Burning Bush. Stephen J. Pyne
deeper taproots drain only after true drought, or following several seasons of cumulative water deficits. Thus, the circumstances that favor grass-driven fires do not promote equally vigorous fires in scleroforest fuels, while, conversely, the prolonged drought necessary to prepare scleroforest strangles the heavy growth necessary to propel grassy fires. In any year large fires affect either the lowland savannas or the mountain forests but not both simultaneously. The intermediate scleroforest varies between those extremes in timing, fire frequency, and fire intensity, its fuels meshing with its biology in rough, sometimes uncanny synchronization.
In dry scleroforest eucalypt litter sets the fuel loads (65–70 percent of the total), with minor contributions from herbs and grasses. Fires are frequent, light, syncopated with fuel buildup. Surface fuels can reach levels of 15 tons/hectare within ten years, 22 tons/hectare after twenty years, and 27 tons/hectare after thirty. But fires typically burn between three and twelve years, between the time at which fuels are adequate to support sustained combustion and the time beyond which it is unlikely that a site can escape ignition. For several years after a burn, a dry scleroforest lacks the fuel to reburn, and this pause allows the woody scleromorphs to maintain their position. More rapid burning would degrade the biota to pyrophytic grasses, if any are present, or choke the forest with coppicing eucalypts and scleromorphic scrub.13
By contrast, wet scleroforest cultivates an understory rich in shrubs, a higher overall productivity, and a greater proportion of available fuels. Where dry scleroforest encourages easy ignition and spread, however, wet scleroforest discourages it with a microclimate that inhibits drying and a dense canopy that shelters the surface from wind. It claims niches protected from desert winds, sites often swathed in maritime moisture. Within their understory are often found the flora of an embryonic rainforest. Short-rhythm fires are unlikely, yet long-rhythm fires essential. When they come they burn with catastrophic fury, the fuels thick and kiln-dried by intemperate drought; fire razes whole forests, from standing eucalypts to lush understory. In what has been described as a near miracle of timing, fire even rejuvenates the mountain ash (E. regnans).14
When surrounded by parched litter and shrubs, mountain ash not only burns vigorously but carries fire briskly to the canopy and hurls smoldering bark far in advance of the flaming front. Unusually thin-barked, lacking a lignotuber, even a modest fire will kill the bole. Its canopy fire, however, scarifies and liberates seeds stored in the crown—as many as 14 million seeds per hectare by some estimates. The tough tissues around the seeds can just hold off, and in fact need, the explosive heat created by a torching canopy. The thick ash left by the surface burn creates an ideal seedbed. After a fierce fire at Noojee, Victoria, nearly 2.5 million seedlings per hectare carpeted the burned lands. Patiently, the wet scleroforest begins its rejuvenation, a renewal dependent on a holocaust fire every two or three centuries. The ages of the wet scleroforests chronicle the ages of their great fires.
Too many fires—the rapid, forced succession of a holocaust by repeated surface burns—and the site will degrade to scrub, perhaps to heath. Too few fires and the rainforest taxa latent in fire-sheltered niches will mature, strangle the scleroforest like parasitic vines, and restore a lost world of green Gondwana. The fire flume ensures that the reconquest is improbable. Only in sodden gulleys, on leeward ridges flanked by castellated rocks, in isolated mountain fire shadows can the relict rainforest persist. Elsewhere the fire flume sweeps it away in floods of flame.
SCLEROSCRUB: OLD AUSTRALIA IN MINIATURE
For nearly all the major Australian biotas there exists a type of heath. There are tropical heaths, alpine heaths, mallee heaths, temperate heaths, dry heaths, and wet heaths. The kwongan—the heath of the sandplain—claims sites intermediate between the mulga and the mallee. The wallum—the heath of the Queensland coast—is a diminutive wet scleroforest. Large swaths of heath dot the Kimberleys and Arnhemland, a distilled rainforest. Patches of heath on the Cape York Peninsula concentrate the flora of the tropical savanna. Throughout the southeast a kaleidoscope of heaths compresses the biomes of the scleroforest. Its heaths form a kind of scleroscrub, Old Australia in miniature.15
If their distribution distills the biogeography of Australian life, their history recapitulates its evolution. Heaths claim sites with wretchedly impauperate soils. Even slight alterations in the nutrient flux—a sudden surge, a steady loss—can destroy the delicate balances that sustain the heath flora. Next, they accommodate water stress. Too much or too little, waterlogged or desiccated—either way, the imbalance can lead to scleroscrub, a wet heath in one case, a dry heath in the other. But these conditions only intensify circumstances to which Old Australia had long adapted in its transformation from rainforest to scleroforest. The final stress was fire.
As fire regimes, heaths resemble the dynamics of those biotas that surround them and of which they are distillations—they concentrate those properties. Their propensity for burning is extraordinary. Leaves are rich in oils, low in moisture, lean in mineral content—particularly in phosphorus—all of which makes for brilliant, flaming combustion. Fuel loads build rapidly. For three to five years surface fuels increase quickly, then taper off while shrubs proliferate, their growth rates peaking between five and ten years but never really ceasing. Biomass can reach 10 tons/hectare within five years. In Banksia-rich heath at Jervis Bay, fuels measured 50 tons/hectare after fourteen years. Virtually all the new biomass is available as fuel. On average, fuel loads in mature heath range between 20 and 35 tons/hectare, values comparable to wet scleroforest. Its open nature, its chronic aridity, its sweeping winds—all make heath fuels ideally available for burning. A heath requires only a few years of recovery from one fire to prepare for another.
Not only fuels but ignition dictates the frequency of fire and the relative proportion of high- to low-intensity burns. Most heaths reflect the fire frequency of their adjacent regimes, and, claiming less than 1 percent of Australia’s land surface, many fires necessarily enter heath from the outside. They accept such fires readily and with tremendous resilience. They accommodate a range of fires, and while those fires influence the internal dynamics of the heath, they do not alter its status relative to surrounding biotas. Fire neither perpetuates nor destroys: nothing else can claim the site. With or without fire, the heath will persist.
If not obligatory, fire is nevertheless frequent, welcome, useful, and inescapable. It modulates the scleroscrub, miniaturizing and intensifying the impacts it makes on scleroforest. Overall, heath fires are conservative; they can recycle and release precious nutrients, seeds, and access to water, but they cannot by themselves augment or diminish the total quantity of those minerals or waters present. Still, there is much to influence, for the floristic richness of heath is legendary. Australian heaths admit about 3,700 typical species, embracing representatives from nearly every prominent Australian family including many of the more flamboyant floras—the grass trees (Xanthorrhea), the ground orchids, the nectar-rich Banksia, the stilt plants Dryandra and Hakea. The kwongan contains 70 percent of the species found in the southwest enclave. This variety is fundamental. Not a dominant species, but their scrubby growth habit is what defines heathland.
There is, accordingly, plenty of flex in the system by which to accommodate fires. Because of their fuel history, even high-intensity fires can occur at almost any time, and floristic composition reflects shifts in fire frequency and seasonal timing. More-frequent fire favors those flora that propagate by sprouting; less-frequent fire, those that reproduce by seed or that aspire to control the canopy. Too frequent fire can remove some flora by not allowing them to mature into seed-producing states. Too occasional fire pushes other flora into senescence, beyond their ability to regenerate from lignotubers or beyond the capacity of their seed to remain viable. Fire early in the summer selects some species; fire in the autumn, others. Fires twist the heath like a kind of biotic kaleidoscope, the pieces always there but constantly reorganized into new patterns. The heath, in turn, testifies to the universality, ease, and subtlety of fire in Old Australia.
RELICT RAINFOREST
That leaves the rainforest.
It endured—secure from swelling drought and blasts of wind from the desert core, preserved in sheltered grottos around the watered seaboard. Like heath it was multiple, a distillation of many biotas. Unlike scleroforest, it was