The scleroforest revolution concluded between 38,000 and 26,000 years ago as the scleromorphs, led by Casuarina, completed their abrupt, all but catastrophic, expulsion of the araucarias. But almost as suddenly, between 20,000 and 7,500 years ago, Eucalyptus did the same to Casuarina. By the time of European discovery forests and woodlands comprised about 25 percent of the Australian land surface; perhaps 70 percent of those lands could be classified as pure eucalypt forest. Eucalypts claimed about 16 percent of the tropical eucalypt and paperbark biomes, and an estimated 11 percent of the cypress pine biome. Across Old Australia eucalypts comprised some 95 percent of the constituent tree species. They thrived almost everywhere—at the snow line of the Australian Alps, along the saltwater tide of tropical mangroves, along desert watercourses, on monadnocks; in relatively wet climates and in relatively dry, on impoverished sites and on more enriched; in Mediterranean climates, in true deserts, in wet-dry tropics, along the margins of rainforest and interpenetrating grasslands. They were absent only in the true, the relict rainforest. With minor exceptions, Eucalyptus dominated Australian forests. Every other organism had to accommodate that fact.4
“The remarkable plurality of the Eucalypts,” as Ferdinand von Müller called it—what staggered Charles Darwin as the “never-failing Eucalyptus family”—prevailed over the Australian continent to an extent unrivaled by any other genus on any other continent. Eucalyptus had exploded so widely that it is considered by some authorities as less a genus than an alliance composed of three suballiances, ten subgenera, and over six hundred species. The plasticity of the genus is extraordinary. Hybrids are common within subgenera, juvenile habits persist into adulthood, and even phantom species (apparently hybrid populations that now exist in the vicinity of only one parent) have been identified.
The eucalypt conveyed to Australia a special character. Marcus Clarke gave it poetic expression as “Weird Melancholy.” Here, where “flourishes a vegetation long dead in other lands,” is found the “Grotesque, the Weird, the strange scribblings of nature learning how to write,” a “phantasmagoria of that wild dreamland termed the Bush.” Others described or cursed it in more prosaic language, but no one could deny that Australia was different and that the eucalypt was to a large extent both cause and symbol of that difference. But if the eucalypt animated the bush, fire animated the eucalypt. The abrupt, smothering rise in Eucalyptus pollen that accompanied the scleroforest revolution paralleled an equally sudden increase in charcoal.5
THE EUCALYPT AS SCLEROMORPH
Eucalyptus was first a scleromorph and then a pyrophyte. Of the three suballiances that comprise the genus, Monocalyptus shows the greatest adaptation to impoverished soils but displays limited tolerance for drought or hostile soil microorganisms. By contrast, Symphyomyrtus avoids the worst soils, but shows considerable tolerance toward drought and microbes. Corymbia falls somewhere in between, and was probably intermediate in the evolution of the alliance. But degraded soils were something to which most members of the Gondwanic rainforest had to adapt. Eucalyptus, however, elevated nutrient scavenging and hoarding to an art form.
The eucalypts typically developed extensive, deep roots, capable of foraging widely. Rather than target particular nutrient niches, rather than hone their search with exquisite refinement, the eucalypts processed soil catchments in volume, partially compensating for the relative poverty of soil at a restricted site. In addition, eucalypts evolved chemical and biological aids to improve access to those nutrient reservoirs, particularly phosphorus, that did exist. Through various biochemical mechanisms, probably involving phosphataze enzymes or organic exudates, eucalypts could extract phosphorus from iron and aluminum compounds. Similarly, it appears that leachates from leaves and litter of some eucalypt species can percolate into the soil and mobilize phosphorus compounds that are otherwise inaccessible. And then there are the biological allies of Eucalyptus, soil microbes and mycorrhizae, that evidently 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.