by Colin Tudge
The fourth of the major plant hormones, the cytokinins, have the opposite effect to ABA: they prompt cells to divide. The cytokinins were first discovered in 1941. They turned up first in coconut milk, but they are now known to occur in all plants. In some contexts cytokinins act in opposition to auxins, and override them: so horticulturalists may use cytokinins to make side-buds sprout, even when the apical bud is still in place.
The fifth of the basic hormones is ethylene, which affects many different aspects of a plant’s life, from the ripening of fruit to the falling of leaves and a great deal more besides. Ethylene is a chemically simple gas, and it may seem an odd choice for a hormone, for gases are wayward clouds of molecules that seem too unruly for precision work. But then, one of the surprises of recent years has been that the physiology of animals – not least of human beings – is profoundly influenced by nitric oxide, which is even simpler than ethylene. Like ethylene, nitric oxide seems to be involved in just about every system that has been looked at. It is the key even to Viagra – which operates by prompting release of nitric oxide, which in turn, in this context, acts as a muscle relaxant, and so allows engorgement. Ethylene does not act simply as a hormone within any one plant. It may also travel from plant to plant and so acts as a pheromone: an airborne hormone that acts on creatures other than the one that produces it. Whether nitric oxide also behaves as a pheromone, as ethylene does, is a most intriguing question.
Ethylene’s role as a hormone was first revealed in the 1880s, when the trees that lined the streets in many a city began to lose their leaves. German scientists were the first to find the reason: the street lights were run on gas, and some of it escaped unburned. In 1901 one D. Neljubov showed that none of the several components of the gas had any defoliating effect, except ethylene. Ethylene was active at astonishingly low concentrations: 0.06 parts per million (or six parts per 100 million). On the other hand, the light from street lights can prompt city trees to retain their leaves, so that the same tree may lose its leaves on the shaded side, but keep them far longer on the side nearer the lamp. City trees in the days of gaslights must have been horribly confused.
Ethylene soon proved to have other effects too. It causes fruit to drop, as well as leaves: and, like auxin, ethylene is used to thin commercial crops of plums and peaches, and to loosen cherries, blackberries, grapes and blueberries in preparation for mechanical harvesting. Ethylene also promotes ripening, and herein lies another bizarre tale. In the early 1900s growers ripened fruit and walnuts in sheds which they warmed with kerosene stoves. But the more go-ahead growers switched to electricity. This was altogether more satisfactory: cleaner, more reliable, more modern. The only trouble was that the fruit no longer ripened. It wasn’t the warmth that did the ripening. It was the ethylene, leaking from the smelly and despised old heaters.
So it is that plants control their form. Darwin wrote of the English wayside as a ‘tangled bank’, and in the jungle the tangle can be beyond unravelling. Yet each plant in the mêlée knows what it’s doing. Each contrives to position its leaves in the light, and send its roots to the ground (except for a few specialist epiphyte orchids, which send some of their roots upwards), while the climbers wrap around others for support. In short, all in the apparent havoc know exactly what they are doing. Each adjusts its shape to the conditions, and to the presence of the others; and all this achieved, it seems, through astonishingly simple chemistry. The workings of trees, like plants in general, are indeed wonderfully elegant.
But trees do not dwell only in the present. They remember the past, and they anticipate the future.
THE PAST AND THE FUTURE: MEMORY AND ANTICIPATION
How trees remember, I do not know: I have not been able to find out. But they do. At least, what they do now may depend very much on what happened to them in the past. Thus if you shake a tree, it will subsequently grow thicker and sturdier. They remember that they were shaken in the past. Wind is the natural shaker, and plants grown outdoors grow thicker than those in greenhouses, even in the same amount of light; and so it is too that parkland oaks grown in splendid but breezy isolation are much more sturdy than those of the forest. Similarly, a larch tree remembers attacks by caterpillars. In the year after an assault it produces leaves that are shorter and stouter than usual. (Larches are among the few deciduous conifers.) Short, stout leaves do not photosynthesize as efficiently as the thinner, longer kind, but they are better at fending off pests. In subsequent years, however, the once-infested larch can and does revert to normal foliage. By then, the population of predatory moths would have plummeted, since the offspring of the original plague of caterpillars (which became moths) would have found nowhere to feed.
Most trees, like most plants of all kinds, are also aware of the passing seasons: what time of year it is; and – crucially – what is soon to follow. Deciduous trees lose their leaves as winter approaches (or, in the seasonal tropics, as the dry season approaches) and enter a state of dormancy. This is not a simple shutting down. Dormancy takes weeks of preparation. Before trees shed their leaves they withdraw much of the nutrient that’s within them, including the protein of the chlorophyll, leaving some of the other pigments behind to provide at least some of the glorious autumn colours; and they stop up the vessel ends that service the leaves with cork, to conserve water.
How do the temperate trees of the north know that winter is approaching? How can they tell, when it is still high summer? There are many clues to season, including temperature and rainfall. But shifts in temperature and rain are capricious; they are not the kind of reliable signal to run your life by. Sometimes a winter may be warm – but frost is never far away. Some autumns and springs are freezing, some balmy. The one invariable, at any particular latitude on any particular date, is the length of the day. So at least in high latitudes, where daylength varies enormously from season to season, plants in general take this as their principal guide to action – while allowing themselves to be fine-tuned by other cues, including temperature. So temperate trees invariably produce their leaves and/or flowers in the spring, marching to the rigid drum of solar astronomy; but they adjust their exact date of blossoming to the local weather. This phenomenon – judging time of year by length of day – is called ‘photoperiodism’. Most of the basic research on photoperiodism has been done on crop plants, which for the most part are herbs. But trees and herbs work in the same way. What applies to spinach and tobacco applies to trees too.
Knowledge of photoperiodism again dates from the 1920s, when agricultural scientists in America found that plants like tobacco, soya, spinach, and some wheat and potatoes would not flower if the days were shorter than a certain critical number of hours (often around twelve). But other plants would not flower if the days were too long: strawberries and chrysanthemums were among those that remained resolutely sterile if the days were longer than sixteen hours. There were some, though, that didn’t seem to mind the length of day. The three groups became known as ‘long-day’, ‘short-day’, and ‘day-neutral’. Long-day plants generally flower in high summer, and short-day plants in spring or autumn. As a further refinement, plants also seem ‘aware’ that absolute day-length has different significance at different latitudes. At very high latitudes, the longest days are twenty-four hours – the sun never sets – and a fourteen-hour day is of modest duration. But in the subtropics, fourteen hours is a long day – as long as any day gets. Sometimes the same species may grow both at high and at low latitudes, including for example the aspens of North America. Then the northern ones will treat a fourteen-hour day as short, and the more equatorial ones will treat a fourteen-hour day as long. Adaptation is all.
In the late 1930s it became clear that plants do not measure the length of the day, but of the night. If the light is turned on even briefly during the night – a minute from a 25-watt bulb would do – then short-day plants such as strawberry will not flower. Contrariwise, a long-day plant that flowers in sixteen hours of light and eight hours of dark will also flow
er with eight hours of light and sixteen hours of dark – if the darkness is interrupted by a brief light. In truth, long-day plants should be called short-night plants; and short-day plants are really long-night plants.
In the next few years the underlying mechanism became clear – and again it is remarkably simple. Inevitably it depends on a pigment – for pigments by definition are chemical agents that absorb and reflect light, and so mediate a plant’s (or an animal’s) responses to it. In this case the pigment is phytochrome. Phytochrome exists in two forms which either suppress or promote flowering; and light flips them from one form to the other.2 Again, these insights have been put to use. Growers of chrysanthemums used to keep the lights in the greenhouse on at night to delay flowering until Christmas – until, in the 1930s, they saw that a brief burst of light at night would produce the same effect, and much more cheaply. Contrariwise, appropriate flashes will bring long-day plants rapidly into bloom, by artificially shortening the nights.
All these mechanisms are evolved – they have been shaped by the experiences of past generations. They can succeed, and serve the plant well, only if present and future conditions are like those of the past. If conditions change slowly over time, then any lineage of creatures, animals or plants, can adapt to the change. But if conditions change rapidly, then creatures that have evolved their survival strategies in earlier and different times find themselves caught out.
Human beings are changing the world profoundly and – by biological standards – with extreme rapidity. In particular, we are altering the climate. Present-day pines and oaks and birches in northern latitudes are adapted to the idea that long days are warm and short days are cold. Everything they do – germination, dormancy, the shedding of leaves (in the deciduous types), the production of flowers and cones – is geared to this assumption. If long days turn out to be cooler than expected, or significantly hotter, drier or wetter, and if the cold days are not particularly cold, then the whole life cycle can be thrown out of kilter. The confusions of urban trees, when light and temperature are out of synch, are just a warning of what may happen to all the world’s forests when the interplay of light, warmth and moisture is altered on the global scale. If plants are seriously incommoded – whether wild trees or domestic crops – then everything else must suffer too. Of all the threats to the present world, this is one that matters most. Yet, as discussed further in Chapter 14, the effects of climate change on plants is extraordinarily difficult to predict. The insights of modern science are wonderful, but absolute knowledge is a logical impossibility. In the end, we are just going to have to wait and see.
This, then, in broad-brush terms, is how plants keep themselves alive. But as living creatures they need to carry out two more tasks. They need to reproduce; and they need to get along with their fellow creatures, of their own and other species. How they do this is discussed in the next chapter.
12
Which Trees Live Where, and Why
California’s coastal redwoods get much of their water from mist
Similar places all the world over pose similar kinds of problems – of light, dark, heat, cold, flood, drought, altitude, toxicity – and all the many varied trees that live and evolve in any one place tend to come up with the same kinds of solutions. Thus the Douglas firs, pines and spruces of the extreme north, and the rimus and kahikateas of New Zealand’s south, are all tall and steeple-like, to catch the light that comes at them from the side; while the cedars and umbrella pines of the Middle East and Mediterranean have flat tops, aimed at the sunlight beamed from overhead. The trees of tropical rainforest grow straight up through the crowd while those of the Brazilian Cerrado, the African savannah, or the Australian bush, spread themselves like cats. So it is that all the world’s forests conform to a score or so of different ecotypes – variations on a theme of boreal, temperate or tropical; wet forest (rainforest) or distinctly dry; seasonal or aseasonal – where seasonal means winter/summer, or wet season/dry season. Within this general framework are a series of specialisms. There are forests that follow rivers (‘riverine’, sometimes known as ‘gallery’ forests). Those in mountains are called ‘montane’. At moderate heights they are ‘alpine’; but in some wet warm places, as in much of South-East Asia, the trees become lost in mist towards the tops and so become ‘cloud forest’. Some forests have their feet in water: swamp forests, with willows, alders, swamp cypresses, and the rest; and mangroves, at the edge of tropical, shallow seas.
Yet no two forests are alike. They are like art galleries: they all have pictures, but they don’t have the same pictures. The forests of South-East Asia are rich in dipterocarps. Eucalypts are virtually confined to Australia – or would be, were it not for human beings, who have planted them virtually everywhere. Africa and Australia both have acacias in their wide open spaces – but they are different acacias. America, China and Europe all have plenty of oaks — but each has its own selection. Oaks and willows in general (with very few exceptions) are confined to the northern hemisphere. Southern beeches (Nothofagus) are indeed inveterately southern. Araucarias too, at least in these modern times, belong exclusively to the south. Some species — and indeed some genera or even families — grow only in particular islands, to which they are then said to be ‘endemic’. New Caledonia has thirteen endemic species of Araucaria out of a world total of nineteen. Madagascar has six of the world’s eight species of baobab, and is the only place with the extraordinary trees of the Didiereaceae. Britain, on the other hand, has a miserable native list of only thirty-nine species, none of which are endemic. All our natives occur else-where as well, mostly in much larger numbers than in Britain. Of course, lists of ‘British’ trees may contain hundreds of species, many growing wild; but the vast majority are imported. The British are supremely acquisitive.
So the first question is ‘Why?’ We would expect each region to contain plants that are adapted to it – for if they were not, then they would soon be ousted by those that are. But why does each region have its own characteristic suite of native species? Why are some species (or genera, or families) very widespread, while others are confined to single islands? Why are some islands rich in endemics (New Caledonia, Madagascar, Hawaii, the Canaries) while others (like Britain) have none?
There’s another kind of puzzle, too. Whatever group you look at – birds, butterflies, fish – you find there are many more species in the tropics than in the north or south; indeed the further you travel from the equator, the more the variety falls off. With trees the falling off is striking. The apparently endless boreal forest of Canada is dominated by only nine native species: a few conifers and the quaking aspen. The US as a whole has around 620 native trees. India (much smaller than the US) has around 4,500. In the Manu National Park of Peru, almost on the equator, twenty-one study plots with a total area of 15 hectares have yielded no fewer than 825 species of tree – about one-fifth the total inventory of all India, and considerably more than the US and Canada combined. As we saw in Chapter 1, the Ducke Reserve of Amazonia has more than 1,000 different trees. Tropical America as a whole, from Brazil, Peru and Equador up to Mexico, has tens of thousands of species. The true number can only be guessed. Why so many?
Both kinds of questions have been exercising biologists for several centuries (at least) and are still a hot topic: I attended the latest international conference on these matters at the Royal Society in London in March 2004. Hundreds of putative explanations are out there which between them encompass every aspect of the life sciences – and of the earth sciences, too. Some have to do with plant physiology, some with genetics, some with history, some with evolutionary theory. All are pertinent; all, indeed, are interwoven. The following is a rough guide to the main threads.
WHY TREES LIVE WHERE THEY DO
Each lineage of trees began with a single tree: the first-ever oak, the first-ever kauri, and so on. So – to begin at the beginning – where did those ‘founders’ arise? What is the ‘centre of origin’ of each species (or genus
or family or order)?
It’s at least commonsensical (and we have to start somewhere) to guess that the founders arose in the places where their descendants now live in the greatest variety. Eucalypts are extraordinarily various in and almost exclusive to Australia and there, surely, is their most likely origin. But of course life is not so simple. Oaks, for instance, span the northern hemisphere, and are most various both in North America and China – which are divided by the Pacific if you go round one way, and by the rest of Eurasia and the Atlantic if you go round the other. Even if we assume that oaks arose in either North America or China, they must at some point have travelled to the other far-distant continent. But if they can make such a journey as that, might they not have begun in the middle, in Europe? Or could they have begun in some completely different place, where they no longer exist, such as Africa? Either way it’s clear that the centre of origin, even if we can work out where that was, does not by itself explain the present distribution. Clearly some trees in the past – perhaps most of them –began in one place and then dispersed to others. If they found their new locations congenial, they could then have formed entire new suites of species – so that these outposts then become secondary centres of diversification. Sometimes, too, the secondary outpost might be the kind of place that encourages the formation of new species. Thus there are many different pines in Mexico, but we need not assume that this is where they first arose. They are diverse because the first to arrive there found it congenial and the mountains provide many different niches where semi-isolated populations can each evolve along their own lines. Just to confuse the picture a little more, any particular lineage of trees might well be extinct in the place where it first arose. The places where particular trees now flourish may well be secondary outposts – or indeed outposts of outposts, or outposts of outposts of outposts.
