improve phosphorus uptake. The scavenging eucalypt can grow where other trees starve.
Getting scarce nutrients is only half the equation. Once absorbed, eucalypts carefully, obsessively retain and recycle them. Seedlings develop lignotubers—enlarged storage organs in the roots. Here nutrients can be collected and stashed until needed. If the shoot is killed, new shoots promptly emerge. Some eucalypts retain their lignotuber into adulthood, and some can send out from it multiple stems. A lignotuber ensures that, when conditions are right for growth, the seedling will have adequate reserves of the nutrients it needs. Likewise, eucalypts store nutrients selectively within their bole. A nutrient gradient exists between inner heartwood and outer sapwood such that phosphorus, in particular, is cached where it will be most useful. If branches are destroyed, new sprouts shoot out from beneath the bark, and the nutrient reserves in the sapwood ensure that this process will be rapid. Thus not only the roots but also the crown are buffered against erratic and ephemeral changes. The effective nutrient reserve shifts from the soil alone to the tree itself and the immediate environs under its biological control. Eucalypts can thus acquire nutrients far in excess of their immediate needs, and they can cache that surplus for years, perhaps as long as a decade. When young, eucalypts prefer mechanisms of internal cycling; when more mature, cycling between the soil and the tree.
Recycling occurs as well in the crown. A eucalypt canopy is dynamic: old branches become senescent and die back, while new branches immediately spring forth from epicormic sprouts lodged just under the protective bark. The crown is thus continually reshaped for maximum efficiency, and nutrients are reabsorbed before the branch is vulnerable to breakage and loss. As an evergreen, the eucalypt retains its leaves, shedding them as infrequently as possible, tenaciously hoarding their precious supply of nutrients. Instead, eucalypts shed their impoverished bark. When leaves do fall, they are drained of vital nutrients to the fullest extent possible before deposition. And once on the ground, leachates from the crown quickly return residual nutrients to the tree through the soil.
These adaptations served Eucalyptus well during the Great Upheaval. The particular mechanisms it favored for the foraging and cycling of nutrients did double duty for water. But there were greater variances in coping with aridity; the range of responses to water stress among eucalypts exceeded their range of responses to soil degradation. In fact, Eucalyptus is not a true drought evader. Eucalypts do not close their leaf stomata, go into seasonal hibernation, or shed their leaves. Instead they tolerate drought. They search out new water sources, hoard existing reserves, shut down nonessential processes. When drought comes, they tough it out.
Like all the scleromorphs, eucalypts have hardened leaves that reduce moisture loss. (The same is true for the operculum, from which derives the name Eucalyptus—from the Greek eu, meaning “well,” and kalyptos, “covered.”) Their canopy drapes downward, evading excessive leaf temperatures. Their vast, plunging root systems; their lignotubers; the capacity of seedlings to reside in apparent dormancy within lignotubers for years, even decades; their ability to shrink their leaf stomata to reduce transpiration and conserve water—all ensure the survival of the eucalypt within a land that is seasonally dry or episodically blasted by drought. But eucalypts have a harder time conserving water than nutrients. Their physical geography is thus limited, in some regions, by cold and in others by water. Where aridity becomes chronic and pronounced, eucalypts surrender to grasses, scleromorphic shrubs like saltbush, and that prolific rival, Acacia.
Its acquired traits were adequate to keep Eucalyptus alive during the eons of soil impoverishment, and they were enough, within the context of the Great Upheaval, to liberate eucalypts among the emergent scleroforest. The reformation in the physical environment meant a reformation in the biotic environment as well, and organisms had to accommodate to both circumstances. The eucalypts were supreme opportunists, infiltrating sites more and more frequently disturbed. As they broadened their domain, entire biotas had to reorganize around the defining properties of Eucalyptus. Eucalypts were too effective as scavengers and as hoarders of scarce nutrients and water to ignore. They were aggressive competitors—and a vital focus for grazing by insects, mammals, and birds. They concentrated bionutrients into particular forms; their hard gum nuts, for example, were accessible to some species and not to others. They created special niches and coevolved unique associations with koalas, termites, possums, and parrots; while eucalypts covered 25 percent of the surface of Australia, they harbored some 50 percent of its avifauna. The patterns of eucalypt forests defined the structure of critical habitats; the processes of eucalypt life determined the flow of nutrients and water.
If they wished to survive, other organisms had to seek out an accommodation with the Universal Australian. But the revolution did not end with the breakup of rainforest into scleroforest. The last 20,000 years—the epoch of the eucalypt revolution—have been marked by massive biotic realignments and extinctions. Each stress inspired others. Selective aridity encouraged fire, and fire fostered another suite of conditions, both abiotic and biotic. If they wished to survive, flora and fauna had to adapt not only to fire in the abstract but to the kinds of fire their scleromorphic neighbors supported. How their associates burned and reproduced determined in no small way the kind of fire they confronted. Amid fire the eucalypts flourished.6
THE EUCALYPT AS PYROPHYTE
The spread of Eucalyptus traced the spread of fire. Charcoal and eucalypt pollen march side by side in the geologic record of the late Pleistocene and Holocene. Fire proliferated across the spectrum of Old Australian biotas—in scleroforest of course, but also in the grasslands, the acacia-splattered savannas, the heaths; it rolled back the rainforest into sharply bounded sanctuaries. The environments were varied, and so, not surprisingly, were the responses even among the prolific eucalypts.
Those inherited traits for contending with deteriorating soils and unreliable water preadapted the genus to survive fire. It knew how to cope with irregular nutrient fluxes, with an erratic tempo of too much and too little. Its quest for water already plunged roots safely out of the way of surface fires. Its weedy ancestry had groomed the eucalypt into an opportunist, ready to seize disturbed, opened sites. Eucalypts could capture nutrients released by fire, could store them until another release, could in emergencies live off internal caches in heartwood and lignotuber. Bark was thick, tough, and it shed as it burned like the ablation plate of a descending spacecraft. If branches were seared off, new ones could sprout from beneath the protected layer. If the bole burned, new trunks could spring from the buried lignotuber. A eucalypt could pour old nutrients into new growth, even as it scavenged liberated minerals from freshly burned ground. Fire could, for a couple of years, purge hostile microbes from the site; it might encourage better percolation of groundwater; it opened an area to sunlight, allowing the sun-worshiping eucalypt seedlings a chance to outgrow more shade-tolerant rivals. For most eucalypts, fire was not a destroyer but a liberator.
There were differences between fire and other pressures toward sclerophylly. Fire acted on a scale of minutes or hours, not over decades or millennia or eons. It was also interdependent with life in ways that leaching and drying were not. Soils degraded regardless of vegetative cover. Droughts arrived and departed whether there was anything on the surface or not; rocks or rainforest, it mattered little, for while organisms could alter the surface concentrations of minerals and water, while they could modulate the force of climate, they could not prevent rain or drought from appearing. But fire could only thrive in the presence of organic fuels. The character of those fuels profoundly influenced the character of the fires that resulted. And those fires, in turn, shaped the kind of biotas on which the fires fed. Fire and flora entered into a process of mutual selection, of positive reinforcement, that was far more rapid, intimate, and compelling than any of the relationships that preceded it.
Eucalyptus was excellent at extracting and hoarding precious nutrients; but so were most of the Australian flora. It was successful at persevering through dry seasons and episodic droughts; but so, again, were the other scleromorphs. Eucalypts, in fact, tended to occupy the relatively better sites of Old Australia—shunning the driest, the worst waterlogged,
