by Colin Tudge
So water is not pumped from below but dragged from above by the leaves, up through the vessels of the xylem, not in a crude and turbulent gush but in millions on millions of orderly threads. Each liquid thread is only as thick as the bore of the conducting vessels: the biggest are 400 microns across (0.4 of a millimetre) and most are far smaller than this. The tension within them is enormous: the threads are taut as piano wires. Yet, except under conditions of severest stress, they do not break. Water molecules cling tightly together. Their cohesive strength is prodigious. Were it not so, trees could not pull water from below, and could not grow so tall; but in practice the forces are such that a tree could grow to a height of three kilometres if the tensile strength of water was the only constraint on its growth. Even as things are, the threads of water may sometimes break – an accident known as ‘cavitation’ – leaving a space in the vessel that a plumber would call an airlock, and a surgeon would call an embolus. Given time and favourable conditions, plants can eventually fill this space again, and normal service is resumed. Otherwise, if cavitation is too great, the tissues that depend on the vessels may die. Parasites such as the mistletoes increase the tendency to cavitation because they take in the conducting vessels of their hosts and extract water from them by transpiring more quickly than the host, thus creating even greater osmotic tension than the host itself. Sometimes in these conditions the water supply gives out. Mistletoes are wonderful and have launched a thousand myths, but they may kill their hosts, not least by desiccation.
The final evaporation of water from the leaves, out through the stomata, should perhaps be seen as a side effect of the whole mechanism. The evaporation may bring benefit because it cools, as sweating does, and it’s hot out in the sun. Some drought-tolerant plants practise a refined form of photosynthesis known as crassulacean (because it was first discovered in succulent plants of the genus Crassula, much beloved of gardeners) that is designed (or evolved) to reduce the loss of water. In such plants, the stomata open only at night. The carbon dioxide that is taken on board during the night is then put into temporary chemical store, and is released again next day when the sun comes out and photosynthesis can resume. No tree that I know practises crassulacean photosynthesis, so it is not directly relevant here – except to say that these plants at least demonstrate that it is possible to live out in the sun without overheating, even when water is not lost by evaporation. So it seems that most plants (including all trees) lose water through the stomata simply because this is very difficult to avoid; or at least, the loss is a price worth paying to maximize the efficiency of photosynthesis. The point of the plant’s architecture – all those conducting vessels, all those perforated leaves – is to bring the Greek elements together: to present water to the sun, in the presence of air. But it is hard to bring them together without losing water, and sometimes losing more than the plant would like.
The overall effect is a flow of water from the roots, through the vessels to the leaves and out to the atmosphere: trees act like giant wicks. The final loss of water by evaporation is called ‘transpiration’; and the total flow of water from soil to atmosphere is the ‘transpiration stream’. The overall magnitude of this stream, especially when several trees are gathered together, can be prodigious; and its effect on soil and climate, and thus on surrounding vegetation and landscape, is critical to all life on earth, including ours. (I discuss this further in Chapter 14.)
So to the earth, the fourth of the four Greek elements. In this context, earth means soil.
THE SOIL
Air and water provide the carbon, oxygen and water that are the most basic materials of plants. The soil provides all the rest: nitrogen, phosphorus, sulphur – which (apart from carbon, hydrogen and oxygen) are the materials required in the greatest amounts – and a host of metals. All these extra elements from the soil are collectively known as ‘minerals’. If any of these essential minerals is lacking (or indeed if carbon or water are lacking) then growth is restrained, if not impossible: whichever ingredient it is that is deficient, and holding up the rest, is called the ‘limiting factor’. In most soils, the most likely limiting factors are nitrogen, phosphorus and potassium: and these are the three standard components of the artificial fertilizers used in agriculture, which accordingly are packed in bags marked ‘NPK’ (K being the chemical symbol of potassium).
In truth, such fertilizers should probably contain sulphur as well: but the fields and forests at least of industrialized countries have been well if dubiously served this past 200 years by the sulphur-rich smoke from coal-burning factories, which has kept crops and trees alike well supplied. Now that factories are burning cleaner fuels, crops could well become short of sulphur and we are likely to see fertilizer bags marked ‘NPKS’. Nitrogen, too, has rained on plants from on high, mainly in the form of the ammonia and nitrate from car exhausts; indeed there has been enough nitrogen from such pollution to keep the forests of Europe ticking over, even when the timber and fruits are harvested. It is indeed an ill wind that blows no good. On the other hand, sulphur and nitrogen in the form of sulphuric and nitric acids fall as acid rain, and then have often proved extremely harmful. To enrich the soil by polluting it is a precarious way to proceed.
Nitrogen is the mineral needed in greatest amounts. As a chemical element, it is tremendously common: it accounts for nearly 80 per cent of all the gas in the atmosphere. But in gaseous, elemental form, nitrogen is of no use to plant or beast. For plants to absorb it, it must first be converted into some soluble form – of which the commonest by far are nitrate and ammonia. Of course, organic gardeners contrive to supply their plants with nitrogen in organic form – which, broadly speaking, means nitrogen in the form of protein (or the broken-down products of protein) in manure and rotting vegetation, and so on. Fair enough. But the plants cannot absorb the nitrogen in the organic material, and hence make use of it, until it too is broken down (by soil bacteria) to ammonia or nitrate. Those two simple compounds are the ultimate currency of nitrogen.
In nature (unassisted by nitrogenous car exhausts), soluble nitrogen comes from four sources. Some may come from ground rock. Organic material – the rotting corpses of animals, plants, fungi and bacteria, and the faeces of animals – is also important. (But the dead leaves that form most of the leaf litter on the forest floor are typically low in nitrogen, for the trees withdraw the nitrogen from them before shedding them.) Then there is ‘nitrogen fixation’, in which nitrogen gas is chemically combined with hydrogen (derived from water) to form ammonia, which may then be oxidized in the soil to form nitrate. This happens in two ways. First, lightning fixes a surprising amount of nitrogen: the necessary chemistry is brought about by the electric flash, and the ammonia thus formed is carried into the soil by rain. But a wide range of bacteria can also pull off this trick, albeit with somewhat less drama.
These nitrogen-fixing bacteria live in a variety of ways. Many cyano bacteria are nitrogen fixers. You often see them on the boughs of trees as a dark bluish slime (hence the misleading soubriquet of ‘blue-green algae’); but you won’t see the ammonia (converted to nitrate) that they produce, which is carried down the trunk in solution when it rains, runs into the soil, and so nourishes the tree. Many nitrogen-fixing bacteria live free in the soil, and to a large extent (it seems) they are nourished by carbohydrates that the tree ‘deliberately’ exudes to keep them happy. This is symbiosis of the mutually beneficial kind, known as ‘mutualism’: the tree provides the bacteria with sugars, which they absorb like any other heterotroph; and the bacteria in turn provide soluble nitrogen, which the tree would otherwise lack.
But about 700 species of tree are known to form much more intimate, mutualistic relationships with nitrogen-fixing bacteria (and another 3,000 tree species are suspected of doing so). In these, the bacteria lodge in custom-built nodules on the roots.
Most of the plants that have such nodules on their roots are in the Fabaceae family, the ‘legumes’ – like acacia, mimosa, Robinia and the trop
ical American angelim. It comes as no surprise to any gardener that these leguminous trees are nitrogen-fixers – for so too are peas and beans, from the same plant family. In all of the legumes, the nitrogen-fixing bacteria in the roots are from the genus Rhizobium (though there are many different species of rhizobia). Most gardeners would be surprised to learn, however, that various species from ten other families of flowering plants are also known to fix nitrogen. Like the Fabaceae, all of those families come from rosid orders. Among the nitrogen-fixing, non-leguminous trees are the she-oak, Casuarina, and the alder, Alnus. In the non-legumes, the nitrogen-fixing bacteria are not rhizobium but from a quite different genus, Frankia.
Whatever the details, nitrogen-fixing trees in general can grow on particularly infertile soil, since they make their own fertilizer: and thus we find alders on dank and impoverished river banks. Nitrogen-fixing trees also tend to provide particularly nutritious leaves, for fodder. The leguminous trees especially are the arborescent equivalents of clover, alfalfa and vetch, which enrich the world’s grasslands and are much favoured by livestock farmers. Since the nitrogen-fixing nodules are leaky, they release surplus nitrogen – and so they serve to enrich the soil at large. For all of these reasons, nitrogen-fixing trees are often of particular use to foresters – and especially to agroforesters, who seek to raise other crops, or livestock, among the trees. Thus acacias and Robinia are highly favoured the world over not simply on their own account but also to help all else that grows.
Clearly, close cooperation (via root nodules) between plants and nitrogen-fixing bacteria has evolved more than once: once in the Fabaceae with Rhizobiutn, and also in other rosid groups with Frankia. We have already seen many times how nature has constantly reinvented the same kinds of structure or modus operandi so this need not surprise us. Indeed, such associations seem such a good wheeze – the plant apparently gets free nitrogen fertilizer – we may wonder why all plants don’t do it. But nothing is for nothing. The nitrogen-fixing bacteria are not altruists. They want something in return – that something being sugars. Hence legumes and alders and the rest must use some of the organic molecules that they create by photosynthesis to feed their nitrogen-fixing lodgers rather than directly for their own growth. Clearly, it’s often worth it. Worldwide, the Fabaceae are a particularly successful family. Leguminous trees are a huge presence throughout the tropics, where soils are often low in nitrogen. There are many places, not least the cold, dank woods of Latvia, where alders flourish. She-oaks too find their special niches. Equally clearly, it is sometimes just as easy to do without bacterial residents, and get your fertility from elsewhere (not least from neighbouring legumes).
Far more common and widespread than such arrangements with nitrogen-fixing bacteria, are the associations between trees and fungi that invade their roots – not as parasites, but as useful and in some cases essential helpmates. These associations are called ‘mycorrhiza’, which means ‘fungus-root’. Most forest trees and many other plants too make use of mycorrizae: some, like oaks and pines, seem particularly reliant on them.
Fungi in general consist of a great mass of threads (known as ‘hyphae’, which collectively form a ‘mycelium’) and a fruiting body that typically appears only transiently, and often manifests as a mushroom or toadstool. Many of the toadstools that are such a delight in autumn, and are avidly collected by gourmet peasants in France and Italy and elsewhere, are the fruiting bodies of fungi which, below ground, are locked into mycorrhizal associations with the roots of trees, and help them to grow. Thus the fungi are even more valuable than they seem. The wild mushrooms and toadstools are often only a tiny part of the whole fungus. The whole subterranean mycelium, including the mycorrhizae, sometimes covers many acres and weighs many tons. Forest fungi, mostly hidden from view, include some of the largest organisms on earth.
Mycorrhizae take various forms. Some simply seem to ensheath the fine roots of the trees. Sometimes the hyphae penetrate between the cells of the root, and often, in various structural arrangements, they invade the cells themselves. The relationship, in short, can be extremely intimate. Often, a tree will form mycorrhizal associations with more than one fungus at once, each with a different invasive strategy. Leguminous trees such as acacias, which harbour bacteria in root nodules, commonly have various mycorrhizae as well. Their roots are a veritable zoo.
The arrangement between trees and fungi, like those between trees and nitrogen-fixing bacteria, is extremely advantageous for both participants. The fungi gain because they take sugars from the tree, the products of photosynthesis. The fungal hyphae in turn are functional (and indeed anatomical) extensions of the roots, and hugely increase their efficiency. The hyphae commonly spread far beyond the normal limits of the roots, and vastly increase their effective absorptive area. They also function in the way that fungi always do – by producing enzymes that digest surrounding materials, and then absorbing them. Thus they make direct use of organic materials in the soil and may also break down phosphorus-containing rock – and lack of phosphorus (in the form of phosphate) is often a huge problem for growing plants. Then again, fungi are heterotrophs – they live by breaking down organic material; and so an oak or a pine or an acacia whose roots are fitted with mycorrhizae has the advantage both of autotrophy (through photosynthesis) and of heterotrophy (via its fungal helpmeets). Furthermore, a single fungal mycelium, sometimes covering several hectares, may interact with many different trees. Thus all the trees in a wood, even of different species, may be linked to others; and each may therefore share to some extent in the benison of all the others. Trees collaborate one with another in several ways, as we will explore in the next two chapters. Here is one: cooperative feeding.
Many trees including pines are as successful as they are largely because they have evolved particularly advantageous relationships with mycorrhizal fungi. Astute foresters commonly supply young trees with cultures of mycorrhizae to set them off. Many tropical trees prefer to grow as far away as possible from others of their own species (for reasons discussed in the next chapter) but young temperate oaks are said to grow best when close to others of their kind. Close together, they gain from each others’ mycorrhizae.
Indeed, although we often think of fungi as pests of plants (and they often are: mildews, rusts, wood-rotters and the rest) they often emerge as key allies. Lichens are associations of fungi with algae: and lichens are found everywhere, on rocks and as epiphytes, in thousands of forms. Indeed, many botanists now feel that the association of plants with fungi is intrinsic to the success of both. Both groups evolved initially in water. It seems at least possible that neither could have invaded the land except by cooperating with the other. There is indeed some fossil evidence that the very first algae that came on land had fungal companions. Since then, fungi have evolved in all directions, not least to produce the magnificent creatures that we now know as toadstools; and plants have evolved this way and that, and now include the world’s wonderful inventory of trees. But the old habits persist. Plants and fungi still stick together to their mutual advantage as, apparently, they always did.
All soils are different, but this, in broad-brush detail, is how all trees cope with all of them: the ground rules. Some soils, however, are more different than others. Some are positively weird. But there are trees to cope with some of the weirdest.
STRANGE SOILS: MANGROVES AND OUTRIGHT TOXICITY
Around the shallower shores of the tropics and subtropics, in 114 countries and territories, are the forests known as mangroves – a word that is also applied to the individual trees and shrubs that live within them. The mangrove forest is typically low, but some trees within it sometimes grow to a height of 50 or 60 metres. The mangroves generally extend only a few kilometres inland and cover only 181,000 square kilometres worldwide, and yet are supremely important. Like any forest, they are habitats for a huge array of land creatures – insects, spiders, frogs, snakes, birds, squirrels, monkeys – plus a host of epiphytes; and they provide loca
l people with food, fuel, and timber for shelter. Like any forest, they lock up a significant amount of carbon and so help to protect the world’s climate (of which more later). Unlike most forests, they lack an understorey of specialist shade-loving plants: at ground level, there are just roots, water and mud. Unlike other forests, too, they serve as the breeding ground for a long inventory of marine creatures, including fish, crustaceans and molluscs, and around their roots are trails of marine algae. Thus the mangroves link the food webs of land and sea. For good measure (in a natural state) they filter the silt that may flow from the land and so they protect the beds of seagrass that generally lie further out, permanently submerged, and the coral reefs that often lie beyond the seagrass. They also protect the land from excessive seas – the tsunami that struck so devastatingly at the end of 2004 might well have been less devastating if some of the shores had retained more of their mangroves. If we take away the mangroves, then all the creatures that they are home to, and all the seagrass beds and reefs and coastal lands that they protect, are liable to disappear as well.
Most plants, like most creatures of any kind, are killed by too much salt: but mangroves spend much of their time with their roots in pure seawater. This is sometimes diluted by rain, but at other times it evaporates to become as saturated with salt as water can be, until the salt begins to crystallize out; and for good measure, much of the tree roots are intermittently exposed as the tides rise and fall. In addition, the mud in which the trees are rooted is often shallow and is invariably starved of oxygen except for the top few millimetres – yet roots need oxygen. In temperate latitudes, willows, poplars and alders are among the trees that cope with waterlogged soils that may similarly be deprived of oxygen – but at least, in their cases, the water is fresh. Salt water raises a whole new raft of problems.