by Colin Tudge
WHY BE A TREE?
A non-living thing is passive. The atoms of which a stone is composed sit there for as long as it endures – until it is melted in some volcano, or dissolved by acid rain. But living things are restless, through and through. As soon as some living cell has constructed some protein, as part of its own fabric, it starts to dismantle it again. This constant self-renewal, powered by an endless intake of energy, is called ‘metabolism’.
Metabolism – the basic business of staying alive – is half of what living things do. The other half is to reproduce. It is not vital to reproduce in order to stay alive – indeed, reproduction involves sacrifice; reproduction, as we will see later in this book, is often the last fling: many a tree dies after one bout of it. But it is essential nonetheless. At least, all creatures that do not reproduce die out. However successfully an organism may metabolize, sooner or later time and chance will finish it off. Everything dies. Only those that reproduce endure – or at least, their offspring do. All individuals are part of lineages – offspring after offspring after offspring.
But then, too, each creature finds itself in the company of other creatures, of its own kind and of different kinds. To some extent they are its rivals, to some extent it needs them – for food, shelter, mates, or whatever. Each successful creature, then – each one that survives at all, that is – must come to terms with the others around it.
All of life’s requirements – metabolism, reproduction and the business of getting along with others – are difficult. Each creature must solve life’s problems in its own way. There is no perfect, universal life strategy. Each has its own advantages and drawbacks.
So it can pay a creature to be very small; or it can pay to be big. Each mode has its pros and cons. A plant that is big like a tree can stretch further up into the sky, and so capture more of the sun’s energy; and reach further down into the earth, for water and minerals. This is the upside. But it takes a long time to achieve large size, and whether you are an oak tree or an elephant or a human being, the longer you take to develop the more likely you are to be killed before you reproduce.
Being big is difficult, too. To hold a ton of leaves aloft in the sun and air requires enormous strength: specialist material like wood, and clever architecture. All trees have wood, by definition (apart from those granted honorary status, like bananas); but as we will see, wood is subtle stuff, requiring much chemistry, and micro-geometry. Trees between them have essayed many architectural forms. Ginkgoes and conifers are built from repeats of a single simple module: a straight trunk up the middle with circles or spirals of branches at intervals. In others, like the elm, the lead shoot bends over and the next shoot in line takes over the lead until it too bends away and the one below that takes over. In others (particularly some tropical trees) the branches that grow upwards from the horizontal branches repeat the form of the whole tree – it’s as if a new, miniature forest grew aloft, from the horizontal branches of the giants below. Others, like oaks or chestnuts, are more free-flowing. There are many basic designs. The point is, though, that such design is necessary. Being big requires a lot of engineering as well as a lot of chemistry, and it takes a long time to put in place. But the bigger trees grow, the more they are vulnerable to wind – and tropical storms regularly cut swathes as big as an English county through the world’s rainforests.
For the purposes of reproduction, creatures in general pursue one of two main strategies. Some, known as ‘K-strategists’, produce just a few offspring at a time which in general are large at the time of their birth to give them a good chance in life; after they are born, typically, the parents take good care of them. K-strategists in general are longlived and reproduce several times in their life, often at long intervals. Orang-utans, elephants, eagles and indeed human beings are classic K-strategists. Other creatures, known as ‘r-strategists’, produce an enormous number of offspring. Inevitably, each individual offspring is small, and so has little chance of survival. But there is safety in numbers. Codfish are noted r-strategists. They produce up to 2 million eggs at a time. The newly hatched fish live for a time as plankton, floating fairly helplessly – and most perish: they just get eaten. But so long as each pair of codfish manage to produce just two surviving offspring in the course of their lives, then the lineage of cod will carry on. Despite the enormous prodigality of their reproductive strategy, its fantastic wastefulness, codfish are immensely successful – or at least were until the North Sea fishermen became too technically proficient, and too ‘competitive’, and disastrously reduced their numbers. Cod live a long time. But many r-strategists, like flies, run through their entire life cycle in a few weeks – birth, growth, reproduction, death. Thus populations of flies may rise and fall from near zero to plague proportions in what seems like no time at all.
Trees seem to get the best of both worlds. Many – most – produce huge numbers of seeds and may do so repeatedly. A mature oak or beech may produce many millions of seeds in a good year (good seed years are known as ‘mast’ years) and although they won’t do this every year, they may well have scores or even hundreds of prolific years in the course of their lives. They are r-strategists indeed – in a good year at least as prolific as codfish. Yet many trees – including oaks – produce seeds that are large and which do not need to germinate immediately: each has a very good chance of survival. To this extent they are K strategists too. To combine the advantages of the K- and r-strategy an organism must be truly mighty. Yet there is a downside too: most trees must grow for several years, and many must endure for several decades, before they can reproduce at all; and all the time they are growing, without yet scattering their seed, they are vulnerable.
We don’t think of trees as r-strategists, because they are so big and long-lived. Their populations do not boom and bust like those of flies. They cannot, we imagine, leap to take advantage of newly created environments as a fly or a mouse may do. Yet we can see that they can and do do this – once we venture beyond our own puny timescale, and take the long view. Thus when the last ice age ended in the northern hemisphere around 10,000 years ago the forests of birches and alders that had been whiling away the time further south were able virtually to race towards the Pole in the wake of the retreating glaciers; and they will surely resume their advance as global warming reduces the polar ice still further. By the same token the huge tropical rainforest of Queensland in the southern hemisphere has not been there for ever, as it may seem. Like the Great Barrier Reef, which stands just off the Queensland coast and is as long from end to end as Great Britain, the rainforest of Australia grew up only after the last ice age, and is a mere 10,000 years old. Macbeth was shocked to see the Great Wood of Birnam shift a few miles across the moor to the Hill of Dunsinane. But if we could take a time-lapsed view of all the world this past few million or tens of millions of years, as cold has followed warm has followed cold, we would see vast and apparently immovable forests flitting over the surface of the globe like the shadows of clouds.
Thus the advantages of treedom are both manifold and manifest. Big plants can metabolize more effectively because they command so much earth and sky; and they can produce literally tons of seed, to be scattered far and wide. Small wonder that a third of all land is covered in forest. But being big is complicated – all that chemistry and architecture – and it is risky, because all the time a tree is growing, time and chance and other creatures are working on its downfall. So it is that many other plants, such as mosses and liverworts, never acquired the means to be big at all; but still they have made a very good living this past 400 million years, just by sticking to damp and easy places. Then again, trees cannot grow where it’s too dry or the soil is too thin, and so they leave scope for many smaller plants that can. So the world’s grasslands are vast too, like the savannahs of the dry tropics, or the prairie of temperate North America and the pampas of subtropical South America, and the steppes of Asia. These grasslands at best have scattered trees, though they grade into open w
oodland – many small trees but with big, mainly grassy spaces in between, as in the dry, tropical Cerrado of Brazil. Furthermore, trees are classic ‘keystone species’: simply by existing and doing their thing, they create niches where other creatures can live. Hence forests create endless scope for small, quick-growing plants – herbs and ramblers – to occupy the ground in between the trees; and a vast variety of plants of all kinds (mosses, liverworts, ferns, and many kinds of flowering plants including many relatives of the arum lily and of the pineapple, some cacti, and most of the orchids) grow on the trees themselves, as epiphytes. Overall, too, there is more room for small plants than for big ones. Whole, viable populations of small plants may need only a few square metres, while a population of wild trees that is numerous enough to endure will generally need many hectares. So although there are tremendous theoretical advantages in being a tree, the species of trees are outnumbered by non-trees by about five to one. The non-trees live in places where trees cannot – and in the niches created by trees.
So now to the third childish question.
HOW MANY KINDS OF TREE ARE THERE?
A simple question indeed – but of course there are complications. To begin with, as the more irritating kind of philosopher would say, ‘It depends what you mean by kind.’
In this context, ‘kind’ most obviously means ‘species’. The common oak is a species: Quercus robur. So is the Scots pine: Pinus sylvestris. The common birch is Betula pendula. And so on. What’s the problem?
One problem is, how you tell the different kinds apart. Any one species is liable to be highly variable, and sometimes different species resemble each other very closely. Sometimes there is more variation within species than there is between species. Or then again: many creatures can be identified definitively only by their reproductive organs, which in the case of flowering plants (including most trees) means flowers. But many trees are not in flower at the time you come across them – a particular problem in the tropics, where flowering often seems to be erratic (or at least, the tree knows when it is appropriate to flower, but the biologist does not). But some trees with similar flowers have different leaves, and both may be needed to make the identification. Willows, however, tend to produce their flowers before they produce leaves – so you never find flowers and leaves on the same tree at the same time. If you want to know what species a particular willow belongs to, you may have to make two visits.
But biologists do not define species purely in terms of what they look like. Much more fundamental, they very reasonably feel, is who mates with whom. If different individuals breed together, then it is reasonable to declare that they are of the same species. Betula pendula will happily breed with other Betula pendula, but not with Quercus robur. So they are different lineages of creatures, living separate lives. Easy.
Still, there are snags. Many species can and do interbreed with other species, and so form hybrids. The example that everyone knows is the mule: the issue of a male donkey and a mare. But horses and donkeys seem to be very different kinds of animals. If they can breed together, doesn’t this mean they are of the same species? No – for although the mule is a powerful animal and ‘mean’, as cowboys were wont to complain, it is nonetheless sexually sterile. Strong though it is, it is not, as a biologist would say, ‘viable’. So we can extend our definition slightly: ‘Two or more individuals can be considered to be of the same species if they can mate together to produce fully viable offspring.’ ‘Fully viable’ implies sexual potency; and also implies that the offspring should be able to compete successfully in the wild. For there are some hybrids (for example among frogs) that are sexually fertile yet generally fail in the wild, unable to compete with either of their parent species. Again, it is reasonable to rank the parent types as separate species, since the hybrids they produce between them are (relative) failures.
Still, there are problems. For example, two apparently different species, which look different, may fail to interbreed in the wild simply because they live in different places. Bring them together, and they may interbreed perfectly happily. Trees provide scores of examples – among oaks, willows, poplars, and many more. Many hybrids have arisen in gardens, where human beings bring plants from very different areas together, perhaps for the first time in many thousands of years. Among the most striking examples is the London plane, Platanus x hispanica (the x indicates its hybrid status). It is tremendously successful in cities, not simply in many a London street and square, but throughout the northern hemisphere. Because it sheds its outer bark (as a eucalyptus or a madrone does), it gets rid of all the soot and other pollutants that can make life so difficult for many other kinds of tree. It is the offspring of the oriental plane from southern Europe and Turkey, Platanus orientalis, and the western plane from North America, Platanus occidentalis, and arose, so tradition has it, in the Botanic Garden of Oxford University, in the seventeenth century. An offspring of the first-ever London plane now stands in a courtyard in Magdalen College, which is next to the Botanic Garden. That offspring, now several centuries old, is huge. For those who would be connoisseurs, it is well worth a diversion (assuming the porters will let you in).
Then there is the extremely important phenomenon of ‘polyploidy’. Genes, as everyone knows these days, are aligned along chromosomes. Every kind of organism has its characteristic number and arrangement of chromosomes. Eggs and sperm (or the appropriate cells in ovules and pollen) contain only one set of chromosomes, and are said to be ‘haploid’. When they fuse in the act of fertilization, the resulting embryo has two sets of chromosomes and then is said to be ‘diploid’. Most organisms (at least of the most familiar kinds) are diploid: for example, human beings have 46 chromosomes – 23 acquired via the egg of the mother and 23 from the sperm of the father. Chimpanzees have 48 chromosomes – 24 from each parent.
Sometimes, however, apparently spontaneously, the chromosome number will double (the chromosomes divide in the normal way they do in preparation for cell division – but then the cell fails to divide). Then the diploid cell becomes tetraploid, with four sets of chromosomes. This does not apparently happen much in animals (or not, at least, in mammals) but it is extremely common in plants. The newly-formed tetraploid organism can breed successfully with other tetraploids of its own kind but it cannot usually breed successfully with either of its parents. So it forms an instant new species. Many plants in nature turn out to be tetraploid, and many more tetraploids have been produced in cultivation. The common potatoes grown in Europe are tetraploid derivatives of diploid potatoes that grow wild (and are cultivated) in the Andes. Many trees, wild and cultivated, are tetraploid. Sometimes the chromosomes of the tetraploid plant double again to produce octoploids. The octoploids again form new, discrete species – generally unable to interbreed with the tetraploid parents who gave rise to them. ‘Polyploid’ is the general term that describes any organism with more than two sets of chromosomes. Sometimes the complications become too much even for the plants and they finish up with an odd number of chromosomes (some having been lost among all the cell divisions and matings). Plants with anomalous numbers of chromosomes are said to be ‘aneuploid’. Aneuploidy in animals generally leads to various degrees of disorder – aneuploid animals usually die and if they live they tend to be compromised at least to some extent. But many plants put up with aneuploidy. Sugar cane is aneuploid; but that doesn’t stop it being an extremely vigorous, major crop.
There is one further complication. As we have noted, diploid organisms that are of different species sometimes mate to produce fully viable offspring (as the eastern and the western plane trees evidently did). But usually such crosses fail, and often this is because the chromosomes of the two parents are incompatible. The two different sets of chromosomes might be able to support body cells that work well enough (as in the mule). But even if cells with two different kinds of chromosomes succeed this far, they will not necessarily produce sound gametes (eggs and sperm, or ovules and pollen) because this requires close
cooperation between the chromosomes.
But if a hybrid organism doubles its chromosomes, it often can produce viable gametes. So we find diploid parents of different species mating to produce diploid, hybrid offspring that are sterile; but the hybrids then double their chromosomes and become tetraploid – and the hybrid tetraploids are then fertile. This happens a lot among plants, and has produced many, many new plant species, both in the wild and in cultivation. Indeed the complications seem endless. For instance, a tetraploid plant might mate with a closely related diploid plant to produce a triploid offspring – two sets of chromosomes from the tetraploid parent, and one set from the diploid parent. Triploids are sterile – they cannot produce gametes at all – but they may still form viable plants. Thus the cultivated banana is triploid. Because it is sterile, its fruits contain no seeds (as wild banana fruits do). So the domestic banana has to be reproduced vegetatively, by planting cuttings. In other cases, though, triploid hybrids double their chromosomes to become hexaploid (with six sets of chromosomes). The most famous and important hexaploid organism of all is bread wheat (as opposed to pasta wheat, which is tetraploid).
If you have been brought up with animals, and are innocent of botany, you may find all this fantastical. But among trees, hundreds of examples of polyploids are now known: the more that botanists look, the more polyploids they find. Some of the polyploids simply represent a doubling (or redoubling) of chromosomes within one species. Others are polyploid hybrids. For good measure, breeders have produced many hundreds of polyploids by artificial means. (Some chemicals induce polyploidy almost to order.)
Willows, genus Salix, provide many fine examples of polyploid trees. There seem to be around 400 species — although there must be many more that are yet unknown, including an entire phalanx in western China, yet to be properly studied. Some willow species have a haploid number of 19 chromosomes, so that the diploids have 38 (2 x 19). But another group of willows has a haploid number of 11 (diploid 22) and the third group has 12 (diploid 24). There doesn’t seem to be much hybridization between willows with different haploid numbers, but there is a great deal of hybridization between different species with the same haploid number, and this has produced a whole array of polyploids with up to 224 chromosomes. Most of those polyploid hybrids are fertile, and some willows have been bred artificially from combinations of up to fourteen different species. For good measure, many of the hybrids are all of one sex and reproduce by suckers, so that all the members of such ‘species’ in fact form a clone (of which more later). Thus, the hybrid known as Salix x calodendron is all female. Many willows, too, both wild and in cultivation, are aneuploid. All in all, identification of the multifarious willows – the diploid types and all their polyploid hybrids – is a nightmare (even when they are not tucked away on some remote Chinese hillside).
