by Colin Tudge
Redwoods are wonderfully fire-resistant, too – and show yet more survival strategies. Their bark is a fireman’s dream: tough, fibrous, and up to a foot thick. Their timber retains a lot of water. They produce a pitch that is almost non-flammable. A severe fire will break through the bark, and thus allow access for subsequent fires. But the charred wounds on the boles of many old redwoods merely attest that they have endured.
Like aspens and jack pines, too, coastal redwoods have a wonderful ability to recover after fire. Almost uniquely among conifers, redwoods can sprout new stems and roots from trunks that have been burnt, cut down or blown over. From the earliest age – indeed from the seedling stage – redwoods carry latent buds around the base and along the trunk. If they are damaged, or irritated, these dormant buds develop into shoots or roots and then into whole new trees. When the parent tree is damaged the buds around the base sprout to form a ‘fairy ring’ around it, of up to a dozen scions; and when the main tree dies they persist, in conspiratorial circles, that indeed seem enchanted. Sometimes the process is repeated: the trees of the inner circle are damaged and more grow up around them. By such means redwoods may return even to areas that have been heavily logged.
The many thousands of cones on each redwood produce prodigious harvests of seeds. They do not germinate well, and redwood groves generally contain few seedlings. But, as with jack pine, the seeds fare best on bare, mineralized soil that has been stripped by fire or newly deposited as silt. Again, then, the redwood is equipped to return after fire – and for sexual reproduction at least may be said to be dependent on it. Again, the principle is common throughout nature: that organisms often delay reproduction until conditions turn nasty, and a new generation is needed to carry the day. Many organisms reproduce only in extremis.
Despite their solitary splendour in the western forests, coastal redwoods are not innately standoffish. They do not set out, for example, to poison their putative competitors, as some plants do. On the slopes, away from floods, they may mix well enough with Douglas fir, tanoak, grand fir or California bay. Tanoaks too sprout vigorously after fires, and tolerate shade, and are in other ways resilient, with a bark rich in tannin that protects them against insects and fungi. But (like Douglas fir, grand fir and California bay) the tanoak succumbs to flooding and silting; so on the floodplains, the redwood reigns supreme.
Overall, I suggest that the tropics show what nature can really do: how various it becomes when conditions are easy, or when the pot is stirred now and again, as by the drying of the ice ages, in a creative way. By contrast, in the high latitudes, which mainly means in the north, nature was repeatedly filleted by the ice ages, and what we see now are the few brave and specialist survivors.
Yet, so far, this book has dwelt mainly on the basic cast list: who’s who, where they all are, and why. Ecology begins when we ask how all the different players interact. That is the subject of the next chapter.
13 The Social Life of Trees: War or Peace?
Figs – all 750 species of them – need their own wasps to pollinate them
‘Nature red in tooth and claw,’ wrote Lord Tennyson in the 1830s. ‘No man is an island,’ said John Donne, about 150 years earlier. Both are right. War and peace are threads that run through all of life, from the cradle to the grave, from the seed to the compost heap. Everything we do reflects the tension between conflict and cooperation.
The tension is evident in reproduction. No individual creature needs to reproduce. Many a bachelor uncle and aunt live perfectly happy lives without offspring. But if any lineage of creatures abandons reproduction all together, it dies out. Yet reproduction is costly. It takes energy to produce gametes. It takes even more to produce viable, fertilized eggs, or live young. Many creatures die after a single bout of reproduction, including most annual plants and even some mighty trees like the big bamboos and some of the greatest palms. Even a few eudicot trees follow such a strategy. Over eighty years or so in the forests of Panama, Tachigalia versicolor of the Fabaceae family grows into a tree imposing enough for any royal park – then blossoms, sets seed, and dies. I have stood at the feet of one already dead. If parent organisms do not die (as most do not), then their offspring may become their assistants, as in many bird families, and in our own societies when they are working well. But children also become rivals. Contrariwise, the greatest threat to the offspring often comes from its parents – and so as we have seen, tropical trees that aspire to grow too close to the mother tree may perish from its diseases. The offspring of Tachigalia, presumably, are better off as orphans. The theme of parent–offspring rivalry runs through all of literature, beginning with the Bible and the Greeks (and doubtless through pre-literate cultures too) – and the greatest themes of literature are those of nature. Reproduction is necessary, to be sure. But it takes a heavy toll.
Sex adds another layer of complication. Sexual reproduction is less efficient than the many asexual strategies seen in nature. It takes two parents to produce even a single offspring by sexual means, whereas cloning requires only one. Yet most creatures practise sex – including all the trees that I know about, even though many of them also reproduce asexually. In truth, sex has nothing directly to do with reproduction. Natural selection has favoured sex for another reason – because it mixes genes from different organisms, and so produces variation: offspring produced sexually are all genetically unique, different from either of their parents, and from their siblings. This offers long-term advantage, since variation is a key ingredient of evolutionary change. But natural selection does not look to the future. Sex must bring short-term advantage – for if it did not, then sexual beings would lose out to asexual ones, who in principle can reproduce twice as quickly. What is the short-term advantage? There are two main hypotheses: one (from the American biologist George Williams) says that if all the offspring are different, then this increases the chances that one, at least, will find itself in favourable conditions, and live to pass on its parents’ genes. The other (from England’s Bill Hamilton) says that short-term variation keeps parasites on the back foot. If all members of a population are the same, then any parasite that can attack one of them will be able to defeat all of them. If they are all different, then each individual poses new problems. There is direct evidence to support this idea. Certainly, modern crops grown as genetically identical monocultures are especially vulnerable to parasites.
For sex to work, each creature must find a mate. For animals this can be dangerous – many a lion and stag has died from battles with rivals; many a male spider has died in his chosen one’s jaws. Trees, rooted to the spot, must find a way to transport pollen from male flower to female. The female flowers (or the female parts of hermaphrodite flowers) must in turn be able to capture pollen. Yet they must not be too promiscuous, and allow themselves to be entered by pollen of the wrong type. In general they reject pollen from other species. Many trees, including domestic plums and apples, also reject pollen from other trees that are too genetically similar to themselves: by such self-incompatibility they avoid the genetic perils of incest. Growers of plums and apples must grow varieties side by side that complement each other. Again, the tensions between males and females are a principal theme of literature, and always have been, running as strongly through the Bible and the Koran as anywhere else.
Yet the relationships between creatures of the same species – parents and offspring, friends and rivals, males and females – are only a part of life’s complexities. Each creature must perforce interact with all the other species that share its environment, and especially for those that live in forests, the catalogue of cohabiting species is very long indeed. Sociology merges with ecology. It is not true (as Tennyson’s line implies) that each individual or each species is inevitably in conflict with all the others. No species, to extend Donne’s metaphor, can ever be an island. Cooperation is often the best survival tactic, as Darwin himself emphasized – and so it is that many pairs and groups of different species are locked in mutualist
ic relationships that are vital to all participants. Yet even here there is tension. Figs need fig-wasps, and fig-wasps need figs. Their interdependence is absolute. But as we will shortly see, no once-for-all peace treaty has been signed, or ever can be. The relationship is always liable to break down as each partner begins to take advantage of the other – and ‘freeloaders’ (a technical biological term) cash in on both. Machiavelli spelled out the intricacies of such relationships in The Prince. The themes of literature are indeed the themes of nature. Still, though, it is not true as has often been argued of late, that human beings need to reject their own biology in order to behave unselfishly, as moral beings. Cooperativeness and amity are at least as much a part of us as viciousness. The point is not to override our own nature, but simply to give the positives a chance.
So what does all this mean for trees?
To begin with, for a tree to reproduce sexually, it must transfer pollen from anthers to stigmas. In theory, a hermaphrodite flower might achieve this easily enough by self-fertilization, but in reality this is rare. Most wild trees are ‘out-bred’. Pollen travels from the male flowers (or male parts of hermaphrodite flowers) of one tree, to the female flowers (or female flower parts) of another tree. Since trees do not move, they must employ couriers. In mangroves, water sometimes serves as the vehicle. In temperate forests, where any one tree is liable to be surrounded by others of its own species, and the weather in general tends to be breezy, the wind does the job. This is hit-and-miss, of course, and trees that do reproduce by wind tend to produce prodigious quantities of pollen. Flick a young male pine cone and the pollen swirls out like orange smoke. One early summer in Oxfordshire I watched two wood pigeons jostle for position in a birch tree. From a field away I could see the thick yellow puffs of pollen that they dislodged – illustrating, incidentally, that wind-pollination may be animal assisted.
In tropical forest, however, where any two trees of the same species may be half a kilometre apart, with thousands of trees of other species in between, it just will not do to scatter pollen literally to the four winds and hope for the best. In the tropics, only some trees of the open savannah or the Cerrado practise wind pollination – apart from a few like Cecropia, which grow only in forest clearings. Most tropical trees rely on animals to carry their pollen. This has led to some of the most spectacular examples of mutualism in all of nature.
ANIMALS AS GO-BETWEENS
Transmission of pollen by animals requires many layers of co-evolution. The flowers, both male and female, must have the shape and colour that the chosen pollinator will respond to, and they must be displayed appropriately. The pollinator in turn must be geared to the plant’s signals. When bees are at work in a rose garden it all looks simple enough, but in a tropical forest the bee or wasp or fly or bird or bat must seek a particular glint of colour, shape or whiff of scent among a cacophony of colours, shapes and scents, as a million different organisms send a million different signals to their potential mates, allies, predators or prey; and there are many other smells besides to sow confusion, including those of general decay.
How is this possible? It’s as if any of us could pick out the reedy squeak of the oboe when the Berlin Philharmonic was in full spate – but then of course we can; or at least, the conductor does. Some biologists have suggested that the ability of a wasp to detect its particular fig, or of a night-time moth to pick up the ultrasound pulse of a bat, implies an advanced ability to filter out all extraneous scents or sounds, as any of us can do (up to a point) when chatting at a cocktail party. But perhaps the truth is the other way around. Perhaps particular animals are geared exclusively to the particular sights, smells or sounds of the flowers they feed from or the predators they seek to avoid, and register nothing else. In the same kind of way we see light, and yet are not at all fazed by the radio waves and ultra-violet and cosmic rays, not to say the swarms of neutrinos, that assail us all the time. Our senses are simply not aware of them. No filtering is required.
We see, too, that co-evolution is an exercise in give and take. The tree must buy the animal’s help. Many primitive plants, such as waterlilies and the trees of the custard-apple family, the Annonaceae, allow or encourage the pollinating insect to eat great chunks of the flower itself. Others offer custom-made especially attractive food, which commonly but not always takes the form of nectar, both sweet and nourishing. Sometimes they lure the pollinators with aromatic oils. Often the pollinating animals eat a lot of the pollen itself. In short, trees that seek insect help must pay a double price. First they must produce the pollen and ovules, and all the supporting apparatus of petals and sepals – but then they must make a surplus, to bribe the pollinators.
Some of the relationships between trees and their pollinators are somewhat loose: many trees solicit the help of several or many animal helpers, and many animals seem happy to pollinate many different trees – domestic honey bees, for example, are generalist pollinators. But often the relationship is specific. Often a particular plant is completely committed to one particular pollinator (a bee, a moth, a wasp, a hummingbird), while each pollinator depends absolutely upon the particular tree. Generalism spreads the options, but reduces precision. Specialization improves the accuracy, but also means that the fate of any one creature is linked absolutely to the fate of another. Lose the pollinator (for example through some over-enthusiastic attack with insecticide) and you lose the plants that depend on it.
Insects and birds are the chief animal pollinators and for them (unlike mammals, which tend to be colour-blind) colour is critical. They each have their preferences. Beetles were probably the world’s first animal pollinators (they pollinated cycads long before flowering plants came on the scene), and beetles prefer white flowers. They ignore red. Bees too prefer white – although bees are perhaps most alert to ultra-violet, which we don’t see at all: often the flowers that seem to us to be plain white turn out to be ultra-violet coloured (and intricately patterned – sometimes with a road map to the nectaries).
Red or purple flowers attract butterflies – and may repel all the insects that prefer white, including the potential freeloaders. Moths are closely related to butterflies and yet, like beetles and bees, they prefer white. The colour preferences of butterflies and moths are reflected in the related Amazonian trees Hirtella and Coupeia. (They are both in the family Chrysobalanaceae, in the Malpighiales.)
The many species of Hirtella are geared up perfectly to butterflies. They open by day; they are pink or purple (rarely white); they have hoods, which provide the butterflies with a place to land; they reward their pollinators with copious nectar; and they have only a few stamens (the organs that bear the pollen), which are neatly and widely spaced. Butterflies of many species, guided by sight as well as scent, land at leisure on the flowers of Hirtella and feed decorously, coating themselves in pollen as they do so. Coupeia puts its trust in moths – particularly big hawkmoths, which fly by night and hover like hummingbirds to feed. Coupeia flowers open at night, just a few at a time, and are always white. They have a great many stamens – from 10 to 300. They coat the hovering hawkmoth (and occasional hummingbird) with liberal quantities of pollen as it probes among the tangled stamens for the nectar – which Coupeia provides even more generously than Hirtella.
As a relative of the custard apple, the Amazonian tree Annona sericea is pretty primitive; and it is pollinated mainly by beetles, which as insects go are primitive too. But ‘primitive’ does not mean ‘simple’, or merely ‘prototype’; and the degree of co-adaptation between A. sericea and its pollinators is extraordinary. To be sure, the flowers of A. sericea are simple: three fleshy petals that never fully open, grouped around a central conical knob which bears both the stigma (female) and the stamens (male).
At about seven o’clock in the evening – soon after dark in the tropics – the flowers begin to warm up, to about 6°C above ambient. You may well find this surprising: after all, we all learn at school that only mammals and birds are ‘warm-blooded’,
able to raise their body temperatures just for the sake of it. In truth, though, many creatures can do this – probably including some dinosaurs, and certainly some modern insects and some sharks. Some flowers can do it too. The rise in temperature helps to intensify their odour. The flowers of A. sericea do not breathe out the sweet smell of violets and honeysuckle that entranced lovers in Shakespeare’s comedies but (says Ghillean Prance) a perfume more ‘like chloroform and ether’. But it serves its purpose. Beetles (of the particular kind known as chrysomelids), and also some flies, come flocking in.
The beetles squeeze their way past the fleshy petals to the cone of male and female organs within. This is a common device among insect-pollinated flowers: provide an obstacle, which only the desirable insects can overcome. The stigmas at this time are ready to receive pollen, but the anthers, which provide pollen, are still closed. Any pollen that the beetle has about its person may thus be transferred to the stigmas. But the beetle, at this stage, cannot obtain pollen from the same flower that it is pollinating; so there can be no self-fertilization. The beetles often stay to copulate within the flowers and as they mill about they transfer even more pollen.
When the beetle has transferred its pollen, the stigmas at the top of the central cone fall off. Then the anthers become erect and release their pollen, and so the beetles become coated in it. Then the stamens drop off. The beetles eat the bases of the petals and then the petals fall off. Then the beetles can escape (they could escape before the petals fall off but they generally do not) and fly to another flower – now carrying pollen from the flower they have just pollinated. The flies that may visit also serve as pollinators in passing, and may lay their eggs on the sepals, but the beetles are the main players. Note, in this account, that the flower is seriously damaged, not to say destroyed by the beetle: the flower sacrifices bits of itself to bribe the beetle. But so what? The flower is only a lure. Once pollination is effected, its job is done.