by Colin Tudge
Through a series of neat tricks, is the answer. The aspen’s whirling leaves resist the wind by not resisting: they ride the blows, go with the flow. It’s wise of the tree to lose them in the winter, when they could do no good. What really counts in the north, though – what ensures that aspens may dominate for hectare after hectare, when trees that are more obviously shaped for the boreal stresses have long since fled – is their ability to bounce back after fire.
For the aspen has long lateral roots which, at intervals, send up suckers that grow into entire new trees. Many other broadleaf trees produce suckers, including the paper birch, another pale and ghostly denizen of the boreal forest. But only the aspen spreads itself so wide: an Alaskan ecologist, Leslie Viereck, has found such suckers more than 80 metres from the parent trunk. Fire inevitably strikes the boreal forest sooner or later and if it occurs in spring or summer it will kill the aspens along with everything else, because it burns the organic matter within the ground, including the aspen’s trailing roots. But if fire strikes in winter when the ground is frozen, or in spring when it is still wet, the roots survive. Then the suckers rapidly grow up to form new trees – rapidly because they already have a vast, established root system to draw upon. Thus the mixed forest, probably mostly conifers with a few aspens, may be replaced in months by a grove of aspens.
The overall form of the aspen – lateral roots with suckers – is reminiscent of the calamite trees, primeval relatives of the modern horsetails (Equisetum) with their long trailing rhizomes (which are underground roots swollen to form storage organs). All the aspen trees in the grove that springs up around the parent, together with the parent itself if it survives, form a clone: a related group of genetically identical individuals. They all come into flower together, and shed their leaves together. Since they also remain physically joined (by the lateral roots) some biologists have suggested that the entire grove should be regarded as a single organism; and since it may be hundreds of metres across in all directions, such a grove would rank as one of the largest organisms on earth. It’s been suggested, too, that some groves date back to the end of the last ice age, since they would have been among the first on the scene after the ice melted. Thus, at around 10,000 years, an aspen clone would also be among the oldest organisms on earth.
This is an intriguing thought, worth musing over, but perhaps not to be taken too seriously. The trees that look like individuals in a grove really are individuals, to all intents and purposes. If some accident or outward-bound gardener were to sever the lateral roots that join the trees to their neighbours, each one would happily live alone – just as young strawberries do when they are cut loose from their runners. What the aspens really illustrate is the power of asexual (or vegetative) reproduction. Most vertebrate animals, including human beings, and many plants, cannot pass their genes on from generation to generation except through the medium of sex. But many organisms can bypass sex, at least some of the time. They simply generate copies of themselves. Human beings can do this only in Greek mythology. But as the aspens demonstrate, asexual cloning can be a useful wheeze.
Two further morals emerge from the tale of the aspen. First, appearance in nature can be highly deceptive. The aspen doesn’t look tough, and yet it may flourish where other trees, more obviously stripped for action with needle leaves and pointed crowns that shed the snow, succumb. But then, many of nature’s most successful creatures seem to go out of their way to look bizarre and delicate. Sometimes the bizarreness is bravado, intended to attract mates, as with the plumes of the peacock and bird of paradise. Sometimes flamboyance is a warning, as in poisonous lizards and caterpillars. But sometimes, when we look at an animal or a plant, we just get the physics wrong. Moths and butterflies look absurdly frail, fair game for anyone, but they are ubiquitous, and in their season in the Amazon the yellow butterflies come at you in blizzards. Nature moves in mysterious ways, which itself is a moral for all who would presume to take nature in hand.
Secondly, many creatures survive when you might not at first sight expect them to because of just one particular trick. The analogy is with an old and arthritic fencing master who wraps up the young athletic tyro with disdainful ease just because he knows a few wrinkles that the novice does not. The aspen thrives where others fail because of its. suckers. It might not seem much, but it works.
But the trick works only if the forest fire spares the lateral roots. If fire breaks out in summer, when the soil is dry and unfrozen, the roots are cooked and the aspen succumbs. Then the jack pine, Pinus banksiana, comes to the fore: a quite different tree with a quite different survival strategy.
J. David Henry in Canada’s Boreal Forest calls the jack pine ‘a scrappy tree’, which in former times (when the timber of nobler trees like the white pine, Pinus strobus, were more available) was often treated as a weed. Now, however, with the alternatives diminished, it is widely used for fence posts, telegraph poles and pit props, and indeed for Christmas trees; and together with the black spruce, says Henry, ‘is a mainstay of the pulp and paper industry’.
The jack pine has several qualities that seem to equip it for a life with fire. As in many conifers, the branches nearer the ground tend to die off as the tree grows taller, not least because they find themselves without light. In the jack pine, these dead branches simply fall off: the tree is said to be ‘self-pruning’. If the dead branches were allowed to persist, they would provide a ‘ladder’ for the fire from the ground to the top. The physics of fire is in many ways counter-intuitive. Crucially, a hot fire that burns itself out quickly can be less damaging than one that’s somewhat cooler, but lasts longer. Jack pine needles are high in resin and often low in water, especially in the droughts of spring and summer, when fires are likely, and so they burn hot but quick. On much the same principle, the jack pine’s bark is flaky. It picks up surface fires but then burns swiftly and does little harm. The stringy bark of eucalyptus in Australia, hanging loosely from the iron-smooth bole beneath, is protective in much the same way. In both cases the discarded bark acts as a decoy, like a hamper thrown from a troika to divert the chasing wolves.
On the other hand, if jack pine bark accumulates on the ground (as it does if there is a long interval between fires) then surface fires – particularly in spring and summer – can be very fierce. Then, most trees of all kinds are killed. But the jack pine is typically the first to spring back. For a very hot fire in the summer burns both the leaf litter on the surface and the organic material in the soil itself, leaving a bare, mineral soil behind. Jack pines germinate well in such soil – and indeed are inhibited by leaf litter: organic matter isn’t always everybody’s friend. They like bright sunlight, too, and appreciate the open space.
Once germinated, the young trees grow happily in sandy soil that is too dry for other species; and for good measure, the young saplings can tolerate drought of a month or so, and sudden drops of temperature of the kind that for many trees are lethal. They grow swiftly when young – more than 35 centimetres in a year. This is a joke by the standards of tropical trees, some of which reach 20 metres in five years, but good for a land so niggardly in bright sunshine and general warmth. By their fourth or fifth year many of the young jack pines are producing their first cones – which by tree standards is markedly young. The Canadian ecologists Stan Rowe and George Scotter asked why they should be so precocious: why not focus their precious energy on more growth, rather than on reproduction? Forest fires often leave a lot of fuel behind, and sometimes a second fire comes hard on the heels of the first. It seems a good idea to scatter a few seeds before the possible follow-up.
But it’s the cones and the seeds of the jack pine that are adapted most impressively and specifically to fire. The cones are hard as iron, their scales tightly bound together with what J. David Henry calls a ‘resinous glue’. Many creatures attack cones; but only the American red squirrel will take on the jack pine cone, and even the red squirrel much prefers the easier, fleshier meat of spruce cone. The cones m
ay persist on the trees for many years, and the seeds within them remain viable: in one study more than half the seeds from cones that were more than twenty years old were able to germinate.
The cones do not open until there is a fire: it takes heat of 50°C to melt the resin that locks the scales together. Then, they open like flowers. Thus the seeds are not released until fire has cleared the ground of organic matter and of rivals, and created exactly the conditions they need. The output is prodigious. After a fire in the taiga (the northernmost forest that then gives way to tundra), the burnt ground may be scattered with 5 million jack pine trees per hectare.
But although the cone responds to fire, and only to fire, it is remarkably fire resistant. Thus in the early 1960s a biologist called W. R. Beaufant found that the seeds inside would survive for thirty seconds even when the cone was exposed to 900° C – the kind of temperature that potters use for firing. At a mere 700° C, the seeds were perfectly happy for at least three minutes. In short, it takes an awful lot of thermal energy to kill jack pine seeds when they are still in their cones. Trees seem to have evolved cork largely as an adaptation against fire; and jack pine cones contain cork too.
Yet there is more. For as J. David Henry has found, the cone does not respond simply to the presence of fire, like some crude unmonitored mechanical device. As it is heated, it releases resin from within. This oozes to the surface, and ‘creates a gentle, lamplike flame around the cone’, which lasts for about a minute and half. All in all, says Henry, ‘It seemed that, once ignited, the cone was programmed to provide a flame for the right amount of time to open the cone… while a forest fire is needed to initiate this process, the cone itself is capable of providing the type and duration of flame it needs to open and disperse its seeds.’ He then showed that, once open, the heated cone does not release its seeds until it has cooled down again – which in field conditions may take several days. So the initial opening is controlled; but when the cone is first open, the seeds are held back. They are not sent out like Daniel into the fiery furnace. Henry suggests a mechanism: perhaps the hairs that coat one side of the seed are sticky when hot, and hold the seed in, but lose their stickiness when cooled again. This is speculation, yet to be tested.
In any case, the adaptations are extraordinary. Jack pines belong among a fairly impressive shortlist of trees that not only resist fire, but have become dependent upon it. They cannot reproduce without it. If there is no fire within their lifetime, they die without issue. After a fire, jack pines may flourish and form a monoculture, for hectare after hectare. But without further fire, the jack pine forest fades away.
Yet there is one final twist. In practice not all of the jack pine cones need the fierce heat of a fire to open them. In the north, about one in ten open just in the warmth of the sun. In the Great Lake states to the south, where there are far fewer fires, most of the jack pine cones are able to open in the sun. Thus jack pines have a ‘mixed strategy’: the genes that make their cones so special are clearly of two kinds – some that gear the cone to fire, and some that enable it to respond to sunshine. Geneticists call this a ‘balanced polymorphism’. Natural selection tips the balance towards the fire-dependants in the north, and to the sunshine-dependants in the south. The jack pine isn’t simply the supreme specialist. To some extent at least it is the jack of all trades. Various other pines have comparable fire-resistance and fire-dependence, but none surpasses the scrappy jack pine in its adaptation.
But perhaps the tree that is most thoroughly adapted to the special conditions of the north – not the extreme north, but the central and northern coast of California – is the coastal redwood, Sequoia sempervirens.
The coastal redwoods inhabit, or rather they create, temperate rainforest in a discontinuous belt, roughly 15 kilometres wide, from Big Sur south of San Francisco northwards to the Oregon border. They are of course magnificent: the height of a cathedral spire, 60 to 70 metres. The tallest, known prosaically as ‘The Tallest Tree’, is in Redwood National Park and is 111 metres high. Its trunk is 3 metres in diameter – although this is quite slim by redwood standards. They often go to up 5 metres. Many live to 1,000 years and some reach more than 2,000.
All forests can be peaceful (you can sleep the sleep of the just in Amazonia without being carted off in pieces by voracious ants), but nothing compares with the tranquillity of redwood forest. The columnar trucks reach far higher than in a tropical forest, where 30 metres is more standard. Tropical forest is a mass of small trees, battalions of poles, with just a few trunks of respectable garden size and only the occasional giant glimpsed through the gloom; all festooned with climbers and epiphytes. But the coastal redwoods for the most part are decorously spaced – except where they form little circles, so-called ‘fairy rings’. They are an army of giants; the biggest on earth. The ground beneath them is littered with the delicate, yew-like branchlets that the trees shed every few years, chestnut brown after death. The mosses and ferns, herbs and shrubs, dotted here and there, stay at the feet of the great trees. There is no importunate clambering. There are few birds. You rarely hear such silence as in a coastal redwood forest. The light is green. The sun shines through in sharp bright shafts. On a warm, late afternoon the Pacific rainforest of coastal redwoods is perhaps the most serene of all earthly environments.
But it’s not always like that.
The first problem is flood. Redwoods like moisture; a ‘mild maritime climate’ as it is sometimes described (though they don’t tend to like salt spray). Indeed they go to great lengths to capture and condense the thick fogs of the cool Californian night and morning in their leaves. It falls as ‘fog drip’, and in the rainless summers, may add 30 centimetres of extra water. So they make their own climate, humid and shady.
But you can overdo the water. In winter, there may be 250 centimetres of rain. Storms are frequent; and with storm comes flooding. In Humboldt County, redwood country, there were severe floods in 1955, 1964, 1974 and 1986. The floods of 1955 swept away sawmills, farms and whole communities along the Eel, Klamath and Van Duzen rivers. Buildings were buried deep under mud. More than 500 redwoods were swept away along Bull Creek, a tributary of the Eel. Elsewhere the forest floor was buried under 1.3 metres of silt.
Then, with the aid of radiocarbon dating, Paul Zinke showed that Bull Creek had often suffered such insults in the past. In fact, a study in 1968 cited by Verna R. Johnston (see Notes and Further Reading) showed that there have been fifteen major floods in the past 1,000 years, and between them they have raised the level of the whole surrounding area by more than 9 metres – the height of a three-storey house.
In short, over time along these north Californian coastal rivers, the banks and surrounding areas are eroded in some places and built up in others: the kind of pattern that is seen for example around the coast of Europe, as the North Sea picks up entire beaches from some places and dumps them somewhere else. The cartographers of eastern England have been particularly busy this past few centuries, and surely will be even busier as global warming strikes.
This is where the redwoods reveal their own set of tricks. Of course, if the ground is removed from around them all together, then they are swept away. But if they are merely buried, to a depth of a metre or so, they are untroubled. Most trees do succumb to such treatment. They are suffocated. But redwoods send up roots, vertically, from their buried lateral roots, into the silt above; and these verticals grow so quickly they sometimes come bursting through the surface.
Coastal redwoods re-root themselves as the silt piles up around them
These rapid-growing verticals, however, are merely the emergency procedure, the front-line troops. Before long, new lateral roots grow from the buried trunk, just below the surface of the newly deposited silt; generally speaking bigger and broader than the previous roots at the lower level. Thus an old redwood, that’s survived many floods – and those a thousand years old or more must often have survived more than a dozen – finish up with a multi-layered root syste
m, like an inverted pagoda; a fairly small set of roots deep down, then a bigger set higher up, and so on, each set corresponding to some earlier flood. The net result is a truly remarkable anchorage. Thus they hold their otherwise precarious trunks steady for a thousand years. Incursions of silt kill most trees, but redwoods have made a virtue of it. Here again is nature’s opportunism.
Human beings are managing to queer the pitch, however. Forest clearance exposes the survivors to stronger winds, and may blow them over despite their anchorage. Roads alter the natural drainage from the hills, and so alter the pattern of flooding, leading to landslides, which nothing can resist. Added silt builds up in the middle of rivers, forcing the currents to the sides and undercutting the trees along the banks. More than 100 ancient trees in California’s Avenue of Giants were killed this way in 1986, when 58 centimetres of rain fell in nine days. And although the redwoods have turned partial burial to their advantage, their roots are damaged if the soil above them is compacted – which it can be by the dedicated tractors (skidders) that are used to extract logs, and by other traffic. In great gardens, as at Kew, soil that has been compacted by the feet of thousands of visitors coming to admire the trees is loosened by pumping in nitrogen gas under pressure. In some of New Zealand’s forests, visitors walk along catwalks a foot above the ground, with little bridges over exposed roots. Humanity needs wild trees. But sometimes we need to tend the wild as carefully as any garden. ‘Managed wilderness’ may seem a paradox, an oxymoron; but it is the reality we have to come to terms with.