by Colin Tudge
Finally, though, we should note in passing that not all autumn colour is due to anthocyanin, or indeed to active production of extra pigments. The yellows and browns of autumn leaves seem simply to represent pigments, notably yellow carotene, that are left behind after the tree has stripped its leaves of chlorophyll. They really do reflect innate inefficiency. But we should be grateful for this too. The reds are beautiful; but are the more beautiful because there are browns and yellows and every other shade alongside.
There are many more twists as the game of chemistry unfolds. An array of plants has been shown to produce various terpenes only after insects have begun to feed on them: another economy. These terpenes discourage the invading pests from laying their eggs. But in addition they also attract the natural enemies of the pests, so that they fly in from far and wide to see the pests off: swarms of parasitic wasps or ladybirds summoned to feast on aphids. So far such effects have been shown in maize, cotton and wild tobacco. I know of no specific examples in trees – but again it would be very surprising indeed if they were not to be found. Most herbaceous plants have trees that are close relatives and, beyond doubt, it’s just a question of finding the time and resources to look. Again there are commercial possibilities. Chemicals that summon help from insects that kill pests could surely be developed to protect crops.
Exactly how plants know that they are being attacked, and so tell their genes to produce more insect-repellent, is not known in detail. It is clear, though, that one essential ingredient in the chain of chemical communication is salicylic acid, which is widespread among plants (although willow bark is particularly rich in it: this is the source of aspirin). One modified form of salicylic acid is methyl salicylate, and this is volatile: produced in one plant it can float off in the atmosphere and so affect another. Thus it may serve as a ‘pheromone’, an airborne chemical signal. This means, though, that a tree that is being attacked can not only induce defences in other parts of the same tree – it can also warn other trees in the vicinity that trouble is afoot. So it is that when elephants in Africa feed from the mopane trees of Africa (Colophospermum mopane) they take just a few leaves before moving on to the next tree. Furthermore (so it is claimed) the elephants move upwind to a new tree. Evidently the mopane increases its output of tannins as the elephants browse, so the leaves become less and less palatable. Evidently, too, they emit organic materials (the identity of which I do not claim to know: perhaps the tannins themselves) that act as pheromones, and so warn other mopanes downwind that an attack is imminent and they too should produce more tannins. Some acacias are said to behave in the same way, in response to giraffes. We cannot hear the trees calling each to each, as T. S. Eliot claimed to hear the mermaids. But the air is abuzz with their conversations nonetheless, conducted in vaporous chemistry, and the ground too, via the bush telegraph of their roots.
But although it is clear that parasites are damaging, and although trees and their various tormentors go to such extraordinary lengths to overcome each other, we still must ask whether the attacks that we perceive to be so heinous are always as destructive as they seem. Thus, many trees – including those of the Rosaceae and perhaps most dramatically lime trees – are attacked every year by millions of aphids. The trees crawl with them. Linden leaves become sticky with ‘honey-dew’ – the surplus sugary sap that the aphids excrete. The stickiness attracts disfiguring sooty black fungi. Aphids also carry viruses. It seems downhill all the way. But perhaps, says Oxford ecology professor Martin Speight, the honeydew that falls on to the place below is good for the soil. Leguminous trees such as acacias, and non-legumes such as alder, harbour nitrogen-fixing bacteria in their roots. Many other plants, including many grasses, exude organic compounds from their roots that feed colonies of bacteria that remain free in the soil, but fix nitrogen nonetheless. Perhaps the honeydew dripping from the leaves of lime trees has just this effect: creating an environment in the soil beneath where nitrogen-fixing bacteria can thrive. Perhaps, taken all in all, the overall effect of the aphids is neutral — or even beneficial. Other insects too may help trees along by their nitrogenous excretions. Termites have nitrogen-fixing bacteria in their guts. Perhaps they bring extra nitrogen to tropical forest trees that are often starved of it, and so on the whole their attacks may be helpful.
Trees that seek to be pollinated by insects must sacrifice nectar, pollen, fats, or parts of the flower itself, all of which are metabolically expensive. Perhaps in the same spirit trees may sacrifice a proportion of their leaves – even half of them in a season – for the sake of soil fertility. Many thrive on marginal soils, after all, and perhaps they would not if it were not for pests, which perhaps are not so pestilential after all. Foresters hate the parasites that reduce growth rate. But does it matter to the tree? Trees, we may assume, are not proud. Does a mahogany mind if it develops a bushy top, instead of a fine straight bole? So long as it is still able to reproduce, why should it? Gardeners of a certain kind typically zap everything that moves with the vilest toxins they can find, and then spend their evenings cutting back the growth they perceive to be superfluous. Why not let the pests do the pruning? Organic gardeners who eschew all vileness often have the best gardens.
However – and it’s a big ‘however’ – the whole equation changes if the conditions change. Pests or diseases that can be shrugged off as part of life’s rich pattern in favourable circumstances, and may even bring net benefit, may be seriously bad news if the tree is under additional stress. For example, and notably, pests are kept at bay to some extent by the simple physical pressure of the sap. In drought, this pressure is reduced. Thus (says Professor Speight) bark beetles commonly make little headway into eucalypts – except in times of drought. Drought, and consequent stress, happen often enough to keep the beetles in business. Contrariwise, waterlogging is bad too.
Here is where our own restless and meddlesome species, Homo sapiens, plays many a part. We have carted entire battalions of pests around the world to fresh woods and pastures new, to where the native plants have had no time to adapt. ‘Exotic’ pests (and weeds) are often the worst. If it is true (as it seems to be) that the enormous heterogeneity of tropical forest is in part a response to disease – no tree can afford to be too near another of the same kind – then it follows as night follows day that monocultural plantations of tropical trees will be especially vulnerable (as is true of all monocultures). I do not know if mahoganies in plantations suffer more from shoot-borer moth than in the wild but it seems likely, and we know that many a mahogany grows straight and tall in the natural forest, out-topping its neighbours, without any help from pesticide.
More broadly, we are changing the climate. General warming is already enabling many an animal pest to move away from the equator into the realms of temperate trees that have had no time to adapt to them, and cannot of course run away. It is impossible to predict in detail how warming the whole globe will affect the climate of any particular spot, but we can be sure there will be plenty more drought, and plenty more waterlogging. Parasites in general adapt to changing conditions many times more quickly than trees can do. Some insects have a generation time measurable in days. In some bacteria it is minutes. Either may go through a thousand and more generations while a tree is still feeling its way. Trees are opportunists, but they are slow opportunists. It is the stock in trade of parasites to seize new opportunities very rapidly indeed. The armed but often amicable truce that has evolved over many a millennium between trees and their hordes of parasites could be horribly thrown off balance over the next few decades. There are a few things we can do – some specific controls, some replanting of more resistant species, and every possible attempt to minimize warming. But for the most part, we will just have to wait and see.
IV
Trees and Us
14
The Future With Trees
O if we but knew what we do
When we delve or hew –
Hack and rack the growing green!
&nbs
p; Gerard Manley Hopkins,
‘Binsey Poplars’, 1879
Humanity is in a mess. The statistics are simple and stark: a billion chronically undernourished, a billion in slums (and growing), more than a billion who live on less than two dollars a day. Even worse, because potentially more final, is the collapse of the earth itself, the place where we live. Soil, air, sea, lakes, aquifers and rivers: all are under stress. Above all, potentially the coup de grâce, lies the reality of climate change. Global warming does not merely imply amelioration –Mediterranean beaches in hitherto chilly northern latitudes; vineyards in Scotland. It will bring a century and more of extremes, and of extreme uncertainty: flooding of reefs, mangroves and fertile coastal strips, of lowland countries and states from Bangladesh to Florida, and of cities from London to New York; stronger hurricanes, more often; the world’s great wheat and corn fields, our ‘breadbaskets’, potentially reduced to dust bowls; absolute loss of ancient habitats, including the beautiful frozen lands of the polar bear, the northern forests of spruce and aspens, and – most destructively of all – tropical rainforest. The first signs are with us. I have two grandchildren and I fear for them.
Worst of all, though, is that the standard solutions to all our problems, bruited from on high by the world’s most powerful governments, are in truth their prime cause. ‘Progress’ is the buzzword. If this meant improvement of human wellbeing and security then it would a fine concept, but in practice it does not. It has come to mean ‘Westernization’ – that all the world should be more like us: more industrialized, which means more global warming; and more urbanized, which means less care for the countryside and even bigger slums. By 2050, on current projections, there will be as many people in cities (6 billion) as now live on the whole earth. The cities cannot cope. Demonstrably, they are losing the race. At the same time the world’s politics has become more abstract – geared not to physical realities, but to the abstraction of money. Governments measure their success in GDP –the total wealth created. Good endeavours can create wealth, of course, like building schools, and planting forest. But bad things create wealth at least as easily – like felling forest and flogging it off, or simply making war. The cash thus conjured up is all grist to the GDP and so, on paper, contributes to the great desideratum of ‘economic growth’. Agrarian economies that have ticked along for thousands of years in perfect harmony with their surroundings are said to be ‘stagnant’, and ‘ripe for development’. ‘Development’ in turn is equated with increase in cash. The fight against hunger, disease, oppression and injustice has been reduced (dumbed down, one might say) to ‘the war on poverty’.
Yet very simple arithmetic shows that the Western goal of urbanized industrialized life and the promise of limitless wealth for everyone cannot be realized. It would need the resources of at least three earths to raise everyone in the world to the material standards of the present-day average Brit (or a less-than-average American). Furthermore, the means by which this dream is being imposed on the world (in the names of ‘freedom’, ‘democracy’ and ‘justice’, as well as ‘progress’) seem expressly designed to undermine the wellbeing of most of humanity. For most people in the world live and work in the countryside, at rural crafts of which the chief is farming. Industrialization simply puts them out of work. If India farmed the way the British do, then half a billion people would be unemployed – more than the total population of the newly expanded European Union; almost twice the population of the present United States. No other industry can employ so many people as farming can, and still does, and no other industry could conceivably be so sustained. Most of Africa is even more emphatically rural than India, and has even fewer realistic options. Yet Westernization – urbanization, industrialization, and a monetarized economy geared to GDP – is promoted by the world’s most powerful governments as the universal algorithm for all of Africa, Asia and South America: the formula which, once applied, will solve all our problems. Even worse, the formula is applied with all possible haste, for the world’s new economy is driven by the perceived need for ‘competition’. But untempered competition is a crude concept indeed, as any worthwhile moral philosopher would attest, and any truly modern biologist could ratify.
The world needs a sea change. It isn’t just a question of changing our leaders’ minds, for that is too exhausting and we don’t have enough time. It isn’t a question of changing our leaders – for we are liable simply to land ourselves with more of the same. The world as a whole needs a different kind of governance; different kinds of people in charge. To ensure this, we (all of us, for it’s no use relying on the current leaders, who are doing well according to the status quo) must find new ways of ensuring that new kinds of people are elected and given real power: people who respond to the real, physical needs of the world, and to the needs of humanity at large. Present-day leaders – politicians and captains of industry – are wont to suggest that any radical initiative, that takes account of the realities of soil, water and climate, is ‘unrealistic’, commonly because such initiatives may inhibit the plans of bullish industries and their governments, and hence inhibit ‘growth’. But the word ‘realistic’ has been corrupted. It ought to apply to the realities that are inescapable – of physics, of biology – made manifest in the declining earth, and the creatures that live on it. It should apply to the realities of people’s lives – whether they have enough to eat, and water, and shelter; whether they have control over their own lives, and worthwhile jobs, and can live in dignity. The ‘reality’ of which our current leaders speak is the reality of cash. But cash is not the reality. Cash is the abstraction.
Sea change, to mix the metaphors, may seem pie-in-the-sky. In truth, however, and encouragingly, most of the big and necessary ideas are already in place, or already being worked upon. Excellent ecological studies are in train. Truly ‘appropriate’ technologies, designed expressly to be sustainable – both low tech and high tech –are developing apace, and many (including many of the simplest) are quite wonderful. And, which is at least as necessary, many groups worldwide are working on new economic models which, though basically capitalist, are geared directly to human wellbeing, rather than to the gross abstraction of accumulating cash. Certainly, there can be no sure-fire formulae. We simply cannot tell, until we try, whether our ecological projections and economic plans will work out the way we want them to. But if we keep our eyes on the right targets – human wellbeing in a diverse world; humanity able to live well, effectively for ever – then we are in with a chance. If we proceed with reasonable caution, we can correct mistakes as we go. If we simply apply the present-day algorithms – high tech applied willy-nilly wherever it will make most money most quickly – then, surely, we and the world as a whole have had our chips.
Most importantly, the future endeavours of humanity must be geared to biological realities. The world’s economies (and the endeavours of scientists and technologists) must serve those realities. Most obviously – once we start to think seriously about the fate of cities, and environmental stress in general, and human employment and dignity – we see that for the foreseeable future, and probably for ever, the economies and physical structure of the world must be primarily agrarian. In the current, crude, unexamined dogma, ‘development’ and ‘progress’ mean urbanization. The prime requirement, in absolute contrast, is to make agrarian living agreeable. It can be. It’s just that at present, all the world’s most powerful forces are against it.
Trees are right at the heart of all the necessary debates: ecological, social, economic, political, moral, religious. It isn’t the case that trees are always a good thing. The wrong tree in the wrong place can do immense damage. It isn’t true that everywhere in the world needs more trees. In some places, there could be too many – lowering water tables that are already too low. The wrong trees in the wrong places can poison rivers, undermine buildings and even cause soil erosion. Yet beyond any doubt, most landscapes and the world in general would benefit from many, many more trees th
an there are now, and the wholesale squandering of present-day forest seems like an act of suicide.
But it also true – marvellously and encouragingly so – that societies can build their entire economies around trees: economies that are much better for people at large, and infinitely more sustainable, than anything we have at present. Trees could indeed stand at the heart of all the world’s economics and politics, just as they are at the centre of all terrestrial ecology. The more I have become involved with trees in writing this book, the more I have realized that this is so. In the future of humanity, and of all the world in all its aspects, trees are key players.
THE BIGGEST CHALLENGE OF ALL: CLIMATE
The world’s climate has fluctuated spectacularly this past few billion years, often to fantastical extremes. At times the whole world has been tropical, with palm trees in Canada, fringing the Arctic Ocean. At other times (long before the recent wave of ice ages) the entire surface may have frozen over. Beneath the streets of London – obligingly revealed by civil engineers this past few hundred years – are fossils of crocodilians and of nipa palms, denizens of tropical rivers; but also of woolly mammoths, creatures of the tundra. Those who suppose, as some of the world’s leaders have chosen to do this past few decades, that the world is more or less bound to be the way it is now, should look at the evidence all around of past deviations, or at least take notice of those who have.