Burning Bush. Stephen J. Pyne

Чтение книги онлайн.

      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, there are variations according to the pattern of fire to which they are subjected.

      It is important to realize that not every fire is identical to every other fire. Fires vary in their physical properties—their intensity, their rate of spread, their frequency, their flame heights, and their size. Different fires act on the same biota with different outcomes. Even two fires with similar physical parameters will yield different ecological outcomes as a function of their timing. If one fire burns in the summer and another in the winter; if one succeeds an initial fire after four years and the other after forty or four hundred; if one eliminates certain species from the site and another permits enough to survive to recolonize; if one occurs amid exotics and another does not; if one burns around a seedling, another around a juvenile pole tree, and another around an adult of the same species, the biological consequences may well differ.

      Thus it is not enough to say that Eucalyptus is adapted to fire. Rather, particular eucalypts are adapted to fires of particular sorts, to fire regimes. Different species of Eucalyptus require different fires. In wet forests, severe fires, even if infrequent, are more important than mild fires. Wet eucalypt forests tend to be even-aged, triggered by episodic holocausts that prescribe the proportion of eucalypts to invading rainforest taxa. In dry forests, fires tend to be more frequent and less intense, and conflagrations, while less likely to incinerate whole stands, may cause shifts within the existing population of eucalypts.⁴

      Nor is fire a singular event. Typically, fires occur as geographic complexes and historical cycles. Once some part of a biota burns, it influences the other parts of an ecosystem. With long-range firebrands, a fire in one site may propagate into others, and by shaping new patterns of fuels it may propagate into the future as well. Real fires do not occur in strict cycles, like returning comets; they burn in eccentric rhythms. They integrate not only seasonal and phenological cycles, but events that are unexpected, stochastic, irrepeatable, and irreversible. A site’s history is rarely wiped clean; almost always the past lingers in ways that bias the future. Once fire insinuated itself into the eucalypt environment, it was not easily expunged. Instead it spread, like a drop of acid etching new and indelible patterns on whatever it touched.

       SUPPORTING SCLEROMORPHS: FIRE BY SYNERGY

      Even where the eucalypts dominate as trees and control the canopy, they share the surface with other organisms, a cast of supporting scleromorphs. Within the scleroforest, all must interact—sometimes as competitors, sometimes as complements. No organism can afford to establish a special relationship to fire one-to-one in biotic isolation. Rather, its success will depend on how it responds to the spectrum of fires to which the site is subjected and which it helps to shape. If few organisms can survive without regard to the eucalypt, neither can the eucalypt ignore those scleromorphs with which it shares a site and with which it often develops a special fire synergy. In broad terms, these include grasses, shrubs, other scleromorphic trees, and a few Australian exotica such as the grass tree (Xanthorrhoea).

      Gramineae—the grasses—are the most extensive fuels in Australia. They interpenetrate with most scleromorphic biotas, and they claim for themselves a great concentric ring between the central deserts and the coastal forests. In woodlands they sustain understory burning; in many drier forests they often replace eucalypt litter as a driving fuel; in deserts, they appear in the form of ephemerals after heavy rains, promoting widespread if episodic fire. Yet grasslands display few adaptations unique to fire. Their fire-hardiness derives from their СКАЧАТЬ