by Colin Tudge
These nitrogen-fixing bacteria live in a variety of ways. Many cyanobacteria 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 seven hundred species of tree are known to form much more intimate, mutualistic relationships with nitrogen-fixing bacteria (and another three thousand 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 acacias, mimosas, Robinia, and the tropical 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, nonleguminous trees are the she-oak, Casuarina, and the alder, Alnus. In the nonlegumes, the nitrogen-fixing bacteria are not rhizobia 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 riverbanks. 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 favored 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 favored 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 Rhizobium, 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 so good for the plant—it gets free fertilizer—that 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 (for instance, from neighboring 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 mycorrhizae, which means “fungus-root.” Most forest trees and many other plants, too, make use of mycorrhizae: 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 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 harbor 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 acres, may interact with many different trees. Thus all the trees in a forest, 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 one another’s 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 onto land had fungal companions. Since then, fungi have evolved in all directions, not least to produce the magnificent creature
s 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 mangrove swamps, named for the trees and shrubs that live within them. The mangrove swamp is typically low, but some trees within it may grow to a height of fifty or sixty meters. Mangrove swamps generally extend only a few miles inland and cover only 71,000 square miles worldwide, yet they 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 local people with food, fuel, and timber for shelter. Also 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 understory 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 mollusks, 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 sea grass that generally lie farther out, permanently submerged, and the coral reefs that often lie beyond the sea grass. 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, all the creatures that they are home to, and all the sea grass beds and reefs and coastal lands 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 millimeters—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.
To be sure, tropical seashores are in many ways favorable for growth (nutrients, water, sunshine), but conditions overall are as tricky in their way as in any desert, or on any frozen tundra. So you might expect that only a very few, extremely specialist plants could live there. Yet mangrove forests worldwide contain many species of trees and shrubs, and there is a core group of thirty or forty that occur in most of them, and these come from several plant families. The core group includes various “white mangroves” from the Combretaceae family; the red mangrove, Rhizophora, and others from the Rhizophoraceae family; Xylocarpus from the mahogany family, the Meliaceae; Avicennia from the somewhat recondite Avicenniaceae family in the order Lamiales (the order of teak and mint); and palms of the genus Nypa.
Independently of one another, the different tree families of the mangroves have evolved a series of physiological tricks to cope with the otherwise disastrous conditions in which they find themselves. Thus, the tissues that form a sheath around the xylem of the roots provide “ultrafiltration,” preventing the salt from entering the conducting vessels and thus polluting the rest of the plant. All mangrove species can do this, but some are particularly good at it, including the red mangrove. Some that are less efficient at filtering out the salt, like Avicennia, do absorb at least some salt; but then they excrete it actively (by processes that use energy) from special glands in their leaves, where it may dry in the sun to form palpable crystals. Many seabirds do the same thing through their beaks.
Mangrove trees in general combat the airlessness of the soil by ingenious anatomy. Most have at least some aerial roots, directly exposed to the air. Their surface is perforated with lenticels, apertures that enable air to enter, and inside the root the tissue is spongy, with huge air spaces between the cells that may account for 40 percent of the total volume. Many have stilt roots, mostly out of the water. In Rhizophora these stilt roots arch away from the trunk, enter the mud, but then may reemerge and form another loop, snaking along half in, half out. In some species, including Avicennia, the stilt roots thicken to form buttresses. Many species from unrelated groups have independently evolved “pneumatophores,” which grow vertically into the air to act as snorkels. In Avicennia the pneumatophores are thin like pencils. In other species they may be secondarily thickened, and develop into tall, substantial cones. As a final refinement, it seems that the air that does get into the roots is not left simply to find its own way around. The rising tide pushes the old air out; and when it recedes, fresh air flows in again through the lenticels and pneumatophores. Thus the roots of the mangrove trees effectively breathe. They use no muscle power to do this, as an animal must. The sea is their diaphragm. The tide serves to aerate their roots; wind and fleets of obliging animals spread their pollen and seeds. Trees just don’t need the elaborations of muscle and blood and nerves on which animals expend so much.
All in all, the main problems for the trees of the mangroves are chemical: all that fierce salt; too little oxygen. Yet, chemically speaking, the seashore is by no means the most hostile environment that the earth provides.
In particular, land that has been polluted by volcanoes—naturally polluted, that is—may contain an array of metals in concentrations that would be lethal to most plants and, indeed, to most life. Some of them are simply innately toxic. Others are present in most soils and may be essential in very small amounts but are lethal in high concentration. Yet again, many plants, from many different families, have evolved tolerance. So it is that an array of plants from various families have been definitely shown to be highly tolerant to nickel, zinc, cadmium, or arsenic. Others have been reported (though not confirmed) to withstand high doses of cobalt, copper, lead, or manganese.
Most outstanding is a tree from New Caledonia, the island that is so extraordinary in so many ways. Sebertia acuminata, of the Sapotaceae family, grows in soils rich in nickel. It does not exclude the metal from its conducting vessels, as mangrove trees generally contrive to exclude salt. Instead Sebertia accumulates the metal. Indeed, it accumulates so much that if the trunk or branches are damaged, the rubbery sap within (the latex) runs bright blue. Analysis reveals that the latex contains 11 percent of nickel by weight—and it accounts for an extraordinary 25 percent of the dry weight.
Exactly why some trees accumulate such metals (although few match Sebertia) is unknown. Clearly it’s a way of coping with metal-rich soils. But some other plants may grow in the same soil without accumulating the metals—simply excluding them. Perhaps the metal-or arsenic-rich stems and leaves are natural pesticides. Some experiments suggest that this is so; others give less clear results. In any case, such accumulators seem to offer a means of replanting and, indeed, reforesting land that has been polluted unnaturally, as by mining. There are many examples worldwide of such reclamation. It has even been suggested that soils might be freed of toxic metals by growing hyperaccumulators and then harvesting and removing them. But this could be too slow to be worthwhile and also raises the problem of what to do with the harvested metal-rich plants. Perhaps the metals might be recovered from them and pressed back into i
ndustrial service. But calculations suggest that this would rarely be worthwhile economically: it might be for nickel and cadmium, but not for zinc. Meantime, the hyperaccumulation of metals remains a botanical oddity.
So trees make use of what the soil and the atmosphere have to offer them; and so too they have evolved to endure the extremes of both. But they do not merely endure. Trees are not passive players. They are much more subtle than that.
HOW TREES KNOW WHAT TO DO (AND WHAT TO DO NEXT)
Trees live simple lives—or so it may seem to us: nothing to do all day but stand in the sun with their feet in damp and nutritious earth. But there’s a lot more to their lives than meets the eye. Trees, like all of us, have to do many different things, and they have to do the right things at the right times. Taken in the round, their lives are as intricate as those of Hamlet or Cleopatra, albeit without such conspicuous drama. Living is innately complicated.
All living things must respond to their surroundings, and trees respond in many ways. Many trees, like all plants, can move bits of themselves as the world changes around them, opening and closing their flowers or the stomata of their leaves; these movements are known as “nastic.” More broadly, all plants—including all trees—shape themselves according to circumstance, their stems growing away from gravity and toward the light, their roots generally doing the same in reverse. Trees do not simply grow: they grow directionally, most economically to fill the space available, and this directed growth is called “tropism.” Growth toward the light (as in stems) is called “positive phototropism”; growth away from the light (as in roots) is “negative phototropism.”
