the process is so well developed that Eucalyptus grows naturally as a coppice.
The lignotuber is particularly important because eucalypt seed is not long-lived. A tree holds its seeds for one or two years and in exceptional cases for as many as four. After a fire, seed predation is heavy, Germination is typically poor unless the seed is buried in mineral soil or in an environment free from competition for scarce water and nutrients in the critical first years. But a fire, paradoxically, can produce ideal circumstances for germination. Seed virtually rains down from the charred canopy, overwhelming the capacity of those invertebrate animals that normally feed upon it. The fluffy ash accepts the falling seed, buries it, encases it in an environment full of mineralized biochemicals and temporarily purged of antagonistic microorganisms.
The ashbed effect is multiple, complex. The fire temporarily sweeps competition away. It sterilizes the soil of microflora and microfauna, most of which resided in the combustible litter. It may burn away or cripple other woody species, thus permitting greater access to the site resources by the phoenix eucalypts. It mobilizes vital trace nutrients like molybdenum that are never more accessible for biological intake than in their disintegrated forms after a fire. It volatilizes leachates in the litter, some of which are packed with inhibitory chemicals. A moderate or severe fire restructures the canopies of forest and scrub to permit greater sunlight and to restrict toxic leaching from rain drip. A burn scours out fuels, permitting a few years of fire-free existence. Although the biochemical details are not altogether understood, the outcome for most eucalypts is incontestable: it is essentially only in such a context that new seedlings emerge, and it is through successive burns that the resprouting lignotuber allows eucalypt seedlings to triumph over less vigorous competitors. While various scenarios exist for regeneration, almost all depend, at some stage, on an intense fire.3
These are generic traits, common to most eucalypts, and it is important to recognize that an extraordinary variation exists within the alliance. Eucalyptus had, over its evolutionary history, acquired a suite of traits to cope with a suite of environmental stresses. Particular adaptations to fire were, after a fashion, grafted on to already existing traits. Defoliation by fire might differ little from defoliation by insects; the decapitation of a seedling by burning, from decapitation by browsing; branch loss by fire, from branch failure by wind; temporary nutrient losses by fire, from soil paupery or drought. Some eucalypts favor seed production; others, vegetative propagation. Some have enormous lignotubers, while others feature lignotubers that seem almost vestigial or persist only through certain stages in their life cycle. Some eucalypt forests tolerate surface fires; others thrive on stand-replacing fires. E. regnans, the mountain ash, is highly sensitive to surface fires but seeds prolifically after a conflagration with the result that the towering mountain ash forests are even-aged. Even within one species,